Wednesday, February 09, 1994
                           SESSION 1:  PERSPECTIVES
                                9:00 - 12:30 PM

Chair(s):  K. Burke


Herbert T. D.   Fischer A. G.*   D'Hondt S. L.
   Cyclochronologic Approaches to the KT Event

The KT boundary crisis and associated events occurred within
Polarity Chron 29R, occupying only a small fraction of that
interval. Milankovitch-band cyclicity recorded in pelagic
sediments promises to provide time-resolution of events
within this Chron, down to the ca. 20,000 year level. As yet
only a few of many sedimentary sequences have been analyzed
but we present consonant data from the South Atlantic (Rio
Grande Rise, Walvis Ridge, Herbert and D'Hondt) and from
Spain (Kate and Sprenger). All sequences show precessional
signals (low-carbonate/high-carbonate couplets), and some
show a bundling of these into ca. 100 ka eccentricity
cycles, some of which show grouping into 400 ka eccentricity
cycles.

Unless the "boundary clay" represents a full 100,000 year
eccentricity cycle, Polarity Chron 29R, beginning after the
second or third precession in an eccentricity cycle, lasted
for about 600,000 years. The Ir event and main extinction
occurred after the third or fourth precession in the third
succeeding eccentricity cycle, about 320,000 years after the
magnetic reversal. The main boundary event thus falls near
the middle of the polarity chron; its seeming upward
displacement in stratigraphic plots of pelagic carbonates
results from a drastic (factor of 2-3) diminution of
sedimentation rates, owing to a crisis-induced drop in
carbonate production.

Cyclostratigraphic analysis of more KT boundary sections is
required to check and refine these results. A
cyclostratigraphic answer to the duration of the "boundary
clay" requires sites at which the 400 ka cyclicity is
developed above as well as below the boundary. A general
cyclochronology of the KT episode should provide timing of
associated isotope excursions (preceding positive and
succeeding negative ^13C anomalies) and biotic events (first
and last occurrences of species, recurrences of disaster
forms).


Jablonski D.*
   Mass Extinctions:  Persistent Problems and New Directions

Few contest that mass extinctions have punctuated the
history of life, or that those events were so pervasive
environmentally, taxonomically, and geographically that
physical forcing factors were probably involved. However,
consensus remains elusive on the nature of those factors,
and on how a given perturbation (impact, volcanism, sea-
level change, oceanic anoxic event) could actually generate
the observed intensity and selectivity of biotic losses. At
least two basic problems underlie these long-standing
disagreements: (1) difficulties in resolving the fine
details of taxon ranges and abundances immediately prior to
and after an extinction boundary and (2) the scarcity of
simple, unitary cause-and-effect relations in complex
biological systems.

(a) Detailed stratigraphic patterns: Local outcrops and
cores are the ultimate source of the data used to analyze
mass extinctions, but the pitfalls to taking local data at
face value are still little appreciated, resulting in
massive overinterpretation of paleontological patterns. The
accumulation of sediments and fossils is discontinuous, and
environmental change is the rule, so that temporal gaps and
stepped extinctions are inevitable on some scale in any
local sequence. Further, reworking and time-averaging mix
fossils from successive intervals; radiocarbon dates on
shells collected from surface sediments and forams in core
tops indicate a time-averaging window of 103-104 years in
marine shelf and deep-sea sediments alike [1]. Statistical
protocols are available to test for artificial extinction
steps and to place confidence limits on stratigraphic ranges
[2,3], but to date these have seen little use in the mass
extinction literature: nearly all workers wants to take
their data at face value and as virtually devoid of local
overprint. Clearly, the answer to this problem is to test
patterns against an appropriate null hypothesis.

(b) Inferring cause from effect: One fundamental obstacle in
linking extinction patterns and hypothesized forcing factors
resides in the nature of complex systems: nonlinearities,
thresholds, and elaborate feedbacks often rule out the
reconstruction of simple cause-and-effect cascades. The same
forcing factor might have radically different effects
depending on the state of the system at the time of
perturbation, and several alternative forcing factors might
produce the same biotic response. The survival of an
evolutionary lineage during a mass extinction, for example,
could be because it (1) lived in a habitat that was not
stressed, (2) possessed a physiology or life habit that
allowed it to survive in a stressed habitat, (3) was so
widespread that its range includes a locality that provided
a refuge, and so on. Urgently needed is not another catalog
of potential reasons for survival or extinction, but the
development of protocols for testing the alternatives. This
is partly a matter of constructing large, robust databases
amenable to statistical analysis, and coming to grips with
the need to integrate local and synoptic databases. With
sufficient tuning, the long list of potential KT killing
mechanisms can indeed account for virtually any conceivable
extinction pattern; therefore, while the mere fact of
observed selectivity clearly is no argument against impacts,
neither is it an argument in their favor. The time for
consistency arguments is past: hypothesized extinction
mechanisms need to generate unique predictions on the
timing, selectivity (taxonomic, biogeographic, ecologic), or
other biotic patterns to advance beyond the status of
plausible alternatives.

Selectivity is still a neglected area of study for mass
extinctions, and even negative results will be important
here. Selectivity is played out, if at all, at lower
taxonomic levels (families and genera), biogeographically,
or ecologically--and the relevant parameters are largely
absent from the synoptic databases [4]. Consider the problem
of multiple causation for one general pattern: the
apparently higher extinction intensities in the tropics. The
question is whether the greater losses of shallow-water late
Devonian corals relative to deep-water genera, or of
symbiont-bearing KT corals and rudist bivalves relative to
nonsymbiotic corals and non-rudist bivalves, occur because
(1) tropical biotas in general are fragile, perhaps because
their species are adapted to a narrow range of climatic and
other conditions; (2) reef communities in particular are
such a tightly woven network of biological interactions that
the initial removal of the same proportion of species as
were lost at high latitudes could be more disruptive; (3)
tropical biotas contain a large proportion of extinction-
prone endemics, so that losses are high here owing to
biogeographic structure; (4) the favored habitat of reef
communities, low-sedimentation, and low-nutrient shallow-
water platforms or ramps, is itself easily disrupted [5,6].
Some support exists for (3) and (4). First, among-province
variation in mass extinction intensities within latitudinal
belts tend to be positively related to the proportion of
endemic genera in the pre-extinction biota [7]. Second, a
global analysis of end-Cretaceous extinction in marine
bivalves found that tropical settings outside of the
carbonate platforms suffered no greater losses than did
extratropical faunas [8]; the reported latitudinal gradient
in plankton extinction intensities may have a similar basis.
The statistical dissection of alternative mechanisms offers
considerable promise for improving our understanding of
extinction mechanisms and biological consequences, both for
the Big Five mass extinctions and for the smaller extinction
maxima that occur throughout the Phanerozoic.

The initial physical and biological conditions must play a
role in the biotic response to a perturbation. Perhaps, for
example, impacts or volcanism at times of low relative sea
level yield greater taxonomic losses than the same event at
times high sea level when the thermal inertia and other
ameliorating effects of shallow seas are prominent. This
frequent suggestion has yet to be modeled rigorously, let
alone tested empirically, but may help to explain why some
geologically detectable impacts evidently had negligible
biotic effects. The global biota or even individual taxa may
vary in relative extinction vulnerability through time. For
example, it may take time to accumulate a new crop of
extinction-prone taxa after a major extinction event has
removed all but the hardiest lineages. Conversely, lineages
may evolve in directions that make them less vulnerable to
successive perturbations. Such biotic lags and long-term
shifts might explain waiting times between extinction events
[9, but see 10], and the differential responses of
individual taxa to successive extinctions [7].

The evolutionary impact of mass extinctions is another
active area of research. Many of the biotic replacements
once thought to represent competitive victories over
inferior lineages now appear to have been mediated by major
extinction events, even though most species extinctions in
the fossil record, probably >90%, occur outside of the five
major extinction events [11]. Mass extinctions have such
profound biological consequences because they bite deep into
standing diversity and disrupt background selection regimes,
not because they account for most species terminations.
Traits that favor survival during mass extinctions need have
little correlation with those that enhance survival and
diversification during background times, so mass extinctions
can have unpredictable and lasting evolutionary effects
[5,7]. Not only do mass extinctions remove taxa and
adaptations well-suited to the background regimes that
represent the great bulk of geologic time, they create
ecological and evolutionary opportunities by removing
incumbant, dominant taxa and enabling other taxa to
diversify in the aftermath of the extinction event. On the
other hand, mass extinctions do not completely reset the
evolutionary clock: many major evolutionary and ecological
trends transcend even the Big Five events (e.g., the
modernization of marine communities, the rise of flowering
plants, and of predatory neogastropods). We need to
understand survivors and rebounds as well as victims and
ecosystem collapse.

References: [1] Kidwell S. M. and Behrensmeyer A. K. (1993)
Paleont. Soc., Short Courses in Paleontology, 6. [2]
Gilinsky N. L. and Signor P. W. (1991) Analytical
paleontology, Paleont. Soc., Short Courses in paleontology,
4, Knoxville, Univ. TN. [3] Sepkoski J. J. Jr. and Koch C. F
(1994) in Global Bio-Events and Event-Stratigraphy (O. H.
Walliser, ed.), Berlin, Springer. [4] Sepkoski J. J. Jr.
(1990) GSA Spec. Paper, 247, 33-44. [5] Jablonski D. (1986)
in Patterns and Processes in the History of Life (D. M. Raup
and D. Jablonski, ed.), Berlin, Springer, 313- 329. [6]
Jablonski D. (1994) Phil. Trans. Roy. Soc. London, in press.
[7] Jablonski D. (1989) ibid., B325, 357-368. [8] Raup D. M.
and Jablonski D. (1993) Science, 260, 971-973. [9] Stanley
S. M. (1990) Paleobiology, 401-414. [10] Sepkoski J. J. Jr.
(1989) J. Geol. Soc. London, 146, 7-19. [11] Raup D. M.
(1991) Paleobiology, 17, 37-48.


Alvarez W.*   Asaro F.   Claeys P.   Grajales-N. J. M.   Montanari A.   Smit J.
   Developments in the KT Impact Theory Since Snowbird II

At the second Snowbird conference, entitled "Global
Catastrophes in Earth History: An Interdisciplinary
Conference on Impacts, Volcanism, and Mass Mortality," held
in October 1988, much of the discussion focused on criteria
for choosing between the competing explanations--impact or
volcanism--to account for the KT boundary mass extinction.
Other participants argued that the paleontological record
does not support a rapid mass extinction, and that neither
impact nor volcanism was needed to explain the
biostratigraphic observations.

At Snowbird II, the proponents of volcanism struggled to
explain the physical characteristics of the KT boundary
stratum (shocked quartz, spherules, anomalous iridium), but
pointed with confidence to the Deccan Traps as the volcanic
center responsible for the extinction. In contrast, the
proponents of impact could reasonably explain the physical
features, but had no candidate crater of anywhere near
sufficient size.

As members of the proimpact group, we present the following
account of our perception of developments since Snowbird II,
well aware that supporters of volcanism will see the matter
differently.

Back at the time of the first Snowbird conference, Signor
and Lipps [1] showed that an abrupt extinction studied with
insufficient sampling of the fossils would appear gradual.
Two major animal groups previously said to have died out
gradually have recently been studied with heavy sampling and
careful statistics (ammonites [2] and dinosaurs [3]) and
their disappearences at the KT boundary are now seen to be
indistinguishable from a sudden extinction. These cases are
clear examples of the Signor-Lipps effect, and weaken the
case for a gradual KT extinction.

The strongest improvement in the volcanist position has come
from isotopic dating of the Deccan Traps, which yielded an
age of 64.96 +- 0.11 Ma [4]. However, this date comes from a
late intrusion cross-cutting the lavas, and earlier ages of
68.53 +- 0.16 Ma and 68.57 +- 0.08 Ma were measured on early
volcanic alkalic complexes. Although the main pulse of
volcanism may have occurred close to the KT boundary at 65
Ma, the Deccan Traps seem to have begun erupting well before
the extinction, making a causal relationship less likely.
Nevertheless, the near coincidence in timing is striking,
keeping alive the possibility of some connection between the
Deccan volcanism and the KT boundary event.

It is probably fair to say that the case for explaining the
physical characteristics of the KT stratum by volcanic
processes is no stronger than it was in 1987. Around the
time of Snowbird II, volcanists were reporting the
occurrence of anomalous Ir, spherules, and shocked quartz in
more than one horizon across the KT boundary at Gubbio
which, if true, would falsify the derivation of these
features from a single large impact, and better permit their
attribution to volcanic sources. A blind sample
investigation was organized to test whether these features
are confined to the KT boundary bed alone. Although the
results of the blind experiment have not yet been published,
the results seem less critical now because of the
recognition, in the interim, of a strong candidate crater in
the subsurface of the Yucatan Peninsula, larger than any
other impact crater known on Earth, and apparently dating
precisely from the time of the KT boundary.

About the time of Snowbird II, the unusual clastic beds at
the KT boundary of the Brazos River in Texas began to be
interpreted as tsunami deposits [5], suggesting that the KT
impact site might be located in the Gulf of Mexico-Caribbean
region. Persistent searching [6] yielded various candidate
sites, one of which--Chicxulub--is now widely favored as the
KT impact crater.

The Chicxulub crater was located by PEMEX geophysicists in
the 1950s [7], drilled as a petroleum prospect, and
abandoned when it was found to contain melt rocks that were
interpreted as andesites. Penfield and Camargo [8] proposed
in 1981 that it was an impact crater, but the suggestion lay
dormant for 10 years, until Hildebrand et al. [9] proposed
that Chicxulub was the long-sought KT impact site.

Recently located samples from the old PEMEX wells include
breccias and crystalline rocks compatible with an impact
origin but very difficult to explain by volcanic processes
[10]. Single-crystal ^40Ar/^39Ar dating places the melt rock
at 65.0 Ma [11, 12].

If Chicxulub is indeed a very large impact crater dating
from KT boundary time, it should be surrounded by extensive
deposits of impact ejecta and tsunami deposits. This is
indeed the case, and the expected deposits have been located
in many outcrops from the southern U.S. states of Alabama,
Mississippi, and Texas, through the eastern Mexican states
of Nuevo Leon, Tamaulipas, and Vera Cruz, and the State of
Chiapas in southeastern Mexico, as well as in Haiti and in
DSDP Leg 77 sites 536 and 540, between Yucatan, Cuba, and
Florida. Many of these outcrops contain bubble-rich
spherules with tektitelike morphology in a basal layer,
overlain by a clastic bed with sedimentological
characteristics strongly indicative of deposition by a
large, complex tsunami event [13]. They are constrained by
biostratigraphy to lie precisely at the KT boundary (unless
the boundary is defined so as to force them to be slightly
older [14]).

Many of the tsunami outcrops discussed above contain intense
enrichments of the element iridium above background, a
characteristic found worldwide at the KT boundary in more
than 100 sites. The Ir distribution at Arroyo el Mimbral in
Tamaulipas [15] shows several maxima near the top of the
3.2-m-thick tsunami deposit, in contrast to marine sections
elsewhere that have a main Ir peak near the bottom of a
typically 1-cm KT boundary clay bed. Multiple peaking of the
Ir and interlayering of different sized sediment fractions
like sandstone and siltstone suggest that tidal-wave seiche
activity was not over at the time the iridium peaks were
being deposited, and so the latter may have occurred within
a few days after the impact. This does not mean, a priori,
that the Ir could not have been deposited somewhat earlier,
since it could have been remobilized by the tsunami action.
Calculations have shown, however, that a bolide large enough
to cause the Chicxulub Crater would contain enough Ir to
supply that found worldwide at the KT boundary, and that
impact dynamics would distribute it worldwide. Although
other sources for the KT Ir are possible, the Chicxulub
impact is sufficient to produce all that has been observed,
and to explain its stratigraphic distribution.

Glass has been recovered from the Beloc outcrop in Haiti
[16-18] and the Mimbral outcrop in Tamaulipas [15]. Two
varieties are common: a black glass of roughly continental
crust (or andesitic) composition, and a yellow glass rich in
calcium. The Haiti and Mimbral glasses give single-crystal
40Ar/39Ar ages of 65.0 Ma, exactly the same value as found
for the Chicxulub melt rock. Isotopic studies by Blum and
Chamberlain [19] demonstrated that the Haiti glass (1) has a
strong variation in oxygen isotopic composition that rules
out a volcanic origin, and (2) has an isotopic and major-
element composition that can be explained by melting of
silicate basement and carbonate cover. In a follow-up study,
Blum et al. [20] found an essentially perfect isotopic match
between the Haiti glass and the Chicxulub melt rock.

Developments since Snowbird II have thus strengthened the
case for the impact explanation for the KT boundary. An
impact crater as large as [9] or larger than [21] any other
on Earth has been discovered in the Yucatan Peninsula and
radiometrically dated as of exactly KT boundary age. Ringing
the Chicxulub Crater are clastic deposits apparently due to
a giant tsunami and rich in tektite-shaped objects, some
still containing glass. The glass has the isotopic
characteristics of impact melt, matches the chemistry of
Chicxulub, and gives radiometric dates of KT boundary age,
and the clastic deposits lie at the biostratigraphic KT
boundary. It would be hard to imagine a more dramatic
confirmation of the essential prediction of the KT impact
theory, that an appropriate crater should be found.
Nevertheless, some recent authors totally reject this view
[22-25]. These last papers represent the skepticism required
for testing any scientific interpretation, but even though
the testing is not yet complete, the impact theory must be
considered to have been notably strengthened since Snowbird
II. It is now essential to explore the Chicxulub Crater with
the drill.

Dedication: This review is dedicated to the memory of S. V.
Margolis. Stan contributed greatly to the developments
described here, and we all miss him.

References:  [1] Signor P. W. and Lipps J. H. (1982) GSA
Spec. Paper 190, 291-296. [2] Ward P. D. et al. (1991)
Geology, 19, 1181-1184. [3] Sheehan P. M. et al. (1991)
Science, 254, 835-839. [4] Basu A. R. et al. (1993) Science,
261, 902-906. [5] Bourgeois J. et al. (1988) Science, 241,
567-570. [6] Hildebrand A. R. and Boynton W. V. (1989) GSA
Abstr. with Progr., 21, A371; Hildebrand A. R. and Boynton
W. V. (1990) LPSC XXI, 512-513; Hildebrand A. R. and Boynton
W. V. (1990) GSA Abstr. with Progr., 22, A280; Hildebrand A.
R. and Boynton W. V. (1990) Eos, 71, 1424-1425; Hildebrand
A. R. and Penfield G. T. (1990) Eos, 71, 1425. [7] Cornejo
A., and Hernandez A. (1950) Bol. Asoc. Mex. Geol.
Petroleros, 2, 453-460. [8] Penfield G. T. and Camargo Z. A.
(1981) Soc. Expl. Geophys. Tech. Progr., Abstr., Biogr., 51,
37. [9] Hildebrand A. R. et al. (1991) Geology, 19, 867-871.
[10] Cedillo E. et al., this volume. [11] Swisher C. C. III
et al. (1992) Science, 257, 954-958. [12] Sharpton V. L. et
al. (1992) Nature, 359, 819-822. [13] Smit J. et al., this
volume. [14] Smit J. et al., this volume. [15] Smit J. et
al. (1992) Geology, 20, 99-103. [16] Izett G. A. et al.
(1990) U.S. Geol. Surv. Open-File Rept. 90-635, 1-31. [17]
Sigurdsson H. et al. (1991) Nature, 349, 482-487. [18]
Maurrasse F. J.-M. R. and Sen G. (1991) Science, 252, 1690-
1693. [19] Blum J. D. and Chamberlain C. P. (1992) Science,
257, 1104-1107. [20] Blum J. D. et al. (1993) Nature, 364,
325-327. [21] Sharpton V. L. et al. (1993) Science, 261,
1564-1567. [22] Lyons J. B. and Officer C. B. (1992) EPSL,
109, 205-224. [23] Jehanno C. et al. (1992) EPSL, 109, 229-
241. [24] Keller G. et al. (1993) Geology, 21, 776-780. [25]
Stinnesbeck W. et al. (1993) Geology, 21, 797-800.


Sigurdsson H.*
   Environmental Consequences of Volcanic Eruptions and Meteorite Impacts

No abstract available.


Ward P. D.*
   Anatomy of the KT Extinction Event

No abstract available.


Olsen P.*
   Vertebrate Behavior Under Crisis:  The Fossil Record

No abstract available.





                          Wednesday, February 09, 1994
                   SESSION 2: CHICXULUB AND OTHER KT IMPACTS
                                 2:30 - 6:00 PM

Chair(s):  G. Ryder
           G. Izett


Camargo A.*
   Historical Overview of the Chicxulub Crater

No abstract available.


Sharpton V. L.*   Marin L. E.   Schuraytz B. C.
   The Chicxulub Multiring Basin: Evaluation of Geophysical Data, Well Logs,
   and Drill Core Samples

In 1981, Penfield and Camargo [1] reported concentric
gravity and magnetic anomalies over a region in northernmost
Yucatan where unusual occurrences of crystalline rocks had
been reported from wells drilled into the carbonate
platform, and they proposed that these observations could
signal a buried impact structure. Over the last few years,
as eyes turned toward the Gulf of Mexico region in search of
the KT impact site, support has mounted for this Yucatecan
crater, now known as the Chicxulub structure [2-9]. Although
uncertainties still exist as to its exact size and how it
affected the global biosphere, it is now clear that the
Chicxulub impact crater is the source of the worldwide
ejecta layer associated with the Cretaceous-Tertiary
extinction event [2,6,8,10,11]. Here we present observations
from geophysical data, well logs and drill core samples that
indicate the Chicxulub structure is a multiring impact basin
~300 km diameter, and propose that it is the sole source of
the impact debris laid down at KT boundary layer.

Lithologic Evidence of Impact. Three Petroleos Mexicanos
exploratory wells are located near the center of the
concentric gravity anomalies over the Chicxulub structure
(Fig. 1) and all intercepted breccias and crystalline
silicate rocks at depths of ~1 km below sea level [e.g., 12,
13]. Samples from the Yucatan 6 and Sacapuc 1 wells include
suevite, a polymict, clastic-matrix breccia (Fig. 2)
analogous to impact-produced breccias observed at other
terrestrial craters such as the Ries Crater in Germany. The
Chicxulub suevite contains clasts of silicate basement and
platform rocks showing abundant and unequivocal evidence of
shock metamorphism [6], including: (1) planar deformation
features in quartz, feldspar, and zircons, indicating
dynamic pressures up to 23 GPa; (2) shock mosaicism in
quartz and feldspar grains; (3) diaplectic glasses such as
maskelynite (30-45 GPa); (4) fused mineral glasses,
particularly of alkali feldspar, indicating shock pressures
above 45 GPa, and (5) impact melts of whole rock composition
(>60 GPa).

Below the deposits of suevite (Fig. 2) are thick sequences
of impact melt and melt-matrix breccia [6], similar to the
clast-free and clast-rich melt zones observed at other
meteorite impact craters such as Manicouagan, Quebec. Melt
samples show ubiquitous evidence of chemical and thermal
disequilibrium and super heating as is characteristic of
impact melt rocks. Undigested basement clasts retain
indications of shock metamorphism but are usually
overprinted with subsequent thermal effects such as
recrystallization and annealing. Elevated siderophile
element abundances in some melt rock samples [6,7,9] we have
studied indicate a meteoritic component nonuniformly
distributed throughout the melt sheet.

Outside the central region of buried crystalline rocks in
the Chicxulub structure (Fig. 2) wells intercepted thick
sequences of breccia composed mainly of anhydrite and
carbonate clasts from the Mesozoic platform sequence. These
breccia deposits are similar to the Bunte Breccia at the
Ries Crater in that they represent primarily the uppermost
target lithologies and show modest evidence for shock
metamorphism.

The Size and Morphology of the Chicxulub Impact Basin.
Penfield and Camargo [1] originally recognized two
concentric zones in gravity anomaly and aeromagnetic data:
an inner zone of ~60 km diameter characterized by a gravity
high and high frequency magnetic anomalies approaching 1000
nT, and an outer gravity trough with low amplitude (5 to 20
nT) magnetic anomalies. Primarily on the basis of these
patterns Hildebrand et al. [2] proposed that the Chicxulub
crater was a double-ring, or peak-ring basin with a rim
diameter of ~180 km.

More recently, however, the gravity data over the Northern
Yucatan have been reprocessed [14] and additional concentric
patterns outside the original two have been identified
(Figs. 1 and 3). The outermost ring is located at a diameter
of ~300 km. Several arguments have been presented suggesting
that this ring corresponds to the modified rim of the
Chicxulub multiring basin [14]. Of particular importance is
the observation that gravity values increase abruptly and
significantly at the inner flank of the ~200 km diameter
ring (Ring 3 in Figs. 1 and 3), previously interpreted as
the rim crest. Such steep gradients define the edge of a
major mass deficiency in the basin center. We believe that
this is more consistent with the deep transient crater
boundary than with the rim crest, which is the outer edge of
a broad zone of modest and shallow deformation located well
outside the deep transient crater. The fourth ring is
probably not an exterior ring (i.e., located outside the
topographic rim), as observed at some large multiring basins
on other planets, because it has a detectable gravity
expression and because of its association with the edge of
the basin's circular gravity low.

There are additional observations that also indicate that
Chicxulub may be ~300 km in diameter. Well log and sample
analysis data have been used to compile the stratigraphic
correlations in the vicinity of the Chicxulub basin shown in
Fig. 2. These data indicate that the boundary between the
platform sediments and the crystalline basement is depressed
~750 m in wells within 150 km of the basin center. Other
major stratigraphic boundaries are depressed as well. In
addition, thick sequences of breccia are observed in each of
these wells. The Ticul 1 well, closest to the intermediate
ring (Ring 3 in Figs. 1 and 3) contains the thickest breccia
unit, even though previously published reports based only on
drill cuttings show no breccias at all [e.g., 12,13].

Major element chemistry [2,5,6] and isotope analysis [11]
indicate that the Haitian impact glass spherules and the
Chicxulub melt rocks are identical and could be formed by a
mixture of 94% silicate basement and 6% platform carbonate
rocks [11]. Given an average thickness of ~2 km for the
platform cover in this region [12], and allowing for
vaporization of carbonates near the impact point, silicate
basement would have to be melted to a minimum depth of 15 to
20 km to provide these proportions. To the degree that depth
of melting constrains the minimum depth of the excavation
cavity d(sub)exc then the transient crater diameter D(sub)t
can be estimated [15] from the equation d(sub)exc ~= 0.1
D(sub)t or, for Chicxulub, D(sub)t ~= 150 to 200 km. The
final crater diameter D resulting from the collapse of the
transient cavity follows from the relationship D(sub)t ~=
0.5 to 0.65 D [15]. This predicts D ~= 300 km for the
Chicxulub crater, in good agreement with the stratigraphic
data from well logs and the geophysical data.

Finally, crater studies show that even at the smallest
possible final rim crest diameter of 170 km, the Chicxulub
basin would not likely be a peak ring basin. Planetary
datasets show a clear progression in the morphological
classes of large craters with increasing size and that the
transition diameter from one class (such as peak ring basin)
to the next (multiring basin) depends strongly on surface
gravity. The planet whose surface gravity and other
properties are most similar to Earth is Venus. Evaluation of
over 1000 impact craters detected by the Magellan space
craft reveal that double ring, or peak ring basins, are
constrained to diameters between 40 km and 110 km [16]. All
larger craters are multiring basins, i.e., basins
characterized by 3 or more concentric rings. This, coupled
with the evidence above, suggests that the previous model of
Chicxulub as a ~180-km diameter peak ring basin is
incorrect.

Consequently, we believe the interpretation most consistent
with all available information is that the Chicxulub
structure as a ~300-km-diameter multiring basin (Fig. 4)
similar to the largest impact landforms observed on the
Moon, Mercury, and Venus [14]. Such events are extremely
rare; there is only one other impact basin of comparable
size produced in the inner solar system within the last
billion or so years: the 280-km Mead Basin on Venus.

The Chicxulub Multiring Basin and the KT Boundary. Melt
rocks within the Chicxulub crater have experienced varying
levels of hydrothermal alteration and albitization [17];
however ^40Ar-^39Ar determinations on relatively pristine
melt rock samples from the center of the basin indicate a
crystallization age at or very near the KT boundary [6,10].
Evaluation of the magnetization of these and other samples
show that they cooled during an episode of reversed
geomagnetic polarity, consistent with a KT boundary age
[6,18]. Analysis of the Rb-Sr, O, and Nd-Sm isotopic systems
confirm a chemical link between the Chicxulub melt rocks and
the impact glasses contained in the KT boundary deposits at
Beloc, Haiti [11]. Unmelted breccia clasts, representing the
silicate basement impacted by the Chicxulub basin forming
event, are medium- to high-grade continental crust similar
to the lithic clasts observed within KT boundary sediments
from around the world [6,19].

Krogh et al. [8] recently determined U-Pb ages of zircon
xenocrysts from a Chicxulub breccia sample are ~545 Ma, with
a small percentage showing a 418 Ma age. These 545 Ma old
zircons match the crystallization age of those within the
upper member (magic layer, fireball layer, etc.) of the KT
doublet in the Raton Basin of New Mexico. This provides
compelling evidence that the Chicxulub impact event is
responsible for both the shocked minerals observed in the
upper member of the KT boundary sequence and the glass rich
lower member. Consequently all evidence to date points to a
singular, extremely energetic, and deadly, impact event at
the KT boundary.

References: [1] Penfield G. T. and Camargo-Z A. (1981) Soc.
Explor. Geophys. 51st Ann. Meeting, Tech Prog. 37. [2]
Hildebrand A. R. et al. (1991) Geology, 19, 867. [3] Pope
K.O. et al, (1991) Nature ,351, 105. [4] Quezada-M. J. M. et
al. (1992) LPSC XXIII, 1121. [5] Kring D. A. and Boynton W.
V. (1992) Nature 358, 141. [6] Sharpton V. L. et al. (1992)
Nature 359, 819. [7] Koeberl C. et al. (1993) Geochim.
Cosmochim. Acta, submitted. [8] Krogh, T. et al.(1993)
Nature, submitted. [9] Schuraytz B. C. and Sharpton V. L.,
this volume. [10] Swisher C. C. et al. (1992) Science, 257,
954. [11] Blum J. D. et al. (1993) Nature, 364, 325. [12]
Lopez-Ramos E. (1993) Geologia de Mexico, UNAM Press. [13]
Weidie A. E. (1985) Geology of Yucatan Platform, in Geology
and Hydrogeology of the Yucatan, (W. C. Ward, et al, eds.),
NOGS Publications. [14] Sharpton V. L. et al. (1993)
Science, 259, 1564. [15] Melosh H. J. (1989) Impact
Cratering, Oxford Press. [16] Schaber G. C. and Sharpton V.
L. (1993) Nature, 362, 503. [18] Urrutia-Fucugauchi J. et
al.(1993) Tectonophysics submitted. [19] Sharpton V. L. et
al. (1990) Geol. Soc. America Spec. Paper, 247, 349.

Fig. 1, which appears here in the hard copy, shows surface
units, ring locations and wells in the vicinity the
Chicxulub multiring impact basin. See Sharpton et al. [6]
for details.

Fig. 2, which appears here in the hard copy, shows
stratigraphic correlations across the Chicxulub basin based
on well logs and sample analysis. Wells are arranged in
distance from the basin center. The samples we have studied
are identified by the numbered arrows. Other lithological
assignments are based on visual inspection of cores updating
previous published and unpublished reports. Ring
designations refer to the three most prominent rings in the
gravity analysis of Sharpton et al. [6].

Fig. 3, which appears here in the hard copy, shows gravity
profiles taken across the Chicxulub basin at 10 degrees
intervals of azimuth. D (sub)t and D (sub)a refer to the
transient crater diameter and final or apparent crater
diameter resulting from this analysis. Profiles are offset
vertically by 10 mgal; the annotated vertical axis shows the
0 mgal value for the designated profiles, beginning from the
bottom with N to S. Dashed profile is due E to West. The
value of the center point of each gravity profile is 10.4
mgal.

Fig. 4, which appears here in the hard copy, shows a
schematic model of the Chicxulub impact basin that we derive
from this analysis. This simplified cross section shows the
general configuration of the crater but does not consider
erosion; erosion at the time of impact could rearrange the
upper crater units significantly and reduce crater
topography. Faults and unit boundaries are simplified.


Cedillo P. E.   Claeys P.   Grajales-N. J. M.   Alvarez W.
   New Mineralogical and Chemical Constraints on the Nature of Target Rocks at
   the Chicxulub Crater

The Chicxulub Crater melt rocks are being found to display
striking mineralogical and chemical features. The most
important mineralogical and textural features are related to
(1) partial melting of the rock and the preexisting minerals
(quartz, plagioclase, and anhydrite), and (2) textural
features developed in pyroxenes, feldspars, and magnetite
that provide clues to the crystallization behavior of the
melts. Chemical analyses of minerals and whole rocks display
a wide range of compositions. Core samples used for the
present study come from the Yucatan-6 (Y-6) and Chicxulub-1
(CH-1) wells.

Preexisting minerals exhibit numerous features indicative of
partial melting. In some shocked quartz grains, this process
has destroyed lamellar features, while in others the
lamellae were only partially destroyed. Quartz xenoclasts
show a rim of melted and recrystallized quartz surrounded by
a reaction corona of pyroxenes, similar to those found in
the Manicouagan impact melts [1]. Plagioclase crystals show
sievelike melting textures consisting of a network of glassy
material following crystallographic directions. Textural
evidence also supports partial melting of anhydrite; the
physical-chemical conditions under which this process took
place are not completely understood.

Skeletal crystals of pyroxene, feldspar, and ilmenite
provide clues to the crystallization behavior of the melts.
The pyroxene is found as small crystals (10 micrometers),
some only partially crystallized, which indicates
supercooling. It is also common to find larger crystals of
pyroxene (70 micrometers) as reaction coronas around quartz.
This texture shows competition for silica in the melt. The
chemical composition of pyroxenes is characterized by high
calcium content; the analyses plot outside the normal
pyroxene quadrilateral, to the calcic side of normal
diopside. Although X-ray diffraction lines of melt pyroxene
are similar to those of diopside, its composition is closer
to fassaite. The abundance of Ca-rich pyroxenes in the melt
rocks is strong evidence for the participation of platform
carbonate sediments in the genesis of melts at Chicxulub.

Feldspars were formed from glass derived from the melting of
basement rocks. The chemical composition ranges from albite
to andesine, supporting a variety of sources for the glass,
as pointed out earlier [2-6]. Chemical analyses of glasses
show the same variation in composition as their associated
plagioclases, indicating that they have a common origin.

The basement rocks were the source of silica for the
pyroxenes and plagioclases, as shown by xenoclasts found in
the melts of Y-6 [4] and by the abundance of shocked quartz
grains of igneous and metamorphic origin found in KT
boundary sediments [7]. The ilmenite shows skeletal growth
indicative of supercooling, which is also supported by the
absence of exsolution.

In summary, the mineralogy indicates that two groups of
rocks (platform carbonates and basement) were involved in
the genesis of melt rocks at Chicxulub. The relative
contribution of the two sources determines the final
composition of melt rocks. On the other hand, the abundance
of xenoclasts in the melt rocks played an important role in
the thermal behaviour of the melts, favoring a greater
cooling rate. This is the case of melts found in the samples
of Y-6, in contrast to those of Chicxulub-1. Work in
progress is focused on the chemical composition of glasses
in order to constrain the importance of target rocks as
source for melt rocks, vaporization products, and tektites
of the KT boundary.

Acknowledgments: The authors wish to thank the permanent
support of PEMEX and the Instituto Mexicano del Petroleo. We
also express our gratitude to the late Dr. S. V. Margolis,
who participated in the microprobe analytical work at the
beginning of the investigation.

References: [1] Floran R. J. et al. (1978) JGR, 83, 2737-
2759. [2] Swisher C. C. III et al. (1992) Science, 257, 954-
958. [3] Cedillo-Pardo. E. et al. (1992) GSA Abstr. with
Progr., 24, A333. [4] Quezada J. M. et al. (1992) LPSC
XXIII, 1121-1122. [5] Sharpton V. L. et al. (1992) Nature,
359, 819-821. [6] Blum J. D. and Chamberlain C. P. (1992)
Science, 257, 1104-1107. [7] Owen M. R. et al. (1990) GSA
Spec. Paper, 247, 343-347.


Ivanov B. A.*   Badukov D. D.   Yakovlev O. I.
   Shock Degassing of Sedimentary Rocks Due to Chicxulub Impact: Hydrocode
   Simulation

We present the progress report of the work concerned the
scenario of the Chicxulub impact event. The work includes
both 2D hydrocode simulation of the impact and study of
geochemistry of shocked rocks.

The specific feature of the target geology at the Chicxulub
site is the presence of 2 to 3 km thick layer of sedimentary
rocks over the crystalline basement. High contents of
anhydrite (CaSO4) may causes the production of SO2 gases due
to shock heating of rocks [1]; CO2 production from
carbonates also may impact the terrestrial environment at
the KT boundary [2].

Our computer simulation shortly reported earlier [3] is now
in progress to model various stages of the Chicxulub event.
All simulations use the supposed rim diameter of the crater
of 180 to 300 km. To create such a crater the silicate
asteroid with a diameter of 10 to 20 km would strike Earth
at the most probable for asteroids impact velocity of 20
km/s [4]. Variations of asteroid's size, composition, and
impact velocity give an intrinsic uncertainty for all
estimates of a single event. An oblique impact may result in
addition complex phenomena [5].

To make first simple estimates for the future geochemical
modeling we calculate the vertical impact of a cylindric
asteroid into the double layer target. Such a geometry
allows us to use the relatively simple SALE code [6]. The
problem to use this code for the Chicxulub event is to
calculate the shock wave propagation in the target with a
top 3 km layer, which is thin in comparison with a
projectile diameter (10 to 20 km). To resolve the shock
front in the hydrocode with an artificial viscosity we need
to have at least 10 rows of cells to describe the
sedimentary layers. So the cell dimension is limited within
the range of a few hundreds of meters. In this case we need
to cover the projectile with several tens of cells. This
mean that the shape of the projectile may play an important
role in the close to projectile zones.

We run several variants of the code varying the equation of
state of target layers. Tillotson EOS has been used with a
minor modification in the expansion range. To model dynamic
properties of the sedimentary layer we used constants for
wet tuff and limestone, and aluminum and granite constants
for the crystalline basement cited by Melosh [7]. The first
output of the simulation is the estimate of a sedimentary
rock volumes compressed above a given shock pressure. The
geometry of isobars has been also mapped. For all our
calculations the volume of sedimentary material shocked up
to pressures 30 to 60 GPa (the typical for the post-shock
thermal decomposition of carbonates) and higher vary in the
range 500 to 1500 km^3. These values may be used as an upper
limit for all scenario of the consequent effect of impact
produced gases to the terrestrial environment.

It is important to note that more than half of this volume
is the "plug" of sediments compressed by a projectile. This
material will be released to the atmosphere relatively
lately in a mixture with a projectile material and may be
with crystalline material. Other part of a highly shocked
sediments may be involved in the early high speed ejecta. So
the scenario of transport of impact-produced gases to the
atmosphere and stratosphere may have several consequent
stages, which should be investigated separately.

Amount of gaseous phases formed by impact devolatilization
of sulfur-bearing minerals such as anhydrite and gyps should
depend on two causes, which act in opposite ways: (1) a
shock-induced heating and (2) an effect of reverse reactions
after decompression and devolatilization. The degree of the
thermochemical decomposition can be obtained as a function
of amount of post-shock specific internal energy provided
that: (1) the reaction of the decomposition takes place only
after decompression, (2) the system is thermodynamically
closed at the ambient pressure of 1 bar, and (3) all
processes in the system are in equilibrium. According to our
calculation incipient outgassing of anhydrite starts by 90
GPa and is completed by 155 GPa. Violation of an
equilibrium, for example due to fast expansion, may change
simple equilibrium estimates.

One must consider this estimation of as a maximum one
because of high ability of new formed CaO for reaction with
SO3. This reverse reaction can take place in cooling ejecta
plumes. On the other hand the completeness of the reverse
reaction can be restricted due to removal of the gas from
plumes and slow rate of this heterogeneous reaction.
Laboratory shock experiments by high parameters can give
constrains on the suggestion. This work is now in progress.

At the next step of the work we plan to use calculated
evolution of various elements of shocked materials to
estimate the possible kinetics of chemical reactions for the
better constraint of the residual composition of impact-
produced gases.

References. [1] Brett R. (1992) GCA, 56, 3603-3606. [2]
O'Keefe J. D. and Ahrens T. J. (1989) Nature, 338, 247-249.
[3] Pope K. O. et al. (1993) LPS XXIV, 1165-1166. [4] Roddy
D. J. et al. (1987) Int. J. Impact Eng., 5, 525-541. [5]
Schultz P. H. and Gault D. E. (1990) GSA Spec. Paper, 247,
239-262. [6] Amsden A. A. et al. (1980) LA-8095, Los Almos,
NM, 100. [7] Melosh H. J. (1989) Impact Cratering, Oxford
Press, 245.


Takata T.*   Ahrens T. J.
   Numerical Simulation of Impact Cratering at Chicxulub and the Possible
   Causes of KT Catastrophe

The chicxulub crater, located on the Yucatan peninsula is
considered as the most likely site of the impact crater,
which was formed by an extraterrestrial object that appeared
to have caused the catastrophic extinction at the KT
boundary [1]. Recent investigations of the gravity field in
this area extended the possible diameter of the crater from
180 km, which had been previously estimated in Hildebrand et
al. [1] to 260-340 km [2]. In order to create a 260-340 km
diameter crater, the impacting object diameter required is
20 km to 30 km, if the meteorite impacted at a velocity of
20 km/s. The impact of the meteorite of this diameter, that
is 2 to 4 times of scale height of the Earth atmosphere,
could result in global effects on the ecology, environment,
and trigger climate changes that may last ~10^4 years.

Geological investigations indicate that the Chicxulub crater
inferred to have been excavated shallow sea platform
containing 2-4 km thick carbonate and evaporitic rocks
(mainly limestone, dolomite, and gypsum) overlying the
basement rock of andesite at KT time (65 Myr) [3]. This
topographical and geological environment for the Chicxulub
crater is expected to be a typical feature of KT impact
craters. This is because of the high ratio of sea surface to
the continental surface, and the warmer climate of
Cretaceous age. These will result in thick carbonate rock
sequences. The production and release of the water vapor and
CO2 from the terrestrial surface to the atmosphere is
expected during an impact event. The increase of the water
vapor and CO2 in the atmosphere may have resulted in
increasing the global temperature of the atmosphere via the
green house effect [4,5].

Moreover the enormous amount of fine dust ejected from the
crater into the atmosphere and its subsequent global
dispersal in the stratosphere could have possibly decreased
solar insolation inducing a short-duration decrease in
global temperature as first proposed by Alvarez et al. [6].
Partial destruction of the ozone layer in the upper
atmosphere may have also been a factor in extinction of some
species. The possible scenarios including breakdown of the
usual annual hemispheric climate cycle due to the
combination of the above mechanisms have been discussed for
a decade. What mechanisms caused extinction of what
organisms have not been conclusively demonstrated. One
approach to this complex problem is to examine the mechanics
of impact cratering in the Chicxulub crater lithology and
estimate the possible environmental or climatic
consequences.

The impact cratering process on the lithology of the
Chicxulub impact site was investigated by a numerical
simulation. We used the Smoothed Particle Hydrodynamics
(SPH) numerical technique [7]. The SPH method is a free
lagrangian method, and suitable for the highly distorted
motion of materials. All the materials are expressed by
particles that satisfy the hydrodynamic equations and are
treated like N-body problems. In our simulations,
axisymmetric geometry is applied in order to expand the
region taken into account in the calculations.

In order to simulate the response of the lithology at KT
time around the Chicxulub crater site in our code, we
assumed a target with basement rock of andesite, overlaid by
3 km thick-carbonate rock layer, and an atmosphere. A
Tillotson equation of state for limestone is applied for
carbonate, and an ideal gas with constant specific heat
ratio is used for the atmosphere. All the target materials
including atmosphere is initially stabilized and is in
hydrostatic equilibrium. The radius of meteorite is assumed
to be 12.5 km. The advantage of the SPH method is that it
can represent the transfer of energy and interaction of
materials among impacting meteorite, the surface materials,
and the atmosphere. Numerical simulations have been employed
since publication of Alvarez's hypothesis [8,9], however,
the uniqueness of our calculation is that we include an
atmosphere in addition to the carbonate systems.

Figures 1 and 2 show position of materials and particle
velocity at 16 seconds after the impact. We can observe that
the ejecta curtain extends to more than one scale height of
the atmosphere and the mixing of carbonate, basement rocks,
and impactor in the ejecta within the atmosphere. Drilling
data from the center of Chicxulub crater demonstrated the
melt sheet was observed beneath a 200-400 m thick breccia
layer. The results of the calculation show that
approximately 10^1 km thick melt sheet is produced. The
total mass of melt is approximately 20 times the impactor
mass.

The energy transfer to the atmosphere takes place via two
mechanisms: (1) Direct impact of the bolide on the
atmosphere accounts for very few percent of the atmospheric
heating and (2) The work done on the atmosphere by the
moving ejecta curtain transfers the bolide energy to the
atmosphere. When the ejecta curtain of target materials
starts to develop after the impact of meteorite, the total
amount of energy transfered to the atmosphere is less than
0.5%, that is, ~20^22 J. However the ejecta transfers an
enormous amount of energy into the atmosphere due to the gas
drag [8]. As a result of the impact, carbonate rock takes up
approximately 10% of the impact energy, 5% of the initial
energy was left in the meteorite as the internal heating,
and the rest of the energy is transfered to the basement
rock.

The amount of CO2 production by the shock-induced
decarbonation of the carbonate rocks is also estimated. The
decarbonation of calcite was observed experimentally to
occur at 20 GPa [10]. Young carbonate rocks should have had
a certain degree of porosity because they were recently
deposited. In the laboratory, shock experiments using 50%
porous chalk induces devolatilization of 90% of the
available CO2 at 10 GPa [11]. Therefore, we assumed that all
the carbonate pressurized to more than 10 GPa devolatilizes.
By determining the mass of carbonate exposed to >10 GPa, the
production of CO2 by Chicxulub cratering is found to be
10^16-10^17 kg. This mass of CO2 is 10 to 100 times the CO2
inventory in the present atmosphere. This is a greater
increase in CO2 than obtained previously by O'Keefe and
Ahrens [5].

The mixing ratio of impact to target material ejected into
the atmosphere is approximately 0.01 to 0.02. This value
agrees with this ratio (0.01 to 0.1) found in the worldwide
platinum-rich element ratio at the KT boundary [8]. We
calculate that all the meteorite material ejected to the
atmosphere is melted or vaporized.

In conclusion, the production of the enormous amount of CO2,
dust of terrestrial and extraterrestrial origin was
confirmed in our numerical simulation of Chicxulub crater.
The climate system should have been affected by the impact-
induced H2O and CO2 and dust on various time scales. Heating
of the atmosphere probably produces higher water and CO2
vapor pressures [4,5]. If a water cloud expanded globally
this may have increased the albedo of the Earth on a several
month time scale. These albedo-temperature feedback systems
may have been disordered by abrupt changes of constituents
of atmosphere.

Acknowledgements: We appreciate the use of the CRAY Y-MP at
JPL for these calculations.

References: [1] Hildebrand A. R. et al. (1991) Geology, 19,
867-872. [2] Sharpton V. L. et al. (1993) Science, 261,
1564-1567. [3] Swisher et al. (1992) Science, 257, 954-958.
[4] Emiliani C. et al. (1981) EPSL, 55, 317-334. [5] O'Keefe
J. D. and Ahrens T. J. (1989) Nature, 228, 247-249. [6]
Alvarez et al. (1980) Science, 208, 1095-1108. [7] Gingold
R. A. and Monaghan J. J. (1977) Month. Not. Astr. Soc., 181,
375. [8] O'Keefe J. D. and Ahrens T. J. (1982) GSA Special
paper, 190, 103-120. [9] Roddy D. J. et al. (1987) J.
Impact. Eng., 5, 525-541. [10] Lange M. A. and Ahrens T. J.
(1983) LPSC 14, 419-420. [11] Tyburczy J. A. and Ahrens T.
J. (1986) JGR, 91, 4730-4744.

Fig. 1. Particle position during impact Chicxulub at t=16
[s] after the impact. The symbols of the particles, bullet,
triangle, +, and times, represent impactor, basement rock,
carbonate rock, and atmospheric particles, respectively. The
impactor size is 25 km and the impact velocity is 20 km/s.

Fig. 2. The velocity field of the impact cratering at the
same time of Fig. 1.


Robin E.*   Gayraud J.   Froget R.
   On the Origin of Regional Variations in Spinel Compositions at the KT
   Boundary

Introduction: Spinels with morphologies and compositions
quite similar to meteoric spinels (the term "meteoric" that
we use here is defined in reference [1]) have been found
throughout the world at the Cretaceous-Tertiary (KT)
boundary. Their presence and their high abundance is a
strong argument in favor of a major extraterrestrial
accretion event at the end of the Cretaceous. These minerals
display a wide variety of morphology (octahedral, dendritic,
skeletal, cruciform,...) and composition [2] which likely
reflect differences in their formation conditions [1-4].
Here we discuss on the cause of the regional variations in
spinel compositions and on the implications to the
understanding of the KT event.

Samples and Procedure: We have searched for meteoric spinels
at 18 KT sites: Sopelana and Caravaca in Spain, Bidart in
France, Stevn Klint in Danemark, Gosau in Austria, Petriccio
and Nago in Italy, El Kef in Tunisia, Brazos River in Texas
(USA), Beloc in Haoti, Mimbral in Mexico, DSDP site 536 in
the Gulf of Mexico, DSDP sites 577 and 465 respectively in
North and South Pacific Ocean, DSDP site 524 and 527 in
South Atlantic Ocean and ODP site 761C and 752 in the Indian
Ocean. Spinels were extracted from sediment samples with a
magnetic separator and recovered on a 0.4 5m nuclepore
filter (see reference [5] for a description of the method).
The filters were then mounted in epoxy and polished for
detailed scanning electron microscopy and electron
microprobe analysis of individual spinels (> 2 micrometers).

Results and Discussion: Meteoric spinels are found at nearly
all sites except at Stevn Klint, Brazos River and DSDP site
465 where their concentration is below detection limit (<
0.1/mg). At other sites, the abundance of spinels varies
from ~ 0.3/mg at Mimbral up to 3000/mg at ODP site 761C.
Their occurrence is generally associated with the maximum of
the Ir anomaly, in coincidence with a sharp decrease of the
carbonate fraction. Results of the average composition of KT
spinels are given in the table except for ODP site 752, DSDP
sites 524 and 527 for which spinels were not analyzed so
far. Several remarks can be done from these results
considering the formation conditions of meteoric spinels
[1]:

KT spinels have high Ni content (>1%) and high Fe^3+/Fe
(sub)total ratio (>70%) showing that they were formed from
undifferentiated material in a very oxidizing environment
[1,3]. These formation conditions are very constraining and
allow to rule out some possible sources for KT spinels. They
cannot derive from terrestrial magmas which are strongly
depleted in Ni and evolve under extremely low oxygen
fugacity [1,3]. Direct condensation of spinel crystals
during continental cratering events is very unlikely because
of the reducing conditions that prevailed in the impact
vapour cloud [4]. The only process known to form spinels
with such a composition is ablation and oxidation of
meteoritic debris in the atmosphere [3]. Spinels from
Pacific and Indian Oceans have an Fe^3+/Fe (sub)total ratio
>90% which require for their crystallization an oxygen
fugacity >0.21 atm or higher than the one existing at sea
level [1]. Although no definitive explanation exists today
[1], such highly oxidized spinels could be indicative of
oceanic impacts [4,6].

Large variations in spinel compositions are observed from
site to site, even for nearby sites. For example, there is
virtually no overlap in the chromium content of spinels
between Italian sites (Petriccio, Nago) and Tunisian site
(El Kef) (see figure 1). Variation in the Cr content of
spinels is also observed between Sopelana (Spain) and Bidart
(France) which are distant by only 200 km. It implies that
very heterogeneous formation conditions prevailed on a local
geographical scale. At the other hand, numerous cosmic
debris containing Al-rich spinels of quite homogeneous
composition are found over at least 2x10^7 km^2 in the
Pacific Ocean [7]. This suggests that more than one object
struck the Earth at the end of the Cretaceous. The largest
ones in the Pacific to account for the wide dispersion area
of Al-rich spinels and smaller ones over Europe and North
Africa to account for the variation in spinel composition on
a local scale.

Spinels from Pacific and Indian Oceans are always more
oxidized than those from European and African sites (see
figure 2). Spinel from the Carribean area seems to have an
intermediate oxidation state though spinel from DSDP site
536 are highly oxidized. The same tendancy is observed for
their Cr content: spinel from Pacific and Indian Oceans have
lower Cr content than those from European and African sites.
Only spinels from Italy have a relatively low Cr content.
The fact that the Cr content and the Fe^3+/Fe (sub)total
ratio of meteoric spinels is controlled by the oxygen
fugacity [1] suggests that spinels from Pacific and Indian
Oceans were formed under higher oxygen fugacities than those
from european and african sites. This is consistent with the
hypothesis of large objects impacting in the Pacific and
Indian Oceans and smaller ones in Europe and North Africa.
Indeed, large objects decelerate at low altitudes and thus
at high oxygen partial pressures while small objects
decelerate at higher altitudes and thus lower oxygen partial
pressures. If so, it suggests a preferential arrival of
cosmic bolides in the hemisphere containing the Pacific and
Indian Oceans.

Conclusions: (1) The high abundance of meteoric spinels in
most KT samples investigated here confirms that a major
cosmic accretion event occurred at the end of the
Cretaceous. (2) The wide range of spinel composition
observed from site to site indicates that several objects
have impacted the Earth at this epoch rather than a single
one. Multiple objects could derive from a single disrupted
bolide. (3) The observed variation of the Fe3+/Fetotal ratio
of KT spinels from Pacific to Europe suggests that the
largest objects would have impacted in the Pacific and
Indian Oceans and smaller ones in Europe and North Africa.
This would also explain the homogeneous spinel composition
over large area in the Pacific and the heterogeneous spinel
composition on a local scale in Europe and North Africa.

References: [1] Gayraud J. et al. (1994) this volume. [2]
Kyte F. T. and Smit J. (1986) Geology, 14, 485-487. [3]
Robin E. et al. (1992) EPSL, 108, 181-190. [4] Robin E. et
al. (1992) LPS XXIII, 1161-1162. [5] Robin E. et al. (1991)
EPSL, 107, 715-721. [6] Margolis S. V. et al. (1991)
Science, 251, 1594-1597. [7] Robin E. et al. (1993) Nature,
363, 615-617.

Table 1, which appears here in the hard copy, shows Aberage
compostion of KT spinels.

Fig. 1, which appears in the hard copy, shows a histogram
showing the spinel chromium content.

Fig. 2, which appears in the hard copy shows a histogram
showing the Fe^3+/Fe (sub)tot of KT spinels.



                              POSTER PRESENTATIONS

Schuraytz B. C.   Sharpton V. L.
   Siderophile Element Distribution in Chicxulub Melt Rocks: Forensic Chemistry
   on the KT Smoking Gun

The magnitude and duration of catastrophic environmental
effects resulting from collision of an extraterrestrial body
with Earth are, to first order, proportional to the energy
released on impact. This energy can be estimated from
knowledge of the characteristics of the projectile, the size
and morphology of the impact structure, and the amount,
distribution, and composition of the ejecta. Significant
progress toward piecing together the KT puzzle has come from
recent studies of distal and proximal ejecta deposits [1-7]
and recognition of the ~300 km diameter Chicxulub impact
basin as the KT source crater [8-13]. The character of the
projectile in terms of its size and correlation with known
meteorite types, however, is still uncertain [14-16]. In
order to constrain better the chemical fingerprint of the
projectile that may have triggered the KT extinctions, we
report the results of analyses on samples of impact melt
rocks from Chicxulub.

Samples of impact melt rock from drill core intervals Y6-
N17, Y6-N19, and C1-N10 have major element and lithophile
trace element abundances characteristic of average
continental crust [11,17], and most elements fall within the
compositional range of impact glasses from Haiti [3]. The
crustal character of Chicxulub melt rocks and their
compositional similarity to proximal KT ejecta are supported
by S, Nd, and O isotopic analyses [6].

Instrumental neutron activation analyses (INAA) reveal that
several melt rock fragments from C1-N10 and Y6-N19 have Ir
abundances well above those typical of continental crust
[11]. Measured Ir abundances in five samples range from 2.5
+ 0.5 ppb to 13.5 + 0.9 ppb. These anomalous concentrations
are within the range of Ir enrichments in melt rocks from
other terrestrial impact structures [15,18], and suggest
that the Ir is of extraterrestrial origin. Iridium was below
our detection limit in six other samples, with 2s upper
limits ranging from <1.5 ppb to <2.9 ppb. No measurable Ir
was found in our sample of Y6-N17 (2 sigma upper limit <15
ppb Ir; 8.1 + 1.8 ppb Au), confirmed by results of [19] for
their analyses of samples from the same core interval. While
some variation in Ir concentration among melt rocks is not
unusual, it has been suggested that if Ir is detected in one
sample, Ir enrichments are generally found in all samples
within a single melt sheet [15]. Consequently, it is
possible that all our samples contain enhanced levels of Ir,
albeit below our detection limit of ~1 ppb. In addition, the
Ir variations we observe reflect sampling on a relatively
small scale, and although the differences in measured Ir
abundances are significant analytically, they may not be
representative of variations among larger samples integrated
over the entire melt sheet.

Iridium variations among the different samples of Y6-N19 may
reflect in part, the heterogeneous lithologic character of
this melt rock breccia [11]. The nine, centimeter-size
fragments were taken from an 18 cm length of core to sample
specifically the variety of melt clasts and melt matrix
types.  However, significant Ir variations were also
observed between ~100 mg and ~60 mg splits of finely-ground
powder from a single fragment of Y6-N19-R (8.5 + 0.5 ppb and
13.5 + 0.9 ppb, respectively). In contrast to the
lithological heterogeneity of Y6-N19, two samples taken from
different biscuits within the C1-N10 core interval exhibit
similar, uniform intergranular textures, virtually free of
unmelted clasts. Thus, the difference in Ir abundance
between C1-N10-1 (6.0 + 0.7 ppb) and two splits of the same
powder from C1-N10-2 (<2.2 ppb and <1.7 ppb) cannot be
attributed to macroscopic lithologic differences.
Furthermore, a split of our C1-N10-1 powder analyzed by a
different laboratory yielded an Ir concentration of 15 ppb
[17]. Additional evidence for the heterogeneous distribution
of an extraterrestrial component in C1-N10 is indicated by
significant variations in Os abundance and Re-Os isotopic
ratios [17].

Chromium and the siderophile elements Co and Au also show
variations within and among some of the melt rock fragments,
however, inter-element correlations are not clear-cut for
the limited number that also contain Ir. There appears to be
a crude positive correlation of Co with Ir, and inverse
correlations of Cr with Co and Ir, but these trends are not
consistently maintained for all samples. The analytical
uncertainties for Ni, As, and Sb are too large to resolve
differences among the samples for these elements.

Figure 1 shows the Cr and siderophile element distribution
patterns relative to CI chondrites based on average measured
abundances within the C1-N10 and Y6-N19 core intervals.
Common to both intervals is the depletion in Ni relative to
the more refractory siderophiles, and progressive enrichment
of the more volatile elements with chalcophilic tendencies.
Although different in detail, these melt rock patterns bear
an overall resemblance to that of the KT boundary fish clay
at Stevns Klint [14], providing additional constraints on
impactor type and the partitioning of projectile material
between the target and the ejecta. For example, arguments
against a chondritic projectile stem mainly from the low
Ni/Ir ratio observed at Stevns Klint [14-16]. The KT
boundary layer in the Raton Basin also appears to have a
sub-chondritic Ni/Ir ratio [20]. If the low Ni/Ir ratio
common to Chicxulub melt rocks and both continental and
marine KT sections is also characteristic of the projectile,
then arguments favoring an Fe or ureilite meteorite might be
strengthened [15]. However, indications of hydrothermal
alteration and the potential for fractionation in the melt
rocks [21], possible post-depositional mobility of Ir in
sediments [22], as well as evidence for fractionation of
siderophile elements in impact melt specimens relative to
the projectile where the projectile composition is known
[23], suggest that attempts to characterize the impactor at
this juncture would be premature.

Small-scale variations, particularly between aliquots of a
single powdered sample, suggest that the Ir is concentrated
in a trace phase that is not uniformly disseminated in the
melt rock. Our initial report on the discovery of Ir in the
Chicxulub melt rocks noted that the higher Ir concentrations
seemed to correlate with the abundance of opaque, highly
reflective phases [11]. No metal grains have yet been
identified in the melt rock. The dominant opaque mineral is
magnetite, however, petrographic examination of thin
sections from C1-N10 and Y6-N19 reveal the presence of <1%
sulfides, ranging in size from 10 to 300 mm and distributed
irregularly in the melt matrix. Significant enrichments of
siderophile elements have been found in association with Ni-
Fe sulfide particles in melt rocks at the E. Clearwater
Impact structure [24,25]. In the absence of discrete metal,
sulfides would seem to be likely carrier phases for Ir and
other siderophile elements.

Work is in progress to separate the sulfides and other
phases to search for the Ir by INAA. Preliminary
characterization of the sulfides by electron microprobe
indicates that most of the grains are pyrite, with
subordinate chalcopyrite. Random analyses of the pyrite
revealed significant enrichments in Co and Ni, with Co
concentrations up to 6 wt%. Figure 2 shows an example of
pyrite from C1-N10, which suggests that the Co and Ni were
enriched by oscillatory growth zoning. Although both
siderophiles appear to be enriched in the same general
regions, close inspection reveals that the zones of highest
Ni concentration (1.1 wt%) are relatively depleted in Co
(0.6 wt%), whereas the highest Co (3.9 wt%) corresponds to
0.2 wt% Ni.  The best estimates of the average
concentrations in this pyrite are 0.7 wt% Co and 0.5 wt% Ni.
These results confirm the concentration of siderophile
elements in a heterogeneously disseminated trace phase, and
indicate that some siderophile element fractionation has
occurred. The paragenesis of the pyrite, and whether or not
this phase contains the iridium requires further
investigation. Clearly, more evidence must be gathered
before the composition and caliber of the KT bullet can be
ascertained.

References: [1] Sigurdsson H. et al. (1991) Nature, 353,
839-842. [2] Izett G. A. et al. (1991) Science, 252,1539-
1542. [3] Koeberl C. and Sigurdsson H. (1992) GCA, 56, 2113-
2129. [4] Smit J. et al. (1992) Geology, 20, 99-103. [5]
Alvarez W. et al. (1992) Geology, 20, 697-700. [6] Blum J.
D. et al. (1993) Nature, 364, 325-327. [7] Krogh T. E. et
al. (1993) EPSL, 119, 425-429. [8] Hildebrand A. R. et al.
(1991) Geology, 19, 867-871. [9] Kring D. A. and Boynton W.
V. (1992) Nature, 358, 141-144. [10] Swisher C. C. et al.
(1992) Science, 257, 954-958. [11] Sharpton V. L. et al.
(1992) Nature, 359, 819-821. [12] Sharpton V. L. et al.
(1993) Science, 261, 1564-1567. [13] Sharpton V. L. et al.,
this volume. [14] Kyte F. T. et al. (1980) Nature, 288, 651-
656. [15] Palme H. (1982) GSA Spec. Paper, 190, 223-234.
[16] Kyte F. T. and Wasson J. T. (1982) GSA Spec. Paper,
190, 235-242. [17] Koeberl C. et al. (1993) GCA, in press.
[18] Grieve R. A. F. (1982) GSA Spec. Paper, 190, 25-37.
[19] Hildebrand A. R. et al. (1993) LPS XXIV, 657-658. [20]
Izett G. A. (1990) GSA Spec. Paper, 249, 100. [21] Schuraytz
B. C. and Sharpton V. L. (1993) Nature, 362, 503-504. [22]
Colodner D. C. et al. (1992) Nature, 358, 402-404. [23]
Mittlefehldt D. W. et al. (1992) Meteoritics, 27, 361-370.
[24] Palme H. et al. (1979) Proc. LPSC 10th, 2465-2492. [25]
Grieve R. A. F. et al. (1980) Contrib. Mineral. Petrol., 75,
187-198. [26] Wasson J. T. and Kallemeyn G. W. (1988) Phil.
Trans. R. Soc. Lond. A, 325, 535-544. [27] Wasson J. T.
(1985) Meteorites, W. H. Freeman and Co., New York, 267.

Fig. 1, which appears here in the hard copy, shows
siderophile element and Cr abundance ratios in Chicxulub
melt rock samples relative to CI chondrites [26]. Elements
along the abscissa are arranged in order of decreasing
nebular condensation temperature [27]. Shown are the average
and range of two samples from C1-N10, and the mean and
standard deviation of nine samples from Y6-N19. Rhenium and
Os data for C1-N10 from [17]. Also shown are the mean and
standard deviation of six samples of the KT boundary fish
clay at Stevns Klint [14].

Fig. 2, which appears here in the hard copy, shows
photomicrographs of pyrite grain in thin section from C1-
N10. (a) Backscatter electron image of pyrite with minor
silicate inclusions surrounded by porous matrix of quartz
and feldspar. Slightly darker-gray grain in saddle along the
upper right margin of the pyrite is magnetite. (b) X-ray map
of Co. (c) X-ray map of Ni.


Sigurdsson H.   Smith S.   D'Hondt S.   Carey S.   Espindola J.-M.
   Crystals, Lithics, and Glassy Ejecta at the KT Boundary: Implications for
   Lithology of the Crust at the Impact Site

Ejecta from bolide impact may contain melt quenched to glass
droplets as well as rock fragments and fractured mineral
grains from the impact terrane. In distal sections of the
ejecta deposit, an impact origin for glass and shocked
minerals may generally be established, whereas the origin of
lithics and unshocked crystal fragments found in association
with impact glass is uncertain, due to possible
contamination from other sources. In addition to 1 to 8 mm
diameter glassy ejecta spherules, the 0.5 to 1 m thick KT
boundary impact ejecta layer in the Beloc pelagic carbonate
sediment formation in Haiti contains silicate mineral
fragments and rock fragments that may provide clues about
the nature of the Earth's crust at the impact site. Because
of the monotonous and relatively pure lithology of the
enclosing upper Createceous and lower Tertiary carbonate
sediments, the Haiti exposures of the KT boundary layer
provide an opportunity to detect mineral ejecta present in
only trace amounts, in addition to the abundant impact glass
[1,2]. New studies of the chemistry of glass spherules from
the Haiti deposit (Fig. 1) further define the mixing trend
already documented between high-Ca yellow glass and
high-silica black glass spherules [1,3]. The high-silica
glasses with SiO2 >62 wt% define a separate trend in terms
of most oxides, such as FeO (Fig. 2), which may reflect
chemical range in the crustal source region. New analyses
show that S content of yellow high-Ca glasses ranges up to
1.25% SO3 [4].

Isolated mineral fragments in the ejecta layer include
amphibole, quartz, clinopyroxene, and plagioclase. Amphibole
is a relatively common and well-preserved crystal component
in the ejecta layer, and is present at all levels, except in
the interval from 1 to 30 cm above the base. The amphiboles
are up to 0.5 rnm in size, and occur exclusively as angular,
euhedral crystals, whose shape is defined by {110} cleavage
planes (Fig. 3). These amphiboles are magnesio-hornblendes
to tschermakitic hornblendes, and define a coherent
compositional trend, with a relatively narrow range from
(Mg/Mg+Fe) = 0.70 to 0.80 (Amph. in table). Variation or
zoning within individual crystals is small, in the range
(Mg/Mg+Fe) = 0.02 to 0.07. The amphiboles are relatively
high in A12O3 (7-12 wt%); with total Al cations in the range
1.3 to 1.45 in the structural formula, these amphiboles
could have equilibrated at total pressure of 3 to 4 kbar (9
to 12 km; [5]). Shocked quartz crystals up to 0.55 mm in
size have already been documented from the Haiti KT section
[6,7]. Fractured plagioclase is very rare and occurs as
solitary and angular crystal fragments. The crystals are of
andesine-labradorite An(sub)5OAb(sub)48Or(sub)2, and
relatively unzoned (Plag-1 in table; An(sub)47-52). Biotite
crystal flakes are very rare.

Solitary clinopyroxene and plagioclase crystals with a
reticulate glass coating are also present. The euhedral
clinopyroxene has the composition
Wo(sub)44En(sub)44Fs(sub)12 (Cpx in table), is 0.7 mm in
length and coated with glass. It contains a rhyodacite glass
inclusion (Glass incl. in table), indicating a magmatic or
volcanic origin of the mineral. However, the glass coating
on exterior of the crystal (Glass-cpx in table) is dacitic
in composition and virtually identical to the common impact
glass spherules found at the KT boundary in Haiti (Haiti
glass in table). The subhedral to euhedral 0.7 mm
plagioclase is of An(sub)47.2Ab(sub)51.6Or(sub)1.2 mean
composition (Plag-2 in table), zoned from An(sub)51 core to
An(sub)44 rim, and also coated with dacitic glass. A single
1.3 mm highly vesicular pumice fragment is also present. In
addition, the KT boundary deposit in Haiti contains minerals
that are authigenic in origin. They include common crystals
of euhedral to subhedral barite, up to 1 mm in diameter,
euhedral and bipyramidal crystals of perfectly formed clear
quartz, and high-Ca-K phillipsite zeolites. Lithic fragments
of graywacke are present but rare in the ejecta deposit, up
to 1.5 mm in size. They consist dominantly of relatively
poorly sorted sedimentary rock, containing large (0.5 mm)
and rounded quartz grains in a silty to clayey matrix.

Studies of mineralogy of upper Cretaceous and lower Tertiary
sediments from DSDP Legs 4 and 15 at ten sites in the
Caribbean provide information on sedimentation of silicate
minerals elsewhere in the region at KT boundary time [8,9].
In addition to authigenic minerals in the latest
Mastrichtian-early Paleocene central Caribbean carbonate
sediments, such as chert, clinoptilolite, barite, these
studies also record grains of hornblende, plagioclase, and
clinopyroxene.

If the silicate mineral assemblage in the Haiti deposit is
of ejecta origin, as indicated by impact glass coating on
clinopyroxene, then it may indicate a crustal section
including both calcalkaline volcanics and possibly mid- to
upper-crustal amphibolites, overlain by greywacke,
evaporites and carbonate sediments. In terms of comparison
with geologic terrane in the Yucatan region, the greywacke
lithic fragments in the Haiti deposit may correlate with the
Jurassic Todos Santos Formation, which is over 400 m thick
in well Y-1 [10,11]. Similarly, the silicate mineral
assemblage in the Haiti deposit may be derived from the
Yucatan crystalline basement rocks of Paleozoic age, which
in the same well include felsic metaporphyry,
crystalloblastic quartz chlorite schist, and rhyolite [10-
12].

References: [1] Sigurdsson H. et al. (1991) Nature, 349,
482-487. [2] Sigurdsson H. et al. (1991) Nature, 353, 839-
842. [3] Sigurdsson H. et al. (1992) EPSL, 109, 543-560. [4]
Chaussidon M. et al. (1994) this volume. [5] Johnson M. C.
and Rutherford M. J. (1989) Geology, 17, 837-841. [6]
Hildebrand A. R. and Boynton W. V. (1990) Science, 248, 843-
847. [7] Izett G. A. (1991) JGR, 96, 20879-20905. [8]
Donnelly T. W. and Nalli G. (1973) Init. Rep. DSDP. XV,
929-961. [9] Donnelly T. W. (1973) Init. Rep. DSDP. XV,
969-987. [10] Bass M. N. and Zartman R. E. (1969) EOS, 50,
313. [11] Viniegra O. F. (1971) Am. Assoc. Petrol. Geol.
Bull., 55, 478-494. [12] Donnelly, T. W. et al (1990)
Geology of North America, H, Geol. Soc. Amer., 37-76.

Table 1, which appears here in the hard copy, shows silicate
minerals and glass in the KT boundary ejecta layer.

Fig. 1. Compositional trends in KT boundary glassy ejecta
spherules, illustrating the smooth compositional trends of
A12O3 and CaO, from high-Ca yellow glasses to the more
abundant highsilica glasses. In total, the dataset
represents analyses of 140 spherules. The compositional
range from 44 to 62 wt% SiO2 can be modeled as a simple
binary mixing trend [1], whereas the highsilica glasses
reflect a more heterogenous source. Curves define best-fit
regression lines through the data, with correlation
coefficient of 0.992 for CaO and 0.91 for Al2O3.

Fig. 2. Two distinct trends of FeO content in KT boundary
impact glasses. The trend of decreasing Fe content with
increasing silica in the high-silica glasses may reflect
heterogeneity in the source region, whereas the trend
between high-Ca glasses (44 to 54 wt% SiO2) and high-silica
glasses can be modeled as a mixing trend.

Fig. 3. Scanning electron micrograph of a crystal of
amphibole from the KT boundary ejecta layer. The 400 micron
euhedral crystal is entirely defined by {110} cleavage
planes and formed by breakage. Mineral chemistry is
consistent with derivation from 9 to 12 km depth in the
crust.


Hough R. M.   Sigurdsson H.   Franchi I. A.   Wright I. P.   Pillinger C. T.
Gilmour I.
   Carbonate Derived Gases in Haitian KT Boundary Glass Spherules

Glass spherules, thought to be tektites from Haiti, have
been analyzed previously for their mineralogy and chemical
composition to identify their origin and mode of formation
[1].

They occur in various colors, dependent upon the composition
of the original target rock, and in many cases are seen to
contain bubbles. To investigate these spherules and the
nature of any gas phase present in the bubbles, several dark
brown glasses have been analyzed for their C content and
isotope composition, using stepped combustion analysis and
static mass spectrometry [2]. Furthermore, brown and yellow
spherules were analyzed for O isotope composition using
laser fluorination and conventional dynamic gas source mass
spectrometry.

Petrographic analysis using a standard microscope
illustrated the presence of "bubbles" within the dark-brown
glass spherules. By simply submerging a fragment in
refractive index oil, placing a glass slide above it, and
applying pressure it was possible to release the gas in the
bubbles. Since on no occasion did the oil move to fill the
void left when the gas was released it can be concluded that
the bubbles were at greater than atmospheric pressure and
may represent a volatile component such as CO2 or
hydrocarbon. Bubbles were also released simply through
fracturing glass fragments that didn't display any signs of
bubbles.

In most cases spherules analyzed for C were treated as whole
entities, but one was broken into fragments for the purpose
of replication. Individual fragments were found to contain
0.2 wt% C two components of different isotopic compositions.
The first component had a release temperature of 350-400
degrees C with a delta^13C of -22 permil, the second
component was released at higher temperatures, 500-600
degrees C and had a delta^13C value of -6.3 permil. The
isotopic composition and release temperature of the first
component strongly suggests that it is organic and probably
represents surface contamination of the sample. To try and
reduce contamination, other spherule fragments were
pretreated with 0.1 M chromic acid to remove organic and
carbonate components. Analyses of cleaned fragments
indicated a variable carbon content from 0.005 to 2.6 wt% C,
but again with two isotopically different components. The
first with a peak delta^13C of -0.8 permil released between
420 and 460 degrees C and the second, a delta^13C of -19.0
permil released between 550-600 degrees C (Fig. 1). Levels
of low-temperature organic contamination were greatly
reduced in the cleaned spherules.

The component with a delta^13C of -19 permil is present in
most of the spherule fragments. A component with a similar
combustion temperature and delta^13C has been encountered in
KT residues containing nanodiamonds [3]. There is currently
no information available confirming its identity, but it
does not appear to be surficial or an oxidizable organic.

The C released between 420-460 degree C is present in most
spherule fragments and has a consistently heavy isotopic
composition (maximum delta^13C of -0.8 permil). This
isotopic composition is remarkably similar to those of
marine carbonates [4], but isotopically heavier than present
day atmospheric CO2 (-7 permil).

If the bubbles are indeed CO2 then one likely origin is that
they represent CO2 released from target rock carbonates by
the impact [5]. Identification of these C components by
future work may reveal a possible source and mode of
formation for the spherules and will also clarify the effect
of the internal bubbles upon the compositions.

Dark-brown spherules selected for O isotope measurements
were broken into fragments to allow repeat analyses on the
same spherule. Due to the smaller size of the yellow
spherules they were analyzed whole. The dark-brown spherules
have delta^180 values between +7.1 and +9.5 permil and
delta^170 between +3.7 and +5.0 permil. The yellow spherules
have a delta^180 of +13.0 permil and a delta^170 of +6.8
permil.

Both types of spherule fall on the terrestrial fractionation
line (TFL) on a plot of delta^170 vs. delta^180 (Fig. 2).
The O isotope data suggests no obvious input from an
extraterrestrial impactor into the glasses but it should be
noted that only a small number of spherules have been
analyzed.

Heterogeneities seen in the C data for the dark-brown
spherules seem to be reflected in the O data with variations
between fragments of the same spherule and between whole
spherules.

References: [1] Koeberl C. and Sigurdsson H. (1992) GCA, 56,
2113-2119. [2] Yates P. D. et al. (1992) Chem. Geol., 101,
81-91. [3] Gilmour I. et al. (1992) Science, 258, 1624-1625.
[4] Faure G. (1986) Wiley & Sons, New York, 496-499. [5]
Sigurdsson H. et al. (1991) Nature, 353, 839.

Fig. 1, which appears here in the hard copy, shows the
stepped combustion profile of a dark-brown glass spherule
from Haiti.

Fig. 2, which appears here in the hard copy, shows the
oxygen isotopic composition of glass spherules.


Evans N. J.   Ahrens T. J.   McInnes B. I. A.   Gregoire D. C.
   New Evidence for Primary Fractionation of Ruthenium and Iridium in the
   Chicxulub Ejecta Cloud

Introduction: Platinum-group element (PGE; Ru, Ir, Pt, Pd,
Ir, plus Au, included in the term PGE for the sake of
brevity) analysis of the Cretaceous-Tertiary (KT) boundary
fireball layer (terminology of [1]) has revealed that the
mean Ru/Ir ratio of the many marine sites studied (1.67 +-
0.38) is close to the chondrite ratio (1.48; [2]) whereas
the value for nonmarine sites (0.76 +- 0.26) is not [3,4]. A
positive correlation between the Ru/Ir ratio of globally
distributed KT sites and 65Ma paleo-distance from the
Chicxulub structure was also observed [4]. This trend
suggested that temperature-dependant, primary fractionation
of PGE occurred in the ejecta cloud during condensation of
vaporized projectile material. However, this previous work
could not negate the dependance of the paleo-distance-Ru/Ir
ratio correlation on environment of deposition because all
the marine sites studied were far from Chicxulub (Europe and
New Zealand) and all the nonmarine sites were in North
America. This work presents additional Ru and Ir analysis of
the fireball layer from marine KT sites proximal to the
impact structure at Chicxulub, Yucutan, other North American
nonmarine KT sites and DSDP cores. With this new data we
assess the dependance of the previously observed trend on
depositional environment and suggest a simple mechanism for
primary fractionation of PGE, prior to their deposition in
the fireball layer.

Methods and Results: Ruthenium and Ir in KT boundary samples
were measured by isotope dilution inductively coupled mass
spectrometry (ICP-MS) and a complete description of the
digestion and analytical procedures have been published
elsewhere [3,4]. Precision for the technique as percent
standard deviation for multiplicate analysis of USGS diabase
standard W-1 are 6% (Ru) and 0.7% (Ir).

The abundances of Ru and Ir for all KT sites is given in
Table 1. The Ru/Ir ratio and nature of the fireball layer
samples analyzed from the following sites have been
previously published [4]; Petriccio, Italy; Knappengraben
and Elendgraben, Austria; Stevns Klint, Denmark; Agost,
Spain; Woodside Creek, New Zealand; Raton Basin, Colorado;
Red Deer Valley, Alberta; Morgan Creek, Saskatchewan and
Lance Creek, Wyoming. New data is presented here for DSDP
577, DSDP 596, Brazos River, Texas and Beloc, Haiti.

DSDP Samples: Typical Ir contents for the boundary in DSDP
577 are 5 ppb [5] over an order of magnitude higher than the
value obtained here. A possible explanation for this
discrepancy may be that the boundary sample analyzed is at
the upper limit of the boundary interval, which is smeared
out by bioturbation over 10 cm in this core [5,6]. Since no
Ru values have previously been presented for this section,
there is no basis for comparison of the present Ru data and
no way to estimate how sampling the outer region of the
boundary interval might affect the Ru/Ir ratio. However, we
have both Ru and Ir data for the entire 10 cm boundary
interval in DSDP 596 and the Ru/Ir ratio only varies by +-
0.06 throughout (unpublished data).

The PGE-rich KT section in DSDP 596 is smeared over at least
10 cm by bioturbation [7] and the values in Table 1
represent a mean for this interval. The physical mixing of
the sediment does not seem to have affected the Ru/Ir ratio
since, as noted above, it varies little over the 10 cm
interval.

Brazos River, Texas and Beloc, Haiti: There is significant
controversy over the location of the KT boundary at the
extensively studied Brazos River site [8-10], however the
observed Ru and Ir data indicate that the degree of
fractionation of Ru from Ir during physical reworking
processes is insignificant and we conclude that the Brazos
River Ru/Ir ratio reflects as primary a value as that for
the other sites analyzed. The fireball layer sample from
Beloc, Haiti (HKB92-56, obtained from A. Hildebrand) is a
medium gray claystone 1.5 cm thick.

There is some uncertainty regarding the composition of the
boundary sample we analyzed for Lance Creek (collected by
others) in that it might include both the underlying
kaolinitic claystone and the fireball layer. This would have
the effect of raising the Ru/Ir ratio because the ratio in
the kaolinitic claystone is always higher than that of the
associated fireball layer for any given site [4]. For this
reason, the Lance Creek value has been excluded from the
calculation of regression coefficients (Fig. 1).
Table 1 gives the paleo-distances (65 Ma) from each site to
the Chicxulub impact structure and the corresponding Ru/Ir
ratios. In Fig. 1 we have plotted all of the Ru/Ir ratios
for the fireball layer worldwide against the paleo-distance
from Chicxulub. In addition to sites analyzed in our
laboratory, data for Chancet Rocks and Neddles Point, New
Zealand from [11] have been included.

Discussion: Previous work has shown that although all the
PGE in the KT fireball layer are somewhat redistributed
laterally and vertically by post-depositional processes [11-
13], Ir and Ru are not significantly fractionated from each
other during redistribution and are the most immobile PGE in
the sedimentary environment [14,4]. The Ru/Ir ratio is
therefore the most useful PGE interelement parameter for
providing insight into primary fractionation processes. The
Ru/Ir ratio of marine and nonmarine KT fireball layer
samples are statistically distinct and nonmarine sites have
nonchondritic ratios. In addition, the fact that all the
marine sites previously analyzed were far from the Chicxulub
impact site and all nonmarine sites were located in North
America resulted in a positive correlation between Ru/Ir
ratio and paleo-distance to Chicxulub. This trend cannot be
explained by input of mantle-derived PGE or addition of PGE
from a simultaneous impact event [3]. A primary mechanism
for fractionation, which operates to change the Ru/Ir ratio
of nonmarine, North American KT sites from chondrite prior
to their deposition in the fireball layer, is therefore
indicated. However, more data was needed to definitively
eliminate the possible dependance of the correlation on
factors related to environment of deposition. The addition
of data from the marine sites, Brazos River and Beloc, which
both plot in the region dominated by nonmarine sites (Fig.
1), negates the dependance of the trend on differential
remobilization associated with differences in the
depositional environment.

In addition, samples of DSDP cores from the Pacific
represent an environment not previously investigated for
Ru/Ir ratios and provide convincing evidence that the
environment of deposition is independent of the observed
trend. The greater then 1000 degrees C difference in the
condensation temperatures of Ir (5017 degrees C) and Ru
(3900 degrees C)[15] leads us to propose that fractionation
of Ir from Ru during condensation of the projectile-rich
component of the ejecta may have occurred in the cooling
ejecta cloud. The first condensates would have a low Ru/Ir
ratio because Ir condensed from the cloud while Ru remained
in the vapor. Settling of early-formed condensates close to
the impact site could explain the low Ru/Ir ratios of North
American nonmarine KT sites (Table 1, Fig. 1). Early
fractionation of Ir from the ejecta cloud combined with
later incorporation of Ru results in later condensates
having higher Ru/Ir ratios [4].

Chemical analysis of ejecta collected after a 6.4km/s
experimental hypervelocity impact of a Fe-Ni-PGE alloy
projectile into a Mo target indicates that temperature-
dependant fractionation of PGE occurs in the ejecta during
an impact event (Evans and Ahrens, unpublished data).
Further study of these experimental results will determine
whether this fractionation occurred during melting-quenching
or vaporization-condensation processes.

Fig. 1. Paleo-distance (km) from KT sample sites studied to
Chicxulub, Yucatan vs. Ru/Ir ratio. 1. Raton Basin,
Colorado; 2. Lance Creek, Wyoming (not included in
regression coefficient calculation); 3. Morgan Creek,
Saskatchewan; 4. Frenchman River, Saskatchewan; 5. Hell
Creek, Montana; 6. Red Deer Valley, Alberta; 7. Stevns
Klint, Denmark; 8. Agost, Spain; 9. Petriccio, Italy; 10.
Knappengraben, Austria; 11. Elendgraben, Austria; 12.
Woodside Creek, New Zealand; 13. Brazos River, Texas; 14.
Beloc, Haiti; 15. DSDP 577; 16. DSDP 596; 17. Chancet Rocks,
New Zealand; 18. Needles Point, New Zealand. Sites 17 and 18
are data of [11].

References: [1] Hildebrand A. R. and Boynton W. V. (1990)
Science, 248, 843-847. [2] Anders E. and Grevesse N. (1989)
GCA, 53, 2363-2380. [3] Evans N. J. et al. (1993a) GCA, 57,
3737-3748. [4] Evans N. J. et al. (1993b) GCA, 57, 3149-
3158. [5] Michel H. V. et al. (1985) Init. Reports DSDP, 86
(G. R. Heath et al. eds.), 533-538. [6] Smit J. and Romein
A. J. T. (1985) EPSL, 74, 155-170. [7] Zhou L. et al. (1991)
Geology, 19, 694-697. [8] Bourgeois J. et al. (1988)
Science, 241, 567-570. [9] Keller G. (1989) GSA Bull., 101,
1408-1419. [10] Montgomery H. et al. (1992) EPSL, 109, 593-
600. [11] Tredoux M. et al. (1989) J. Geol., 97, 585-605.
[12] Schmitz B. (1988) Geology, 16, 1068-1072. [13] Izett G.
A. (1990) Geological Society of America Special Paper 249,
100. [14] Cousins C. A. and Vermaak C. F. (1976) Econ.
Geol., 71, 287-305. [15] Weast R. C. (1989) CRC Handbook of
Chemistry and Physics, 69th ed. [16] Schmitz B. (1985) GCA,
49, 2361-2370.


Chaussidon M.   Sigurdsson H.   Metrich N.
   Sulfur Isotope Study of High-Calcium Impact Glasses from the KT Boundary

High-Ca yellow glasses form a volumetrically minor component
of the impact spherules found in the KT boundary deposit in
Haiti, but these glasses contain valuable information about
lithology of the geologic terrane at the impact site. The
high-Ca glasses define a mixing trend with abundant high-
silica glasses, which led to the hypothesis that they are
derived from fusion of Ca-rich sediment in presence of
silicate melt [1]. Two different sources of Ca have been
proposed for the yellow glasses, either anhydrite-rich
evaporitic terrane, as suggested by their shigh S content
and elevated delta^34S (+13.2 +- 4 per mil, one sample) [2],
or platform carbonates as suggested by their high delta^18O
values (+13.6 +- 0.8 per mil, 3 samples) [3]. These two
hypotheses have important and radically different
implications for atmospheric and other environmental effects
following bolide impact: either global cooling due to
sulfate aerosol formation [4], or greenhouse effect because
of increased CO2 concentration in the atmosphere. Because of
the correlations observed between S and CaO in the yellow
glasses [2], we conducted a detailed S isotopic study of 20
high-Ca yellow glass particles in order to constrain sources
of S and Ca.

Sulfur isotopes were determined with ims 3f ion microprobe,
using an O-primary beam and analyzing secondary S-ions at a
mass resolution of ~3500 [5,6]. Instrumental mass
fractionation was determined (alpha(sub)instr = 0.9547 +-
2.8 per mil) on high-Ca synthetic glass standards with 0.8
to 1.2 wt% SO3, dissolved mainly as sulfate and having
delta^34S of +24.4 per mil. Analytical uncertainty of the
delta^34S values is between +1.5 and +5 per mil, depending
mainly on S content. The yellow glasses are heterogenous in
terms of both S and delta^34S values. Their S contents range
from 0.3 to 1.25 wt% SO3. The spectral position of the S Ka
line in the electron microprobe shows that S speciation is
~94.5% sulfate in the high-Ca glass (sulfate/sulfide =
17.2), in agreement with high O fugacity inferred from
Mossbauer spectra [7]. Melting experiments with gypsum and
andesite have reproduced the high-Ca glass trend, both for
major oxides and S content [2]. A S geothermometer,
calibrated by melting experiments, indicates that the
crystal-free glasses were quenched in the range 1257-1360
degrees C.

The delta^34S values range between +2.9 and +14.5 per mil,
confirming earlier results [2]. A strong isotopic
heterogeneity is observed at all scales, between +2.5 +- 5.1
per mil and +10.9 +- 1.9 per mil within a single glass
particle, and between +2.9 +- 6.8 per mil and +12.3 +- 3 per
mil between different particles. This degree of isotopic
heterogeneity is not known in any terrestrial volcanic
glasses [8] and was not observed in melting experiments [2],
which produced glasses with homogenous delta^34S values,
within analytical uncertainty (+2.8 per mil). The variations
of delta^34S are not clearly correlated to other parameters,
however, all the samples show a general tendency of increase
of delta^34S values with increase of S and Ca content (Fig.
1). The only previous yellow glass measured for delta^34S
[2] follows the same trend.

The delta^34S values of the yellow glasses can be used to
constrain the origin of S. Clearly sedimentary rocks are the
only ones which have high enough S contents to account for
the high S content of the yellow glass. The hypothesis of an
anhydrite-rich evaporitic source was tested by modeling, on
basis of mass fraction of S degassing calculated from known
quenching temperatures of the glasses, isotopic
fractionation of S, and the observed delta^34S of the yellow
glasses. Assuming mixing between anhydrite (41% CaO and 59%
SO3) and a silicate melt from a crustal component with
roughly andesitic composition (5% CaO, 0.3 wt% SO3), the
fraction of anhydrite required to account for the CaO in the
yellow glass was calculated from mass balance, ranging from
0.3 to 0.6. This would have produced a silicate melt plus
anhydrite mixture with ~20 to 36 wt% SO3. Initial delta^34S
value of this mixture, before S degassing, is assumed to be
that of the anhydrite. Loss of S from the melt during
degassing/volatilization lowered their S content to 0.3 to
1.25 wt% SO3, causing isotopic fractionation. We have
calculated the degree of fractionation on basis of quench
temperature (from the S geothermometer), and from dependence
of the isotopic fractionation on temperature at a dissolved
sulfate/sulfide ratio of 17.2 in the glasses [9]. The
results shown in Fig. 2 are for a delta^34S of +12 per mil
(the minimum value for Cretaceous evaporite [10]). It is
clear from Fig. 2 that some of the glasses have delta^34S
values in agreement with the hypothesis of isotopic
fractionation during anhydrite volatilization, while others
have too low delta^34S values. This might be related to the
different trends observed in Fig. 1. It is clear that a
high-S source such as anhydrite is required, but these
initial results indicate that in addition to anhydrite,
other high-Ca sediments, such as carbonates, may have
participated in the formation of the yellow glasses,
contributing to the large delta^34S range. As discussed
previously [4] the geology of the Chicxulub region of the
Yucatan peninsula in Mexico is consistent with the above
scenario, with interbedded Cretaceous evaporites and
carbonates. Further S and O isotope measurements are in
progress in order to quantify the role of these two
components in the genesis of the yellow high-Ca impact
glasses.

References: [1] Sigurdsson H. et al. (1991) Nature, 349,
482-487. [2] Sigurdsson H. et al. (1991) Nature, 353, 839-
842. [3] Blum J. D. and Chamberlain C. P. (1992) Science,
257, 1104-1107. [4] Sigurdsson H. et al. (1992) EPSL, 109,
543-560. [5] Chaussidon M. and Lorand J.-P. (1990) GCA, 54,
2835-2846. [6] Deloule E. et al (1992) Chem. Geol., 101,
187-192. [7] Oskarsson N. et al. (1991) LPS XXII, 1009. [8]
Taylor B. E. (1986) Reviews in Mineralogy, 16. [9] Zheng Y.-
F. (1990) Terra Nova, 2, 74-78. [10] Claypool G. E. et al
(1980) Chem. Geol., 28, 199-260.

Fig. 1. Mean CaO content vs. delta^34S (per mil CDT) in
high-Ca and high-S yellow impact glass particles from the KT
boundary in Haiti. Open circle shows results from an earlier
study [2].

Fig. 2. Isotopic fractionation model of delta^34S evolution
in high-Ca yellow impact glasses. Calculated delta^34S is
based on fractionation factors from [9], quench temperature
determined from S geothermometry and measured
sulfate/sulfide ratio in the glass. Open circle shows
results from previous study [2].


D'Hondt S.   Sigurdsson H.   Hanson A.   Carey S.   Pilson M.
   Sulfate Volatilization, Surface-water Acidification, and Extinction at the
   KT Boundary

It appears that the severity of environmental effects
related to the Cretaceous-Tertiary (KT) boundary impact may
have been largely due to the unusual geochemistry of an
evaporite-rich impacted terrane. KT tektite composition,
experimental results and comparison to Yucatan stratigraphy
all indicate the presence of gypsum or anhydrite at the
tektite source (Chicxulub crater). Thick (~1 km) Late
Cretaceous evaporite sequences occur in the region of the
Chicxulub KT impact structure [1]. Ca-rich KT tektites
contain high SO3 concentrations (0.83% [2]; 0.53% [3]; range
= 0.20 to 1.0%). They also exhibit delta^34S values typical
of evaporites (13.2 [2,4]). Experimental results have
duplicated the high-Ca tektite composition by high
temperature melting of evaporite + andesite [2,5].

Several studies have estimated the amount of SO2 volatized
from target evaporites by the KT impact. An estimate of 1.3
* 10^16 was based on the proportion of unaltered high-Ca
glass in Haiti and the global thickness of the KT boundary
clay. This assumed that 1000 km^3 of impact glass was
created and 2% was high-S glass derived from evaporite
source [2,5]. This represents a minimum estimate, since it
is limited to SO2 released by tektite formation and does not
include SO2 released from solid rock by shock, or SO2
released by initial volatization of the target. More
comprehensive estimates have been derived from
reconstruction of Chicxulub geology and assumed shock
pressures required for sulfate release. These rely on
variable estimates of transient crater diameter (80 to 146
km), stratigraphy of Chicxulub region (0.5 to 1.5 km
anhydrite), and shock pressure of sulfate release (20 to 40
Gpa). Based on such criteria, Sigurdsson et al. [5]
estimated that 2.4 * 10^18 to 8.4 * 10^18 g SO2 was
released, Brett [6] estimated that 4 * 10^17 g SO2 was
released, and Pope et al. [7] estimated that 5.4 * 10^17 to
1.6 * 10^18 g SO2 was released.

Combined with the great optical depth loading previously
estimated to result from the KT impact "dust" cloud, the
resultant stratospheric sulfur aerosols may have contibuted
to a rapid decline in global surface temperatures to near-
freezing in about one week [5]. Time-dependent conversion of
stratospheric SO2 to H2SO4 would have prolonged this cooling
for several years [5]. These stratospheric sulfate aerosols
may also have caused global "blackout," preventing
photosynthesis for months and disrupting it for years [7].
These relatively long-lived effects of the estimated SO2
release (global cooling and global darkness) appear likely
to have been major causes of the global KT extinctions.

The estimated mass of sulfuric acid generated by this event
is equal to or greater than the most tightly constrained
estimates of nitric acid formed by the same event (HNO3
yields corresponding to 10^15 moles NO(sub)x--perhaps higher
if the impact was oblique [8]). Depending on the
stratospheric conversion rate of SO2 to H2SO4 and the rate
of transport to surface waters, these combined sulfuric and
nitric acids may have led to varying levels of acidification
in surface marine and freshwater systems. Instantaneous and
uniform global rainout of maximum KT sulfuric acid estimates
(10^17 moles H2SO4) would have temporarily destroyed the
buffering capacity of surface marine waters (the upper 100 m
of the watercolumn) and driven the pH of nearsurface marine
and fresh waters below 3. Assuming complete atmospheric
conversion of SO2, most estimates of KT SO2 release would
create ~1016 moles of atmospheric H2SO4. Instantaneous and
uniform global rainout of this mass would have driven marine
pH below 6 in the upper 25 m of the watercolumn. Its mixing
throughout a 100 m watercolumn would have depressed marine
pH below 7 and unbuffered freshwater values below 3. Similar
rainout of 10^15 moles of H2SO4 would have depressed surface
marine pH from 8.2 to 7 only in the upper 10 m of the
watercolumn, but would have depressed the pH of unbuffered
fresh waters to about 4.5 throughout a 100 m watercolumn.
Similar rainout of 10^15 moles of HNO3 would have had a
still smaller effect on watermass acidification. In all
cases, seasurface pH would have subsequently increased by
varying amounts as the pCO2 of the atmosphere and surface
oceans re-equilibrated (on timescales of a few months to a
few years). On slightly longer timescales, seasurface pH
would closely approach pre-impact values as surface and deep
waters exchanged.

Possible environmental effects of KT surface ocean and
freshwater acidification include (i) increased availability
of free metal ions in surface waters, (ii) loss of CO32- and
HCO3- from surface marine waters, and (iii) selective
extinction in surface water environments [9,10]. Nontheless,
acidification of the marine and freshwater photic zone may
not have been a primary cause of global KT extinctions. Even
assuming maximum estimates of KT SO2 volatization, our
maximum estimates of global KT watermass acidification are
probably unrealistic since the time-dependent conversion of
SO2 to H2SO4 may preclude instantaneous global injection of
the total H2SO4 mass into surface waters. Furthermore,
analysis of vertebrate fossil records indicates that
freshwater organisms of the western interior United States
exhibited much higher KT survivorship than terrestrial
organisms [11], suggesting that extinction in that region
primarily resulted from factors other than freshwater
acidification. Finally, extant carbonate-secreting marine
phytoplankton and planktic foraminifera have respectively
survived pH values of less than 7.6 [12] and less than 7.2
[13] in culture experiments. This probably precludes rapid
global and uniform rainout of 10^15 moles of H2SO4 and/or
HNO3 from consideration as a primary cause of global
extinction of carbonate-secreting marine plankton. However,
it allows the possibility of regional or depth-related
extinctions resulting from acidification of marine or
freshwater photic environments if rapid nonuniform acid rain
occurred. Furthermore, photic zone acidification remains a
possible cause of global carbonate-secreting plankton
extinction in the case of instantaneous and uniform global
rainout of more than 10^16 moles of H2SO4 and/or HNO3.

References: [1] Lopez Ramos E. (1979) Geologia de Mexico,
Tomo III, 2nd ed. Mexico. [2] Sigurdsson H. et al. (1991)
Nature, 353, 839-842. [3] Koeberl C. and Sigurdsson H.
(1992) GCA, 56, 2113-2129. [4] Chaussidon et al. (1994) this
volume. [5] Sigurdsson H. et al. (1992) EPSL, 109, 543-559.
[6] Brett R. (1992) GCA, 56, 3603. [7] Pope K. O. et al.
(1993) LPSC XXIV, 1165-1166. [8] Zahnle K. (1990) GSA Spec.
Paper, 247, 271-288. [9] Lewis J. et al. (1982) GSA Spec.
Paper, 190, 215-221. [10] Prinn R. and Fegley B. Jr. (1987)
EPSL, 83, 1-15. [11] Sheehan P. and Fastovsky D. (1992)
Geology, 20, 6, 1992. [12] Griffis K. and Chapman D. (1990)
Lethaia, 23, 379-383.[13] Spero, 1993, personal
communication.


Rocchia R.   Robin E.   Froget L.   Gayraud J.
   Ni-rich Spinels (Meteoric Spinels) as Indicators of the KT Event Timing

Introduction: Highly oxidized Ni-rich spinels are minerals
formed when a piece of extraterrestrial matter (rich in Ni)
is melted and oxidized in the Earth atmosphere (high O
fugacity). Such minerals have no counterpart in terrestrial
rocks but are systematically found in the fusion crust of
meteorites [1,2], in cosmic spherules [2,3,4], in the
ablation products found in a Jurassic horizon [5] and in
particles associated with oceanic impact debris from late
Pliocene sediments [6]. They are specific markers of
meteoritic material which interacted at high temperature
with the Earth atmosphere [2,7]. Moreover they are easily
synthesized in the laboratory by simple fusion of meteoritic
material [8]. Therefore, they can be labelled meteoric
spinels since their formation is associated with the
occurrence of meteors.

Relevance to the KT event: (1) KT boundary spinels. In
addition to high concentrations of PGEs (Platinum Group
Elements) KT boundary sediments also contain Ni-rich spinels
[9,-11]. These minerals, found in nearly all KT sections,
cannot be distinguished, by their morphology and
composition, from meteoric spinels derived from pure
meteoritic material. The appearance of these minerals in KT
boundary sediments, in exact coincidence with the planktonic
crisis, is a major argument supporting a cosmic origin for
the KT boundary biological crisis [2].

(2) A short catastrophic event: In all marine KT sites where
the Ir anomaly has been observed, the Ir enrichments are not
confined to the boundary clay layer: overabundances are also
observed on both sides of the boundary. Such extended
distributions are sometimes considered as the indication of
a long duration event. This observation is used to support
very different scenarios: the occurrence of a shower of
comets or a period of intense volcanic activity. However,
the extent of the Ir distribution is not necessarily due to
a long event and can be explained in another way. It might
also be due to the chemical properties of elemental Ir which
can stay for long in the ocean reservoir [12] and may have
diffused in the sediments after deposition. KT meteoric
spinels can be used to remove the ambiguity. These minerals
have a quite simple behaviour: they are locked in rather big
spheroids, a few tens or hundreds microns in size, which
cannot stay for long in sea water and which, once settled,
cannot move inside the lithified sediment. Therefore, the
stratigraphic distribution of meteoric spinels can provide a
less questionable information about the duration of the KT
event than Ir and other chemical markers.

In all KT sites analyzed for Ir and spinels, both markers
have their maxima at the same stratigraphical level. This is
a consequence of the fact that both have the same cosmic
origin. However, as expected, Ir and spinels stratigraphical
distributions are quite different. Ir exhibits broad
distributions but spinel distributions are always very
narrow. Data from the sites of El Kef, Caravaca and Hole
761C, leg 122, speak by themselves: anomalously high Ir
concentrations are found over about 1 meter when the
distribution of spinels never exceeds a few centimeters in
thickness (Fig. 1a, b and c). Two sections deserve a
particuliar attention.

El Kef: This section, the stratotype of the KT boundary in
the mesogean domain, is considered as the most complete one
for paleontological studies. Sediments of the KT transition,
nearly totally deprived of coarse detrital component, appear
to have been deposited under extremely quiet conditions. The
section offers the advantages of a rather low bioturbation
and a high sedimentation rate permitting the study of the KT
sequence with the highest resolution. Actually, this is the
section where we have observed the most different Ir and Ni-
rich distributions. The Ir anomaly is extremely dissymetric
and characterized by a rather narrow (2 cm wide) peak
followed by a low concentration tail extending in the basal
Danian over about 2 m. Overabundances are also oberved in
the last tens of centimeters of the upper Maastrichtian. The
integrated Ir flux, ~70 ng/g, is comparable with the world
average value. The maximum concentration of 18 ng/g is found
in a 1-2 mm thick brown-reddish layer in exact coincidence
with the mass extinction level (drop of carbonate). This
layer contains 5-10% of the total Ir flux. Ni-rich spinels
have a quite different distribution: 95 % of them are found
in this millimetric layer [13]. Taking into account the
sedimentation rates evaluated in the upper Cretaceous and
early Danian, this pulse-like distribution correspond to a
deposition time which cannot exceed 100 years. The infall of
meteoric spinels at El Kef is the result of a quasi-instant
event.

Gubbio Sections (Botaccione, Gubbio): This historical site
has been the subject of deep investigations. The finding of
shocked quartz and Ir secondary maxima in levels distinct
from the boundary layer has pushed Crocket et al. [14] to
propose that multiple events, probably explosive volcanic
eruptions, occurred over a period of more than 3.10^5 years.
These results initiated additional studies and a blind
sampling investigation by different laboratories. Available
data [15] show that secondary Ir maxima are real. However,
it is not demonstrated that these maxima result from
multiple Ir events or from post depositional diffusion and
dissolution. Meteoric spinels can provide conclusive
informations about that point. Our preliminary data show
that, in the Bottacione section, these minerals are not
present in secondary Ir levels: they are found only, like in
other sections, in the boundary clay. The stratigraphical
distribution of meteoric spinels, even if the cosmic origin
of these minerals is ignored, does not support the volcanic
hypothesis of Crocket et al. [14]. Long duration mechanisms
are excludes: all data points to a single cosmic event
occurring right at the paleontological boundary.

Conclusion: In all KT sections analyzed so far, the
stratigraphic distribution of meteoric spinels is extremelly
narrow. This indicates that the meteoritic material,
responsible for the formation of spinels and also for the Ir
enrichments, was accreted during a single and extremelly
brief event, or series of events, which occurred in exact
coincidence with the planctonic crisis. This result is
inconsistent with scenarios based on continuous or
repetitive long duration events. In particular it excludes
the occurrence of a comet shower over several 10^5 years
produced by the perturbation of the Oort cloud at the time
of the KT transition.

References: [1] Brownlee D. E. et al. (1975) JGR, 80, 4917-
4924. [2] Robin E. et al. (1992) EPSL, 108, 181-190. [3]
Robin E. (1988) Thesis, University of Paris XI, 131. [4]
Koeberl C. et al. (1989) GCA, 53, 937-944. [5] Jihanno C. et
al. (1988) LPSC 18, 623-630. [6] Margolis S. et al. (1991)
Science, 251, 1594-1597. [7] Robin E. et al. (1994) this
volume. [8] Gayraud J. et al. (1994) this volume. [9]
Montanari A. et al. (1983) Geology, 11, 668-671. [10] Smit
J. et al. (1984) Nature, 310, 403-405. [11] Kyte F. et al.
Geology, 14, 485-487. [12] Goldberg, E. D. et al. (1986)
Applied Geochem., 1, 227-232. [13] Robin, E. et al., (1991)
EPSL, 107, 715-721. [14] Crocket et al. (1988) Geology. [15]
Rocchia et al., (1990) EPSL, 99, 206-219.


Robin E.   Froget J.   Gayraud J.   Rocchia R.   Turpin L.
   Characteristics and Origin of Spinel-bearing Spheroids at the KT Boundary

Introduction: Spinel-bearing spheroids have been reported at
several KT boundary sites throughout the world [1]. Although
the origin of these spheroids is not clearly established,
they are interpreted as condensed products deriving from the
cloud of vaporized projectile and target material produced
by a large asteroid impact [1-5]. The occurrence of numerous
meteoric spinels in these spheroids shows that at least some
of them could derive from meteoritic debris heated and
oxidized in the atmosphere [6]. Here we report on the
mineralogical and chemical features of the spinel-bearing
particles at the KT boundary and we discuss on their
possible origin(s).

Samples and Procedure: From the 18 KT sites we have
investigated so far [7], only DSDP site 577 still contain
well-preserved spinel-bearing spheroids. At the other sites,
the spheroids have experienced intensive alteration and in
most cases only spinels have survived. However, few
spheroids from Beloc (Haoti), Petriccio (Italy), El Kef
(Tunisia), and Caravaca (Spain) have also been recovered and
studied. At site 577 about 100 particles were analyzed for
Ir, Ni, Co, Cr, Sc, and rare-Earth elements with a high-
purity Ge gamma-ray detector [8]. At other sites, particles
were analyzed only for Ir. Debye-Scherrer and x-ray
diffraction analyses have been performed on a set of
particles in order to characterize mineralogical phases.
Individual particles were then mounted in epoxy and polished
for detailed scanning electron microscopy and electron
microprobe analysis.

Results: DSDP site 577 (North Pacific Ocean). Two distinct
populations of well-preserved spinel-bearing particles have
been found right at the boundary [8]: spheroids with
dendritic spinel textures uniformly dispersed in the whole
particle and irregularly shaped fragments with tiny spinels
essentially confined to the rim. Chemical analyses of
spinels contained in the particles are consistent with those
previously reported [4]: they have high Ni (> 1%) contents,
low Ti contents (<1%) and a high Fe oxidation state (Fe3+/Fe
(sub)total ~ 96 atom%, see reference [7]). In addition to
spinel, augite crystals are observed and pure augite
spherules, some of them containing spinels, are found.
Although most of the minerals originally present in the
particles are now replaced by smectite, their morphology and
composition have been preserved. In 86 analyzed spheroids,
Ir contents range from less than 2 ng/g up to 610 ng/g,
consistent with the range of Ir contents observed in cosmic
ablation spheres [8]. In fragments (11 particles analyzed as
yet), Ir contents range from 95 ng/g to 1200 ng/g, with an
average value of 530 ng/g, very close to the Ir abundance of
meteorites. We also found systematic Cr, Co, and Ni
enrichments in spheroids and fragments as well, with
abundances sometimes approaching those in meteorites, and
low REE contents, with a relatively flat pattern (Fig. 1),
which excludes a mixing with more than 10% of terrestrial
material. The small Ce anomaly may be attributed to
postdepositional contamination by the surrounding REE-rich
sediment.

Beloc (Haiti): Only spheroids are found. Spinel are embedded
in a smectitic matrix enriched in iron oxide (goethite). The
spinel composition differs from the one measured at site 577
[7]. No augite crystals are observed. Ir analyses of 10
spheroids reveal concentrations ranging from less than 2
ng/g up to 100 ng/g.

Petriccio (Italy) and El Kef (Tunisia): At both sections,
many flatttened spheroids are found but few of them contain
spinels. At Petriccio, the particles are altered and filled
with glauconite, K-feldspaths and iron hydroxide. At El Kef
they are mainly composed of goethite and smectite. Rare
kaolinite and K-feldspaths are present. No augite crystals
are observed. Ir analysis on a set of spheroids from
Petriccio gives an average content of 40 ng/g, slightly
lower than previously measured [2]. Spinel composition at
these sites still differs from the one observed at site 577
and at Beloc and also differs between them [7].

Caravaca (Spain): K-feldspaths spheroids are abundant [2]
but none of them contain spinels and their Ir content is
lower than in the surrounding matrix. Numerous lenses almost
entirely made of spinels are present and their Ir content is
comparable to the one in the surrounding matrix (~70 ng/g).

Discussion and Conclusions: From the study of the spinel-
bearing particles at site 577 we can draw out the following
conclusions: (1) morphological, mineralogical and chemical
evidences show that spinel-bearing particles at the KT
boundary derive from nearly pure meteoritic debris. If KT
spheroids are formed in the cloud of vaporized projectile
and target material, then the target contribution is
negligible. (2) The occurrence of partially melted
meteoritic debris is inconsistent with a condensation
process.

This rule out some proposed mechanisms for the origin of
spinel-bearing spheroids at the KT boundary. Formation
during a cratering event on land is unlikely as no
meteoritic material survived in such an event. In addition
the contribution of the target is always high in impact on
land and the amount of meteoritic material in impact-melt
rocks is in general <1% relative to C1-chondrites and still
lower in the long range ejecta. At last but not least,
impact products, i.e., tektites and microtektites are
extremely reduced objects free of any crystalline inclusion.
Condensation from the bolide itself is also unlikely as
spinel crystallization requires high O fugacities [9] and
that there is no available O in meteorites.

Considering the formation conditions of spinels [9] we can
envisage two possible origin for KT spheroids: (1) ablation
and oxidation of meteoritic debris in the atmosphere [6].
This hypothesis implies that several objects resulting
either from an oblique impact or from a fragmented comet are
involved [7]. (2) Oxidation of meteoritic debris in the
water vapor cloud produced by an impact in the ocean. In
such an event pure meteoritic debris can be recovered and
highly oxidized spinel can be formed [10].

References: [1] Smit J. and Romein A. J. T. (1985) EPSL, 74,
155-170. [2] Montanari A. et al. (1983) Geology, 11, 668-
671. [3] Smit J. and Kyte F. T. (1984) Nature, 310, 403-405.
[4] Kyte F. T. and Smit J. [1986] Geology, 14, 485-487. [5]
Bohor B. F. et al. (1986) EPSL, 81, 57-66. [6] Robin E. et
al. (1992) EPSL, 108, 181-190. [7] Robin E. et al. this
volume. [8] Robin E. et al. (1993) Nature, 363, 615-617. [9]
Gayraud J. et al. this volume. [10] Margolis S. V. et al.
(1991) Science, 251, 1594-1597.

Table 1, which appears here in the hard copy, shows
elemental contents determined by INAA for 97 spinel-bearing
debris (86 sperules and 11 fragments) from site 577.

Figure 1, which appears here in the hard copy, shows REE
contents relative to chondrites.


Kyte F. T.   Bostwick J. A.   Zhou L.
   The KT Boundary on the Pacific Plate

We will review our results to date on the geochemistry and
impact mineralogy of the Cretaceous Tertiary (KT) boundary
on the Pacific Plate. Over the past several years we have
been relatively successful at locating the KT boundary at
Pacific sites. In addition to the two KT boundaries in
calcareous sequences at DSDP Sites 465 and 577, the boundary
is now confirmed in three pelagic clay cores--LL44-GPC3,
DSDP 576, and DSDP 596--and in the relatively barren
boundary sequence at ODP Site 803. At each locality the KT
boundary is characterized by an Ir anomaly, spherules, and
shocked quartz. We have extracted and analyzed
magnesioferrite spinel from each site except DSDP 465.

These six sites encompass a region of ~20 million km^2 (4%
of the Earth's surface), but they must be used as
representative of a region at least five times larger. The
pre-Cenozoic portion of the Pacific Plate was situated
generally southwest of the Yucatan Peninsula at 65 Ma and
these sites were ~6500 (GPC3) to ~10,000 km (DSDP 596)
distant from the Chicxulub impact structure. Of the six
sites, all except DSDP 465 are bioturbated and sediments
from this site were severely disturbed by drilling. Thus it
is nearly impossible to perform detailed analyses of fine-
scale stratigraphic units in this portion of the world.

In DSDP Hole 465A we were fortunate to recover a clast
containing 1.3 cm of stratigraphy including the KT boundary.
In this sample, a distinct boundary clay ~2-3 mm thick was
recovered. Detailed geochemical analyses were performed on
four layers from this clast in conjunction with leaching
experiments and isotopic analyses [1]. Siderophile element
abundances are quite variable across the clast (e.g., Ir/Au
varies by a factor of 65). However, as at other sites [2],
integration of the samples results in siderophile element
abundances approaching chondritic values with element/Ir
ratios for Ni, Pd, Os, Pt, and Au within a factor of 2 of
chondritic ratios. These data are consistent with a
chondritic source for the siderophiles, but require
fractionation by impact or terrestrial processes, such as
diagenetic mobilization.

The KT boundary at each site is characterized by a large Ir
anomaly. Ir fluences range from 61 ng cm^-2 in DSDP Hole
577B [3] to 320 ng cm^-2 in DSDP Hole 596 [4]. At each of
the three pelagic clay sites (GPC3, 576, and 596) Ir
concentrations have been measured across the entire
Cenozoic. These Ir profiles are sufficient to rule out the
existence of global Ir anomalies comparable to the KT signal
at any time since 65 Ma. These data confirm earlier results
[5] and cast serious doubt on hypotheses of periodic comet
showers as a cause of mass extinctions.

These sediments are particularly amenable to trace mineral
analyses. Sediment samples (typically 0.5 g) are
disaggregated and dispersed, the fine-fraction pipetted off,
and biogenic and hydrogenous components dissolved by
sequential leaching. This procedure should quantitatively
retain trace mineral fractions >30 micrometers in size.
Residues typically consist principally of quartz and
magnesioferrite spinel. Quartz is defined as shocked if
planar deformation features (PDF) are observed during SEM
analyses. Approximately 40% of the quartz grains do not have
PDF, but these are also apparently derived from the KT
impact event. Large grains of unshocked and shocked quartz
from DSDP 596 [4] and DSDP 576 [6] are restricted to the Ir-
enriched interval. Terrigenous eolian components apparently
do not contribute to the >30-micrometer fraction at these
deep-sea sites.

The largest shocked quartz grain recovered was from GPC3
with a maximum diameter of 230 micrometers. We assume that
larger grains have been deposited on the Pacific Plate, but
our ability to define a maximum size is inhibited by small
sediment sample sizes. Size distributions of shocked quartz
at three North Pacific sites are similar, with mean maximum
diameters of 58, 61, and 76 micrometers (for the >30-
micrometers fraction) at DSDP 576, GPC3 and DSDP 465,
respectively. This is considerably less than the mean
maximum diameters (typically ~200 micrometers) reported for
North American sites [7], but consistent with the ~70
micrometers reported for DSDP 596 [4]. We estimate a total
fluence of 1200 grains cm^-2 of shocked quartz >30
micrometers at DSDP 576, comparable to the 1800 grains cm^-2
at South Pacific DSDP site 596. Our data are not yet
sufficient to quantify the fluence of grains at other sites,
but we expect similar results.

Electron microprobe analyses of magnesioferrite spinel
confirm previous results that indicated regional variations
in KT boundary spinel compositions [8]. Our extensive data
(349-point analyses) from all Pacific sites except DSDP 465
show that spinel on the Pacific Plate is compositionally
distinct from non-Pacific sites (Furlo, 127 points;
Caravaca, 19p; DSDP 524, 7p; ODP 761, 13p). The most
striking differences are higher ratios of Fe2O3/FeO and
Al2O3/Fe2O3 at the Pacific sites with the most extreme
values observed at northwestern DSDP sites 576 and 577.
These data indicate a clear compositional asymmetry of KT
impact debris relative to the proposed impact site in the
Yucatan. Models to explain these variations could include
asymmetric ejection from Chicxulub resulting from processes
such as low-angle impact or heterogeneous target materials,
or heterogeneous accretion of multiple projectiles [9,10].
The compositions of Pacific spinel indicate crystallization
at high O fugacities and high temperatures. We suspect that
these compositions, in part, reflect chemical fractionation
during vapor condensation.

Mineral and chemical data from the KT boundary on the
Pacific Plate must be accounted for by models that are used
to explain KT boundary phenomena. Key constraints imposed by
data from this and earlier studies include (1) significant
amounts of shocked debris and spherules thousands of
kilometers from any continental source, (2) chemical
asymmetry of high-temperature mineral phases, and (3)
absence of global Ir anomalies during the Cenozoic. The
Mesozoic portion of the Pacific Plate is now only a small
remnant of a vast region of the Earth s surface, most of
which was lost by subduction. However, the KT boundary
appears to be very well preserved over most of the plate,
commonly buried by only 20 to 60 m of clay, and readily
accessible by ocean-drilling or long-piston coring.

Supported by NSF EAR-9118701.

References: [1] DePaolo D. J. and Kyte F. T. (1984) LPS XV,
220-221. [2] Kyte F. T. et al. (1985) EPSL, 73, 183-195. [3]
Michel H. V. et al. (1985) Init. Rep. DSDP, 86, 533-538. [4]
Zhou L. et al. (1991) Geology, 19, 694-697. [5] Kyte F. T.
and Wasson J. T. (1986) Science, 232, 1225-1229. [6]
Bostwick J. A. and Kyte F. T. (1993) LPS XXIV, 157-158. [7]
Izett G. A. (1990) GSA Spec. Paper, 249, 100. [8] Kyte F. T.
and Smit J. (1986) Geology, 14, 485-487. [9] Kyte F. T. et
al. (1980) Nature, 288, 651-656. [10] Robin E. et al. (1993)
Nature, 363, 615-617.


Holsapple K. A.
   An Estimation of the Measures of the Chicxulub Cratering Event

1. Introduction: The impact of a large asteroid or comet
into the Earth sets in motion a very complex event with
severe consequences. The energy from the impact of a 10 km
body at 20 km/sec velocity is 10,000 times greater than the
sum of the energy from the simultaneous detonation of all
nuclear devices in the world [1]. That energy is deposited
into what is, compared to the range of the effects, a single
spot in the lithosphere. The consequences of such an impact
are well outside any known experience, and require extreme
extrapolations of limited data to estimate.

The most important link to the estimation of the other
aspects of the crater formation is the remaining final
crater. Here we review the existing methods of estimating
the relations between the conditions of the impact: the
impactor composition, size and velocity; and the resulting
effects: crater size, ejecta deposition, dynamic flow
fields, melt volumes, and so on.

2. The Database: The quantification of cratering phenomena
are based on results of four types. Those types and some
comments on their importance and limitations follow: (1)
Experiments in the laboratory. Since the 1960s experimenters
have formed centimetersized craters in a variety of
materials at velocities up to about 6 km/sec. In the 1980s,
experiments were also performed at high gravity on a
centrifuge [2]. Primarily from these data, two distinct
physical regimes for cratering have been identified,
depending on the relative magnitudes of material strength c
compared to gravitational stresses rho-gh at a crater depth
h. The ratio c/rho-gh is large when the crater depth h is
small. For even very weak materials such as a dry alluvial
soil, c is on the order of 10^5-10^6 (cgs units). Thus, a
crater must be meter-sized before gravity becomes
significant. On the other hand, for craters tens to hundreds
of meters diameter in soils, and km in size in rocks, the
material strength plays little role.

That is the primary limitation of laboratory experiments.
With the exception of a dry sand with zero cohesion, the
testing of a geological material in laboratory experiments
is by necessity in the strength regime, and governed by
different physics than events in the gravity regime. Even
with the centrifuge methods testing into the gravity regime
is barely possible, and only for the weakest materials.
However, the entirety of data does convincingly support the
concepts of the existence of the two distinct regimes, and
makes clear that all significant planetary events are well
into the gravity regime. Since the data for the dry sand
materials are always in the gravity regime they are thought
to furnish good estimates for cratering phenomena for other
materials including rocks in the gravity regime [3].

2. Field Experiments With Conventional and Nuclear Explosive
Craters: The field experiments for cratering are a
consequence of interest in nuclear cratering over the
decades since the 1950s. The majority are limited to
explosives with no more than a few hundred pounds of
conventional explosives, while the threshold of the gravity
regime requires on the order of a couple of tons of
explosives for near-surface events. Many were buried much
deeper than meaningful for impact cratering. Only a handful
of the larger nuclear events were conducted for cratering on
the Earth's surface in meaningful geologies, due to the
nuclear test-ban treaty. As a consequence, the use of
explosive cratering results for estimates of large impact
structures is very uncertain. Corrections must be made for
burial depth effects, for the vast differences in specific
energy between conventional explosives, impact events and
nuclear events, for differences in the geologies, and for
the differences between strength and gravity dominated
events. An overview and summary of size estimates for the
entirety of explosive cratering has been given in [2].

3. Code Calculations: It is plausible that meaningful
calculations of the entire history of an event could be made
from "first principles" using computers. That hope must be
tempered with some caution. The difficulties arise from our
lack of knowledge of meaningful models for the behavior of
geological materials over the vast ranges of physical
regimes involved in a cratering event. Therefore, the
calculation of an entire cratering event from the basic
principles cannot be considered very precise. There is
little hope of correctly tracing all of the earlier parts of
an event to correctly predict the latter and final stages.
Even when the models might be complete, there is a dearth of
data to calibrate them for the range of regimes where they
are exercised. Code calculations are useful primarily for
the more qualitative features of impact events.

4. Observations of Planetary Craters: The variety of craters
on the planetary bodies is well documented in the
literature. It is clear that cratering on the size scale
common on planets must involve physical processes not
involved in the data discussed above. The craters exhibit
morphologies of multiple rings, central depressions and
uplifts, and others; which have never been reproduced in
normal geological materials in the lab, in the field, or in
code calculations. The transformation of craters from bowl-
shaped, "simple" craters into the morphologies of the
observed large craters and basins requires additional
guesses about the relevant physics and estimates of its
quantitative aspects.
3. INTERPRETATIONS 1: SIMPLE CRATERS. The "magnitude" of a
cratering event must be determined by the impactor size,
mass, and velocity, and perhaps to a lesser extent on other
conditions such as impact angle and composition. It has
become accepted that all firstorder estimates should be
based on a single scalar measure of the impactor mass,
diameter, and velocity. Such a measure can be derived from a
melding of the theoretical considerations of "point-source"
solutions with results of the laboratory experiments, field
events, and code calculations (see [1,4]). All are
consistent with a single scalar measure of the power-law
form aU^micrometer delta v involving the impactor radius a,
velocity U, and mass density delta. For impacts into
relatively non-porous materials micrometer~0.55.

If the dominant measure of the impact conditions is this
single parameter, all subsequent effects are determined by
this same parameter. Its use in a dimensional analysis for
any aspect of the event will yield a so-called scaling law:
some restricted form for the functional dependence of one
aspect of the cratering event on the initial conditions. For
example, in the gravity regime the crater volume must depend
on the impactor according to the specific power-law form

equation 1 appears here

using the impactor mass m, mass density delta, gravity g,
and planetary mass density rho. Many other specific scaling
laws are given in a recent review [1]. The figure 6 of that
reference is reproduced here as Fig. 1. Unfolding the
dimensionless forms given in this figure gives for the
crater volume in soft rock geologies the specific equation

V = 0.48 m^0.78G^-0.65U^1.3

(using units of kg for mass, km/sec for velocity, gravity G
in earth multiples and volume in m^3). This is for the
gravity regime only, for impactors with a diameter above a
few tens of meters. The crater diameter at the original
ground surface for a simple crater can be estimated from

D = 2.4 V^1/3 to give:

D = 1.9 m^0.26G^-0.22U^0.43

The diameter at the top of the resulting lip is a factor of
1.3 times greater.

4. COMPLEX CRATERS. Terrestrial craters larger than about 3
km (and lunar craters larger than about 15 km) in diameter
undergo additional processes, presumably gravity-driven,
"late-stage" collapse that modifies an initial transient
bowl-shaped "simple" crater into the flatfloored complex
craters and basins observed. Thus, in addition to strength
scaling for small craters, and gravity scaling for larger,
there is a final complex crater regime for very large
craters. The complete scaling relations are as shown
schematically in Fig. 2. The relations for crater depth and
radius need significant corrections compared to those for
simple craters. This complex crater regime has not been
successfully calculated with codes, and requires much larger
combinations of gravity-scaled size than possible for
experiments. It is commonly assumed that such a
gravitational-driven collapse of a transient bowl-shaped
crater is a process that preserves volume. Based on this,
the relation between the final complex crater diameter D,
the earlier transient diameter D(sub)t and the threshold
diameter D(sub)* at the onset of complex crater morphologies
can be put into the form [1]:

equation 4 appears here

The function has been explicitly determined from a
comparison of the measured crater shapes for simple craters
to those observed for the complex craters on the Moon [5].
The final results of this analysis are as shown in Fig. 3.
Over most of the domain of interest the result is given by

equation 5 appears here

Using (3) for the excavation diameter and (5) one can get
that

D = 1.41 m^0.28U^0.47     (6)

using the units as before.

5. CHIXILUB APPLICATION. For the Chixilub basin, assuming D
= 300 10^3, one can use this relation and others [1] with an
assumed normal velocity of 20 km/sec for a rocky impactor to
determine the entries in the following table. Other
possibilities in assumed size, velocity or impactor
composition will modify these values.

References: [1] Holsapple K. A. (1993) Annu. Rev. EPS, 21,
333-373. [2] Schmidt R. M. et al. (1986) Defense Nuclear
Agency Report DNA-TR-86-182. [3] Schmidt R. M. and Housen K.
R. (1987) Intl. J. Impact Eng., 5, 543-560. [4] Holsapple K.
A. and Schmidt R. M. (1987) JGR, 92, B7, 6350-6376. [5]
Holsapple K. A. ( 1993) LPS XXIV.

Fig. 1, which appears here in the hard copy, shows volume
scaling relations for simple craters showing the strength
regime on the left (constant IIV for each impact velocity)
and the gravity regime on the right.

Fig. 2, which appears here in the hard copy, shows schematic
of crater radius scaling relation showing the complex crater
regime for very large craters.

Fig. 3, which appears here in the hard copy, shows the
diameter enhancement for the transformation from a transient
simple to a final complex crater.

Table 1, which appears here in the hard copy, shows the
results of the application of the scaling method to an
impact basin of 300 km diameter created by a 20 km/sec rocky
asteroid impacting the Earth.






Blum J. D.   Chamberlain C. P.
   Geochemical Constraints on the Composition of Volatiles Released During the
   Formation of KT Impact Melts: Implications for Extinction

It has now been firmly established that an asteroid or comet
struck the Yucatan Peninsula 65 Ma ago forming the ~300 km
diameter Chicxulub impact structure [1-6]. Major element and
Nd, Sr, Pb, and O isotopic studies have shown that impact-
induced mixing and melting of the target rocks produced both
the melt ejecta found in KT Boundary sediments and melts
encountered in several drillholes into the Chicxulub
structure [5-7]. Major element variation among the melts
indicates that they formed by mixing of a silicate endmember
(of granodiorite/dacite composition) with a high-CaO
endmember [8-10], which is easily understood because the
pre-impact target stratigraphy included a ~2 km platform of
limestone, marl (i.e. limestone + shale), dolomite, and
sulfate evaporite overlaying crystalline silicate basement
[11,12]. Thus, the high-CaO endmember is likely to be a
mixture of devolatized calcite (CaCO3), dolomite (MgCO3),
and anhydrite (CaSO4) with minor shale. The relative
proportion of carbonate to anhydrite excavated by the impact
is obscured by the cratering event, but a drillhole (Y-4)
that is 50 km outside of the outer ring structure
encountered ~1.25 km of Cretaceous limestone, marl, and
dolomite overlaying ~0.30 km of anhydrite, which overlays
crystalline basement [12]. Determination of the relative
proportion of the minerals that devolatized during impact is
an essential factor in assessing the influence of the impact
on the chemistry of the Earth's atmosphere and in turn the
potential climatic and environmental changes induced by the
impact.

On Fig. 1 we have plotted SiO2 + Al2O3 vs. CaO + MgO (these
four components account for =>90% of the rock compositions)
for 1) KT impact glasses from Haiti [10], 2) melt clasts,
melt matrix, and melt rock from the Y-6 and C-1 drillholes
into the Chicxulub structure [3,5], 3) a high-CaO endmember
calculated in [10] that accounts for all of the major
element variability in the Haitian glasses, and 4) average
values for platform carbonates [13] and sulfate evaporites
[14] from the literature. The covariation between the
Haitian glasses, Chicxulub melts, and calculated endmember
strongly support a common origin by mixing of two
endmembers. However, the most important question at hand is
the relative proportion of carbonate (calcite + dolomite) to
sulfate in the high-CaO endmember, which can only be
addressed with the aid of O isotope studies because they
provide a sensitive measure of the relative contribution of
O from carbonate vs. sulfate.

The O isotopic composition (delta^18O(sub)SMOW) of calcite
(and dolomite) precipitated from seawater is 28 to 30 permil
and global compilations of delta^18O values of Cretaceous
carbonates display a range from 26 to 29 permil [15]. In
contrast, the delta^18O value of sulfate precipitated from
seawater is 12 to 15 permil and global compilations of
delta^18O values of Cretaceous sulfates show a range of 13
to 15 permil [17]. The average delta^18O values and major
element compositions of anhydrite, carbonate, granodiorite,
and shale are plotted on Figs. 2a and b along with analyses
of impact glasses from Haiti. Also plotted are thick
horizontal lines that indicate average compositions of
platform carbonates and sulfate evaporites. The arrows
indicate that mixing pure carbonate with shale (to produce
marl) explains the difference between the carbonate and
platform carbonate fields. On Figs. 2a and b lines are drawn
following the variation trend of the impact glasses and
extending out toward the high-CaO endmember. The most
plausible interpretation of this data is that virtually all
of the CaO in the impact glasses was contributed by
devolatization of platform carbonate. Short line segments
are shown for mixtures of 10% and 50% anhydrite with
platform carbonate. If the high-CaO endmember contained
these amounts of anhydrite, the chemical variation trend of
the impact glasses would trend toward these line segments.
Based on these data we conclude that the anhydrite
contribution to the high-CaO endmember was 10% or less, a
conclusion consistent with trace element mixing calculations
[18].

The concentration and isotopic composition of S in the high-
CaO Haitian glasses requires the involvement of some
oxidized S (i.e., anhydrite) in the target material, and
thus there is no question that some sulfate aerosol was
released to the atmosphere by the impact [17]. However, the
SO3 content of <=1% in the Haitian glasses requires the
high-CaO endmember to contain only ~2% CaSO4, not 100% as
has been recently suggested [17,19]. Therefore, we suggest
that estimates of the amount of sulfate aerosol released,
which are based on the assumption that the high-CaO
endmember in the target material was mostly anhydrite, are
overestimates by a factor of 10 or more. In addition to
devolatization by melting, sulfate and carbonate
devolatization is also assumed to have taken place in
unmelted target rocks due to the impact shock pressure
[17,19,20]. Recent experiments on the shock devolatization
of sulfate minerals, however, find that sulfate
devolatization reactions occur to a far lower degree under
shock conditions than previous calculations have assumed,
and suggest that the amount of S devolatized is several
orders of magnitude lower than previous estimates [21].

The amount of SO3 and CO2 released during the KT impact
relates directly to several of the proposed extinction
mechanisms including (1) cooling of the Earth's surface on a
~1 year timescale due to increased sulfuric acid aerosol in
the stratosphere, (2) increased precipitation acidity on
land and lowering of the pH of surface ocean water due to
sulfuric acid rainout, and (3) an enhanced greenhouse effect
for 10^4 to 10^5 years due to release of CO2 into the
atmosphere from melting and shock devolatization of
carbonates. Our results and those of [21] suggest that the
volume of SO3 released by the KT impact has been
overestimated by at least one and possible two orders of
magnitude, implying that inputs of SO3 to the atmosphere may
not have been as lethal as previously proposed.

References: [1] Hildebrand A. R. et al. (1991) Geology, 19,
867-871. [2] Swisher C. C. et al. (1992) Science, 257, 954-
958. [3] Sharpton V. L. et al. (1992) Nature, 359, 819-821.
[4] Sharpton V. L. et al. (1993) Science, 261, 1564-1567.
[5] Kring D. A. and Boynton W. V. (1992) Nature, 358, 141-
144. [6] Blum J. D. et al. (1993) Nature, 364, 325-327. [7]
Premo W. R. and Izett G. A. (1992) Meteoritics, 27, 413-423.
[8] Sigurdsson H. et al. (1991) Nature, 349, 482-487. [9]
Sigurdsson H. et al. (1991) Nature, 353, 839-842. [10] Blum
J. D. and Chamberlain C. P. (1992) Science, 257, 1104-1107.
[11] Lopez-Ramos E. (1983) Geologia de Mexico, Univ. Nac.
Auton. de Mexico, 259-282. [12] Sharpton V. L. et al. (1994)
this volume. [13] Wedepohl K. H. (1969) Handbook of
Geochemistry, 1, 250-269. [14] Stewart F. H. (1963) USGS
Prof. Pap., 440Y, Y1. [15] Veizer J. and Hoefs J. (1976)
GCA, 40, 1387-1395. [16] Claypool G. E. et al. (1980) Chem.
Geol., 28, 199-260. [17] Sigurdsson H. et al. (1992) EPSL,
109, 543-559. [18] Koeberl C. (1993) Geology, 21, 211-214.
[19] Brett R. (1992) GCA, 56, 3603-3606. [20] O'Keefe J. D.
and Ahrens T. J. (1989) Nature, 338, 247-249. [21] Chen G.
et al. (1993) submitted to Nature.


Urrutia-Fucugachi J.   Marin L.   Sharpton V. L.
   Reverse Polarity Magnetized Melt Rocks from the KT Chicxulub Structure,
   Yucatan, Mexico

We report paleomagnetic results for core samples of the
breccia and andesitic rocks recovered from the Yucatan-6
Petroleos Mexicanos exploratory well within the Chicxulub
structure (about 60 km S-SW from its center), northern
Yucatan, Mexico. A previous study has shown that the rocks
studied contain high Ir levels and shocked breccia clasts
and an Ar/Ar date of 65.2 +- 0.4 Ma (Sharpton et al., 1992).
Andesitic rocks are characterized by stable single-component
magnetizations with a mean inclination of -43. Breccias
present a complex paleomagnetic record characterized by
multivectorial magnetizations with widely different initial
NRM inclinations. However, after alternating field
demagnetization, well defined characteristic components with
upward inclinations are defined.

IRM acuisition experiments, comparison of IRM and NRM
coercivity spectra, and the single component magnetization
of the andesitic rocks indicate the occurrence of Fe-rich
titanomagnetites of single or pseudo-single domain states as
the dominant magnetic carriers. Mean inclinations from the
andesitic rocks and most of the breccia samples give a mean
inclination of about -40 to -45, indicating a reverse
polarity for the characteristic magnetization that is
consistent with geomagnetic chron 29R, which spans the
Cretaceous/Tertiary (KT) boundary. The inclination is also
consistent with the expected value (and corresponding
paleolatitude) for the site estimated from the reference
polar wander curve for North America. We suggest that the
characteristic magnetizations for the andesitic and breccia
rocks are the result of shock heating at the time of
formation of the impact structure and that the age,
polarity, and paleolatitude are consistent with a time at
the KT boundary.

Recently, we have acquired samples from well Yucatan 6 from
three different intervals (N9, N17 (depth: 1295 5-1299m),
and N19 (depth: 1377-1379.5m)). We are currently determining
their paleomagnetic signature.


Koeberl C.
   On Source and Origin of Haitian KT Boundary Impact Glasses

1. Haitian KT Boundary Glasses Are Not Tektites: There has
been an unfortunate tendency to designate any newly
discovered glass that is suspected to be of impact origin as
a tektite, or glassy spherule as microtektite, even if these
glasses do not fulfill several of the criteria that
designate tektites (e.g., [1]). The "definition" of a
tektite has a somewhat historic aspect; there are obvious
differences between normal impact glasses and tektites (see,
e.g., [1]). Tektites are of course impact glasses, but the
reverse is not true: not all impact glasses are tektites.

Glasses found in Haiti that have been shown to be associated
with the KT boundary are among the glasses that have been
termed "tektites." These glasses are, beyond doubt, of
impact origin. However, considering the size of the ~300 km
diameter Chicxulub structure, and the distance of Haiti at
the time of the impact, these glasses have been deposited at
only about 1-2 crater diameters distance from the crater--
much closer to the source crater than any splash-form
tektites. A number of arguments have been cited that
indicate that tektites are shock-melts of surface rocks
(e.g., the high contents of ^10Be coupled with very low
^26Al abundances in tektites are evidence for a derivation
of tektites from terrestrial surface rocks, and preclude an
origin from greater depth in the crater).

Blum et al. [2] have shown that the Haitian glasses have
isotopic characteristics that are indistinguishable from
those of impact melt rock from the Chicxulub crater. The
Haitian glasses have therefore originated from much deeper
in the crater, analogous to some other impact glasses (e.g.,
at the Ries crater), and not from the surface as tektites.
The descriptive term "impact glasses" should thus be used
for these glasses and the term "tektites" reserved for those
glasses that fall in the proper definition of tektites.

2. Evaporite Was Not Involved in Making Yellow Haitian
Glasses: Sigurdsson et al. [3-5] suggested that the rare
yellow Haiti glasses formed by mixing carbonates and
evaporites with rocks of andesitic composition. No andesites
have been recovered so far from the Chicxulub crater, as
rocks initially thought to be andesites seem to be impact
melts. The black Haiti glasses seem to be representative of
the target rock composition and are thus used for
comparison. Blum and Chamberlain [6] and Koeberl [7] have
presented data that are not in agreement with a major
evaporite component in the yellow glasses. An argument for
the incorporation of evaporites is the high Ca and S content
of the yellow glasses; however, no mixture between
evaporites from Chicxulub and black glass that yields a good
fit for Ca gives values for S and most other elements that
are similar to those of the yellow glasses [7].

Some mixing studies have been performed for evaporites and
pure anhydrite from the Chicxulub crater ([7], and Koeberl,
unpublished data). Resulting discrepancies are illustrated
by the REE patterns shown in Fig. 1, and by the comparison
of trace element abundances between mixtures of evaporites
and anhydrites with a target rock composition, and yellow
glasses (Fig. 2). The REE pattern for average yellow glass
is very similar to and only slightly lower than that for
average black glass [8], while the pattern for evaporitic
rocks from Chicxulub is very different (Koeberl [7]). Thus,
a mixture of a rock of with a composition similar to the
black glass with a significant amount of evaporites, as
proposed by Sigurdsson et al. [3-5], yields REE patterns
that are different from, as well as REE abundances that are
far lower than, those of the yellow glasses. In addition,
other trace elements show no consistent pattern at all.
Evaporites did not contribute significantly to the chemical
composition of the yellow glasses.

The boron isotope ratio provides another argument against a
large evaporite component in yellow glass. The delta^11B of
the yellow Haiti glass is -2.1 permil [4], compatible with
normal terrestrial sediments (e.g., shales). However,
evaporitic rocks (elsewhere) are known to have very high
delta^11B values (+20 to +60 permil) because of the seawater
signature (e.g., Vengosh et al. [9]). The boron isotopic
composition of the yellow glasses is thus not consistent
with an evaporitic source.

Trace element data, O and Sr isotopic composition [6], and
boron isotopic composition [4] all argue against a major
anhydrite component in the yellow impact glasses from Haiti.
This is in agreement with observations of Chen and Ahrens
[10], who found that anhydrite devolatilizes much less than
carbonate. The Ca and S abundances in the yellow glasses can
be explained consistently by admixture of limestone and
sandstone, instead of anhydrite.

Acknowledgements: This contribution was supported by the
Austrian FWF, Project P8794-GEO.

References: [1] Koeberl C. (1994) In Large Meteorite Impacts
and Planetary Evolution (B. O. Dressler et al. eds.), GSA
Spec. Paper, in press. [2] Blum J. D. et al. (1993) Nature,
364, 325-327. [3] Sigurdsson H. et al. (1991) Nature, 349,
482-487. [4] Sigurdsson H. et al. (1991) Nature, 353, 839-
842. [5] Sigurdsson H. et al. (1992) EPSL, 109, 543-559. [6]
Blum J. D. and Chamberlain C. P. (1992) Science, 257, 1104-
1107. [7] Koeberl C. (1993) Geology, 21, 211-214. [8]
Koeberl C. and Sigurdsson H. (1992) GCA, 56, 2113-2129. [9]
Vengosh A. et al. (1992) Geology, 20, 799-802. [10] Chen G.
and Ahrens T. J. (1993) LPS XXIV, 273-274.

Fig. 1, which appears here in the hard copy, shows REE
patterns of Haitian black and yellow impact glass [8],
evaporite [7], and anhydrite (this work) from Chicxulub, and
resulting mixture.

Fig. 2, which appears here in the hard copy, shows a
comparison of the composition of yellow impact glass from
Haiti with that of mixtures between evaporite or pure
anhydrite from Chicxulub with "average" target composition
as shown by the composition of black Haitian impact glass.


Marin L. E.   Sharpton V. L.   Urrutia-Fucugachi J.   Sikora P.   Carney C.
   The 'Upper Cretaceous Unit' in the Chicxulub Multiring Impact Basin: New Age
   Based on Planktic Foraminiferal Assemblage

The Chicxulub structure is now recognized as one of the
largest multi-ring impact basins on Earth with a
corresponding age of 65 million years old (Sharpton et al.,
this volume). It has been interpreted as the source of the
KT ejecta layer. However, some doubt has been expressed as
to the age of the structure because "Upper Cretaceous"
fossils were reported 355 m above what is now known to be
the impact melt in the well Yucatan 6 (Lopez Ramos, Geologia
de Mexico, 1973). However, samples recently released by
Petroleos Mexicanos (Pemex) to the Universidad Nacional
Autonoma de Mexico have allowed us to examine closely the
core immediately above the impact rocks in the Yucatan 6
well. The so-called Upper Cretaceous limestone unit is
represented by core interval (nucleo) 12 recovered from a
depth of 1000 m to 1003 m below sea level. This unit is a
fine-grained well-indurated white limestone with visible
laminations demarcated by dark gray fine-grained material,
possibly clay. Original records indicate that a 2-cm thick
bentonite layer is located near the top of this core
interval. The next core interval down from N12 was recovered
from a depth of 1100 m to 1103 m. N13 was previously listed
as a green bentonitic sandstone unit of Upper Cretaceous
age, but our analysis indicates that this is a sand-sized
impact breccia containing shocked mineral clasts and
partially altered glass in a a carbonate-rich matrix.

Thin sections of two N12 samples (part 11 and part 12) near
the base of the core interval were examined. The fossil
assemblage consists mainly of abundant planktic foraminifera
of the genera Globigerina and Globorotalia. The following
age-significant species were identified: Globorotalia
pseudobulloides, Globorotalia trinidadensis (or possibly
Globorotalia uncinata), and Globigerina triloculinoides.
Each of these forms has the base of its stratigraphic range
in the lower Danian; the youngest base is that of G.
compressa. The tops of their ranges are mostly in the lower
Thanetian; the oldest top is that of G. trinidadensis in the
lowermost Thanetian. Thus, the most likely age for these
cores is Middle-Lower Danian to lowermost Thanetian. These
age assignments rely on the stratigraphic ranges of these
species in Amoco's Corporate Composite Standard Database,
but can be referenced to (Blow's) Planktonic Foram Zones
(e.g., Bolli et al., Plankton Stratigraphy, 1985); either
middle P1 through Middle P2 if we have G. trinidadensis or
Lower P2 through Lower P3 if we have G. uncinata. Either
alternative yields Lower to Lower Middle Paleocene (Fig. 2).

At the time of deposition, the water depth at this spot was
at least 100 m. This is based on the abundant occurrences of
planktonic forams in general. Frequent occurrences of
Globorotalias suggest somewhat deeper water environments,
maybe outer neritic--approaching 200 m. We observe a single
specimen that appears to be Nuttallides (benthic). If this
interpretation is correct, an upper bathyal environment is
implied. More work is needed, however, before a positive
identification of Nuttallides can be made.

The work presented here shows that the so called "Upper
Cretaceous" fossil assemblage found above the impact melt
has an age corresponding to the Lower Tertiary. Thus, the
age of this unit is consistent with Chicxulub being
temporally linked to the KT boundary, as radiometric and
paleomagnetic data have indicated.

Acknowledgements: The authors acknowledge Pemex for
providing the cores to UNAM. This work was funded through
grants to Marin by DGAPA (IN106891). We also thank the
Johnson Space Center and Amoco Production Company for
providing additional support. This work is part of a
cooperative research agreement between the Lunar and
Planetary Institute and Universidad Nacional Autonoma de
Mexico.


McKinnon W. B.   Alexopoulos J. S.
   Some Implications of Large Impact Craters and Basins on Venus for
   Terrestrial Ringed Craters and Planetary Evolution

If the Cretaceous Period closed with a bolide impact [1],
evidence of the resulting catastrophic effect on standing
terrestrial vegetation should be detectable in the
sedimentary record. Around 25% of terrestrial biomass was
consumed in the conflagration, according to a recent
estimate [2] and indeed soot has been detected at KT
boundary sites throughout the world [3,4]. With the
Chicxulub site in northern Yucatan now recognized as the
likely point of impact [5,6] and the discovery of megawave
deposits in the vicinity [7-9], the search for evidence of
the impact aftermath in organic sediments can be
intensified. One site in particular (Arroyo el Mimbral,
Taumalipas, Mexico) is of special interest, as its KT
sequence contains apparent megascopic plant remains [10].
The Mimbral KT deposit consists of 3 m of coarse clastics,
interrupting a pelagic marl sequence [10]. Organic material
(OM) was isolated by acid digestion (HCl/HF) and floatation
from 6 Mimbral samples. While most samples yielded only
traces of OM, one sample contained 0.2% by weight, which was
sufficient for organic petrologic and geochemical analyses.

Petrographic examination in reflected white light revealed
an unusual organic assemblage containing 90% inertinite
(fossil charcoal). The inertinite particles are large,
approaching a millimeter in size in some cases, and show
extraordinary preservation of fine botanical detail. The
remaining 10% is vitrinite (diagenetically-altered, but
uncharred terrestrial plant remains). Flash pyrolysis-gas
chromatography/mass spectrometry of the OM concentrate
indicates a predominance of polyaromatic structures, often
unmethylated. These chemical features are indicative of
severe thermal alteration due to partial combustion and
therefore consistent with the petrographic recognition of
fossil charcoal.

While most sedimentary OM assemblages contain a few percent
of inertinite, it is most unusual to see inertinite as the
predominant component. It is even more unusual to find such
an occurrence within a pelagic sequence, as inertinite
originates from charring of land plant material. (Instead
fossil marine algal or bacterial remains would be expected.)
Thus the inertinite is "out of place", as are the coarse
clastic sediments. A case can be made for the transport of
the clastics by a tsunami of large magnitude, perhaps of
seismically induced. However, when the additional evidence
of the fossil charcoal (and the paucity of non-charred plant
remains) is considered, then a coincident firestorm must
also be evoked. A nearby bolide impact would provide the
mechanism to explain the origin of the Mimbral deposit.

References: [1] Alvarez L. W. et al. (1980) Science, 208,
1095-1108. [2] Ivany L. C. and Salawitch R. J. (1993)
Geology, 21, 487-490. [3] Wolbach W. S. et al. (1990) GSA
Spec. Paper, 247, 391-400. [4] Venkatesan M. I. and Dahl J.
(1989) Nature, 338, 57-60. [5] Pope K. O. et al. (1991)
Nature, 351, 105. [6] Hildebrand A. R. et al. (1991)
Geology, 19, 867-871. [7] Bourgeois J. et al. (1988)
Science, 241, 567-570. [8] Hildebrand A. R. and Boynton W.
V. (1990) Science, 248, 843-846. [9] Alvarez W. et al.
(1992) Geology, 20, 697-700. [10] Smit J. et al. (1992)
Geology, 20, 99-103.





                          Thursday, February 10, 1994
                   SESSION 3:  KT RECORD: THE GULF OF MEXICO
                                8:30 - 12:00 PM

Chair(s):  E. Shoemaker
           G. Longoria


Hildebrand A. R.*   Asaro F.   Attrep M. Jr.   Bermudez-Santana J. C.   Bonis S.
Cedillo-Pardo E.   Claeys P.   Gonzalez-Casildo V.   Grajales-Nishimura J. M.
Gregoire D. C.   Ortiz-Aleman C.   Pilkington M.   Sanchez-Rios M. A.   Smit J.
Stansberry J. A.
   The Chicxulub Crater and its Relation to the KT Boundary Ejecta and
   Impact-Wave Deposits

Introduction: The Chicxulub crater is the largest known
Phanerozoic terrestrial impact crater and the environmental
degradation associated with its formation was probably the
cause of the mass extinction associated with the end of the
Cretaceous Period [1-3]. The Chicxulub projectile impacted a
shallowly submerged pedestal of continental crust located
between the proto-Caribbean and the Gulf of Mexico. The
resulting crater was immediately flooded and eventually
buried leading to apparently complete preservation of its
intracrater deposits subsequent to the effects associated
with the backwash. Because the crater is well preserved and
because its ejecta blanket is known from hundreds of
globally distributed localities, this impact affords an
unprecedented opportunity for impact studies, including the
effects of large impacts on the terrestrial environment.

Chicxulub Crater: The Chicxulub crater is a 165- to 180-km-
diameter peak-ring crater based on drill-hole data,
magnetic- and gravity-field data, seismic reflection
profiles, and surface fracture patterns; a 170-km diameter
is the most likely size, slightly smaller than that
originally proposed by [1]. A preliminary structural cross
section produced based on these constraints [4] has the
features expected for a large complex crater analogous to
other examples such as Popigai, Manicouagan, and Manson
(Fig. 1). The three main structural elements are an inner
zone of brecciated and melted target material roughly
corresponding to the transient cavity (~90 km diameter), a
central uplift (~40 km diameter) resulting from inward and
upward movement of material following rebound of the
transient cavity floor, and an outer faulted, inwardly
slumped zone (~170 km diameter) formed during collapse of
the transient cavity. None of the observations support
suggestions of larger crater diameters [e.g., 5].

Impact Angle and Projectile Type: The angle of impact is
constrained to >10 degrees by the circularity of the crater
and to >45 degrees by the azimuthal symmetry of the crater's
ballistically distributed ejecta [6]. Additional discoveries
of proximal KT ejecta yield thicknesses consistent with
azimuthal symmetry. The independent constraints of the
global fluence of Ir (250,000 tonnes) and the energy
required to excavate the 90-km-diameter transient cavity
(based on energy scaling relationships) are best satisfied
by a cometary projectile [7,8]. An asteroidal projectile is
allowed if most of the projectiles' Ir escaped the Earth in
high-energy ejecta or was sequestered in the crater's melt
rocks as reported by [3]. However, analysis of a sample of
the Y6 N17 melt rock yielded only 29 ppt of Ir, indicating
that not all the melt rocks carry anomalous Ir. Assuming a
melt volume of 20,000 km^3 requires an average concentration
of 5 ppb in the melt rocks if they are to hold an amount
equivalent to that found in the global fireball layer.
Further analyses will clarify Ir melt-rock concentrations,
but, as a total amount of Ir several times greater than the
current fireball-layer inventory is required for an
asteroidal impact, a cometary projectile is indicated.

Proximal Deposits: Discoveries of proximal deposits of
ballistic ejecta and impact-wave disturbances continue at an
accelerating pace. The ballistic ejecta deposits (exposed at
the surface only >500 km from the crater) are comprised
almost entirely of tektites (now generally pseudomorphed)
with a minor component of relatively fine-grained shocked
and unshocked debris. Ejecta thicknesses observed both at
newly discovered and previously described localities (Fig.
2) are consistent with thicknesses predicted by the power-
law relation of [9]. Near the crater the ballistic ejecta
have been stripped and/or diluted by impact waves in
shallow-water environments. In contrast, the fireball layer
appears to maintain its uniform thickness of ~3 mm. Deep-
water environments appear to offer the best chance for
preservation of minimally disturbed ejecta near the crater.

Sequences of sedimentary breccias (and other clastics) up to
>100 m thick occur at numerous KT localities surrounding the
Chicxulub crater. These deposits are interpreted as the
result of erosion and transport by giant impact waves and
submarine slumps triggered by the seismic energy deposited
by the impact. This class of deposits is volumetrically
probably several times larger (>100,000 km^3) than that of
the ballistic ejecta. These deposits invariably show
transport of material to deeper water and may provide
"snapshots" of life at KT time. Across the Chiapas Basin,
Mexico, these deposits show significant variation in clast
lithologies controlled by varying source terranes on the
margins of the basin. In the Peten Basin, Guatemala, a
dramatic deepening (order of 100 m) occurs across the
boundary breccias possibly indicating large-scale slumping
at the platform margin at KT time. These deposits are a
record of the dramatic erosional effects near the point of
impact and serve as a model for proximal deposits produced
by other marine impacts.

Extinction Mechanisms: The Chicxulub impact is a one-in-100-
million-year event based on observed crater and impactor
populations. The impact released approximately 10^31 ergs of
energy in a fraction of an hour. Deposition of this amount
of energy allowed lethal changes in oceanic and atmospheric
systems. Release of devolatized C and sulfur dioxides from
the impacted rocks of the Yucatan Platform probably made the
Chicxulub impact particularly deadly for its size.
Understanding the contributions of different mechanisms to
the extinction of species will be more difficult than
establishing the location and size of the impact, but having
the latter information constrains modeling of the impact's
effects.

References: [1] Hildebrand A. R. et al. (1991) Geology, 19,
867-871. [2] Swisher C. C. et al. (1992) Science, 257, 954-
958. [3] Sharpton V. L. et al. (1992) Nature, 359, 819-821.
[4] Pilkington M. et al. JGR (submitted). [5] Sharpton V. L.
et al. (1993) Science, 261, 1564-1567. [6] Hildebrand A. R.
and Stansberry J. A. (1992) LPSC XXIII, 537-538. [7]
Sigurdsson H. et al. (1992) EPSL, 109, 543-559. [8]
Hildebrand A. R. (1992) Ph.D. dissertation, Univ. of
Arizona, 358 pp. [9] McGetchin T. R. (1973) EPSL, 20, 226-
236.


Bourgeois J.*
   Tsunami Deposits and the KT Boundary:  A Sedimentologist's Perspective

Tsunamis are impulse generated waves and can be generated by
any event that displaces a significant amount of water in a
short period of time, on the scale of seconds to minutes.
The most typical source of tsunamis are subduction zone
earthquakes, when deformation associated with low angle
thrusting displaces water during the event. Tsunamis are
also generated by volcanic events and subaqueous landslides;
bolide impacts in water will produce the largest tsunamis in
Earth history. Tsunamis are wave trains, rather than single
waves, and they may be locally damped, focused, amplified,
and reflected.

Our knowledge of typical (seismogenic) tsunamis and their
deposits has increased significantly since Snowbird II.
These tsunamis typically generate near field run ups on the
scale of meters, to 10s of meters in rare cases. Recent
tsunamis in Nicaragua (1992), Indonesia (1992), and Japan
(1983, 1993) have allowed workers to generate detailed
information about the nature of these tsunamis, their
erosional and other destructive effects, and their deposits.
Paleo tsunami deposits from subduction zone earthquakes have
been documented from Chile (1960), Alaska (1964), the
Pacific Northwest U.S. (late 1600s), and elsewhere. I will
present a brief summary of the Nicaraguan, Chilean, and
Pacific Northwest examples.

Tsunamis generated by bolide impacts ("mega tsunamis") are
at the other end of the tsunami scale, potentially
generating tsunamis several orders of magnitude more
energetic than seismogenic cases. Our understanding of these
kinds of events is much more theoretical because there are
no historic cases. We can expect that an impact in deep
water could propagate a wave equal to the water depth, and
thus mega tsunamis may affect the floor of the ocean at
depths much greater than seismogenic tsunamis, which only
have an effect in the coastal zone. Certainly shelf
environments (depths of c. 30200 m) would be affected by
mega tsunamis, and in proximal regions, the continental
slope would also be subject to the effects of such a wave,
or set of waves.

Coarse grained KT boundary deposits around the Gulf of
Mexico and Caribbean have been variously interpreted as
(mega)tsunami deposits, transgressive sequence boundaries,
and lowstand fan or delta deposits. The first published
examples of each of these interpretations preceded the
identification of a likely impact structure of possible KT
boundary age in the Yucatan (Chicxulub). The postulate that
Chicxulub is a KT boundary impact structure has generated
even more interest in these coarse grained beds. One
interesting aspect of the possibility of this event is that
the impact would have occurred in shallow water, and thus
the tsunami or tsunamis generated would have been smaller
(than for a deep h) 0*0*0* water impact) and more complex,
possibly related to platform collapse or large subaqueous
landslides. Locally, slide generated tsunamis can generate
catastrophic run ups, of 100s of meters, as documented in
Lituya Bay, Alaska, and in prehistoric examples in the
Hawaiian islands. Other phenomena such as the blast wave and
fallout of the ejecta blanket would also create
disturbances. In the complex geometry of the Gulf and
Caribbean, such waves would have been locally damped,
focused, amplified, and reflected; seiches (sloshing
effects) would have been set up. The waves themselves could
generate destabilization, e.g., of delta fronts, generating
large turbidity currents.

A number of talks and posters at this meeting will address
the sedimentological interpretation of coarse-grained
deposits of KT age around the Gulf of Mexico and Caribbean,
presenting diverse points of view. While biostratigraphers
may still and forever disagree with regard to the "precise"
placement of these beds biostratigraphically, it is up to
sedimentologists to produce physically plausible
explanations for these beds, irrespective of their age.
Careful examination, description, and analysis will
elucidate the sedimentary processes responsible for
generating these beds, and will help us to determine, for
example, the water depths at the time of deposition (and
during bracketing times), and the amount of time represented
by the beds. My examination of the KT boundary unit in Texas
(Brazos) and my preliminary study of outcrops in
northeastern Mexico lead me to conclude, as have a number of
others, that the nature these beds is compatible with a mega
tsunami scenario. Many exciting details in the
reconstruction of this scenario are yet to be worked out!

*(address until June 1994: EAR Room 785, National Science
Foundation, 4201 Wilson Boulevard, Arlington VA 22230;
Phone:703-306-1551; Fax:703-306-0382; E-mail:
jbourgeo@nsf.gov].


Stinnesbeck W.*   Keller G.   Adatte T.
   KT Boundary Deposits in NE Mexico: Bolide Impact or Sea-Level Lowstand?

In northeastern Mexico, channelized clastic deposits of near
KT boundary age occur over a distance of 300km in a NNW-SSE
direction. We report here on the stratigraphy, lithology,
and depositional environment of these deposits at El
Mimbral, La Lajilla, El Penon, El Mulato, La Sierrita, La
Parida, and Los Ramones, in the Mexican States of Tamaulipas
and Nuevo Leon (Fig.1) in order to evaluate impact generated
tsunami [1] vs. normal deposition during sea-level lowstands
[2].

In all sections examined, the clastic member discontinuously
truncates the Maastrichtian Mendez Formation, which consists
of rhythmically bedded sandstones, shales, marls, and marly
limestones. Foraminiferal assemblages suggest that sediment
deposition occurred in upper slope to outer neritic
environments during the late Maastrichtian. This is
indicated by the presence of planktic foraminiferal
assemblages characteristic of the A. mayaroensis Zone,
including the index taxon, in the Mendez Fm. as well as
throughout the clastic deposit [1-3]. A thin layer of
Maastrichtian marl containing the A. mayaroensis Zone fauna
also caps the clastic deposit at El Mulato and La Lajilla
[4]. This also indicates that deposition of this clastic
member occurred during the latest Maastrichtian. The first
Tertiary planktic foraminifera are present in the shales and
marls directly overlying the clastic deposit at Mimbral [1-
3] and La Parida or overlying the thin Maastrichtian marl
layer at La Lajilla and El Mulato [4]. In all sections
examined, except El Mimbral, this fauna belongs to the early
Paleocene Pla (P. eugubina) Zone. At El Mimbral the basal
Tertiary Zone PO is present in a 4cm clay layer at the edge
of the clastic channel deposit [3].

We have subdivided the end-Maastrichtian channel fills of
northeastern Mexico into three units, which are defined by
lithology, sedimentology, and mineralogy [2,5].

Unit 1--Spherule-rich Layer: The basal Unit 1 is
characterized by abundant poorly cemented calcareous
spherules, chlorite, quartz, and glauconite. Glauconite
commonly fills chambers of foraminifera. A well cemented
discrete sandy limestone layer (S.L.L.) with convolute
surface is present within this unit and contains only few
graded spherules (El Mimbral, outcrop I and El Penon,
outcrop I) (Fig. 2). At El Mimbral the convolute laminated
bedding surface of this limestone is draped by the overlying
spherule-rich sediments. Moreover, the S.L.L. is in contact
locally with the overlying massive sanstone of Unit 2
(outcrop near meter mark 36), which also drapes around it.
This suggests that the S.L.L. was at least semi-lithified
prior to later deposition. At La Lajilla and El Penon II
this S.L.L. is at the top of Unit 1 due to an erosional
disconformity. The sperules of Unit 1 are of multiple
origins including organic, volcanic, and oolitic. They are
transported from shallow neritic environments, as also
indicated by the common presence of benthic foraminifera and
glauconite. No glassy spherules have been observed and glass
fragments are extremely rare. They are similar in
composition to those of Beloc, Haiti, which have been
variously interpreted as of impact or volcanic origins
(1,2,7-9).

Unit 2--Massive Sandstone: This unit consists of a massive,
commonly laminated, sandstone with mudclasts at its base. At
Mimbral, distinct layers of wood and leaf debris are present
near the bottom of this unit. If deposition occurred in a
few hours to days, this plant debris could not have settled
in distinct layers at the base of Unit 2 prior to deposition
of coarse sands.

Unit 3--Interlayered Sand-Silt Beds: Unit 3 consists of
interlayered sand, shale, and siltstone beds with diverse
sedimentological features such as horizontal laminations,
ripple marks, small scale crossbedding, flaser bedding, and
convolute lamination. Thin layers with normal hemipelagic
sedimentation and distinct layers containing zeolites [5]
were detected within this unit. It is generally topped by
resistant rippled sandy limestone (R.S.L.) layers containing
bioturbation, most commonly Chondrites, Zoophycos and y-
shaped Thalassinoides burrow networks.

This three-part depositional sequence is surprisingly
constant and allows correlation not only of the three units,
but also of individual beds such as the sandy limestone
layer of Unit 1, the rippled sandy limestone of Unit 3 [2]
(Fig. 2), or the zeolite-enriched layers of Unit 3 [5].
Variable thickness or absence of the three units depends
largely on which part of the channel is exposed. Sections
are most expanded in the center of the channel where usually
all three units are present. Towards the edges, first Unit 2
disappears followed by Unit 1 and finally Unit 3. The sandy
rippled limestone at the top of Unit 3 is laterally most
continuous (Fig. 3). We propose that the channelized near-KT
boundary clastic deposits of northeastern Mexico were
deposited over an extended time period, rather than hours or
days, and by normal sedimentary processes that include
gravity flows and turbidity currents, rather than impact-
generated tsunami waves. Our interpretation is based on:
(1) Lithologically, sedimentologically, and mineralogically
distinct units, subunits and individual zeolite-enriched
layers that can be correlated over more than 300km. Such
systematic correlatability is not compatible with a
catastrophic event and chaotic sediment deposition. (2)
Semi-lithification of the sandy limestone within the
spherule-rich Unit 1 prior to deposition of the upper
spherule-rich sediments and prior to the deposition of Unit
2 sandstones. (3) Multiple origins of the spherules,
including volcanic, organic, and oolitic; these spherules
also indicate transport from a shallow neritic environment
rich in glauconite. (4) Presence of plant debris in discrete
horizons at the base of the sandstone of Unit 2 that could
not have settled within hours or days and prior to the
coarse sand grains of this unit. (5) Discrete layers with
hemipelagic foraminifera in Unit 3. (6) Bioturbation in Unit
3 is of Maastrichtian and not Tertiary origin. At least part
of these benthic communities lived during deposition of the
rippled sandy limestone, and burrows are usually infilled
with the same sediment. (7) Deposition of the clastic
deposit occurred prior to the KT boundary as indicated by
the presence of thin marls on top of the sequences in La
Lajilla and El Mulato, which contain A. mayaroensis Zone
faraminifera and no Tertiary elements.

We interprete the channelized clastic beds of northeastern
Mexico as fan deposits, which accumulated during the major
eustatic sea-level lowstand near the end of the
Maastrichtian [10]. In our scenario, the top of the clastic
member marks the end of this regressive cycle followed by a
sea-level transgression that began a few thousand years
below the KT boundary as indicated by the Maastrichtian
bioturbation and subsequent deposition of the thin marl
layer present at La Lajilla and El Mulato. In northeastern
Mexico as elsewhere, the trans-KT interval is marked by a
sea-level transgression, sediment starvation and a short
hiatus [11,12].

References: [1] Smit J. et al. (1992) Geology, 20, 99-104.
[2] Stinnesbeck W. et al. (1993) Geology, 21, 797-800. [3]
Keller G. et al. (in press) Palaios. [4] Lopez Oliva J. G.
and Keller G. (this volume). [5] Adate T. et al. (this
volume). [6] Bohor B. F. and Betterton W. T. (1993) LPSC 22,
Houston. [7]  Jehanno C. et al. (1992) EPSL, 109, 229-241.
[8] Lyons J. B. and Officer C. B. (1992) EPSL, 109, 205-224.
[9] Izett G. A. (1991) JGR, 96, 20879-20905. [10] Haq B. U.
et al. (1987) Science, 235, 1156-1166. [11] MacLeod N. and
Keller G. (1991) Geology, 19, 497-501. [12] Keller G. et al.
(1993) GSA Bull., 105, 979-997.


Smit J.*   Roep Th. B.   Alvarez W.   Claeys P.   Montanari S.   Grajales M.
   Impact-Tsunami Generated Clastic Beds at the KT Boundary of the Gulf Coastal
   Plain: A Synthesis of Old and New Outcrops

The KT boundary is marked in Gulf coastal plain outcrops,
from Alabama to Guatamala [1] by unusual clastic deposits.
These clastic deposits are interpreted as tsunami deposits
[2-4] probably generated by the Chicxulub impact. These beds
have also been interpreted as storm beds, lowstand sands
[5], and turbidite or debris-flows [6]. Sedimentologic
analysis of the clastic beds at Moscow Landing, Alabama,
Brazos, Texas, and los Ramones, Rancho Nuevo, Sierrita-Las
Rusias, Penon, Porvenir, Mulato-Bruselas, La Lajilla, Mesa
de Llera, Mimbral, La Ceiba and possibly Bochil, all in
eastern Mexico, show that the buildup of the clastic beds
follows a consistent pattern, well recognizable all over the
Gulf, but modified by local variables, such as waterdepth,
distance from shoreline, and eustatic sealevel changes. In
all outcrops, we can discern four distinct, successive, in
energy decreasing sedimentary units, each representing a
successive phase in the course of the tsunami activity in
the Gulf of Mexico.

The basal unit 1 consists of shallow channels, filled
laterally with bubbly spherules, all similar to the
spherules containing impact glass in Mimbral and Lajilla,
and Beloc (Haiti). New localities with bubbly tektite like
spherules are in Darting Minnows creek [2], Moscow landing
(Alabama), and La Ceiba, Poza Rica (Mexico). Preliminary
analysis did not yield glass from these spherules. Those
localities also contain similarly sized platform limestone
clasts. These were first found in Mimbral, and are not
locally derived material, but non or partially molten
ejecta. A 87Sr/86Sr ratio of 0.707502 +- 0.000008 (K. Beets,
pers. comm) of the limestone clasts is compatible with a
Campanian age of these clasts. The ejecta are mixed with
locally derived material from the seafloor, such as soft
upper Cretaceous chalk and marl clasts, fossils, and washed-
out foraminifera. The unit 1 sediments are relatively poor
in detrital sand, in comparison with the overlying units [2-
4]. Commonly (Moscow landing, Ramones, Rancho Nuevo,
Sierrita, Mimbral) due to the weight of the channel-filling,
the channels themselves sink down into the soft chalky and
marly oozes of the seafloor, pushing oozes away from the
channel centers to the edges, where they form ~mud-diapirs.'
Diapirism continues also during the fill of unit 2 channels,
showing that diagenetic lithification of certain beds in
unit 1 postdates the deposition of the clastic beds.
Possibly preceding, and continuing through the deposition of
unit 1 are slumping and sliding movements, probably induced
by seismic waves arriving from the impact site. These slides
are observed in outcrop in Moscow landing (Alabama), but
also in Guatamala large slides must have occurred, as the
carbonate platform limestones underlying the clastic beds,
are followed in the earliest Danian pO and P1a zones by
pelagic deposits [1]. The loading of unit [1] or the seismic
shaking also causes ~injections' of spherule rich material
down into the soft Mendez oozes (Lajilla, Sierrita,
Mimbral). Paleocurrent direction measurements are seldom
possibe, but in Mimbral we observe lateral infilling of
different channels from different directions.

The next unit 2 overlies unit 1 channels with a sharp
contact, which mark a transition to detrital rich sands,
often mixed with water saturated wood fragments. At Mimbral
(Fig 1) we observe at least 6 successive channels in unit 2,
each with a coarse basal part scouring in the top of the
preceding channel. The coarse layers of all these channels
contain tektite like spherules, obviously reworked from unit
1 channels. Preliminary sediment petrologic analysis shows
interesting differences between the unit 2 channels, not
obvious in outcrop. The upper channels are better washed out
(contain less mud) than the basal ones. Overall, unit 2 is
grading upward, with coarse rounded and armoured clayballs
mainly at the base. Burrows do not occur in any of the unit
1 and 2 sedimentary layers, which might have indicated an
extended amount of time between the sediment layers.
Sedimentary structures are of a wide variety, owing to the
variations in current strength and suspension load in
different localities. The most conspicuous structures are
parallel laminations with primary current lineation,
climbing ripples, and small scale trough cross bedding. Note
that unequivocal wave ripples have not been observed in any
of the outcrops and that all cross bedding is caused by
currents.

Unit 2 sedimentary structures in almost all sections contain
a wealth of indications for paleocurrent directions, but
most were found in the Lajilla section (Fig. 2). Current
directions were measured on flutecasts, ripple migration,
and primary current lineations, and threedimensional
analysis of crossbedding forests. The measured current
directions are bi-directional, different by 180 degrees, and
interestingly, those directions point toward and away from
Chicxulub. At least 12 times the currents switch direction
in ascending order in Lajilla, showing at least six passages
of giant waves. Also the Brazos river, Ramones, Rancho
Nuevo, Penon, Mulato, Mimbral, and La Ceiba outcrops show
these opposite current directions in unit 2, demonstrating
that this is not a local phenomenon, but a common
characteristic of the KT clastic beds, invalidating a
mass-flow or turbidite origin.

Unit 3 rests conformably on unit 2 at the first thin
laterally continuous silt layer. Several of such silt layers
are intercalated in between fine sandstone ripples. The fine
sandstone ripples decrease in thickness, while the silt
layers increase in thickness upsection. The first silt layer
is also the first with significant Ir ennchrnent and is an
indication that more time (minutes to hours) becomes
available for settling of fine sediment in between the
arrival of weakening tsunami or seiche waves. Burrows abound
in the topmost sandstone layer and some can be observed to
penetrate into lower silt layers. At Penon, spectacular
unknown burrow structures can be observed, penetrating up to
60 cm down into unit 3 and then spreading radially in
dichotomizing 1 cm tubes of over 2 meter length. All the
bioturbation demonstrably postdates the deposition of the
unit 3 sandstone ripples. The last unit 3 sandstone
flaserbeds are often only a few mm thick.

The top unit 4 consists of micritic limestone and mudstone,
seemingly homogeneous in outcrop, but showing a gradual
decrease in grainsize. The mudstone is interpreted as the
settling out of the suspension cloud in seawater, stirred up
by the passing tsunami waves. Ir abundances are still high
and multi-peaked in this interval, and thus apparently the
fine grained Ir carrier simultaneously settled out from the
atmosphere and seawater column. The transition from unit 4
to the overlying hemipelagic oozes is often inperceptible in
the field, but varies from 9 cm at Mimbral to 25 cm at
Brazos river. The rich foraminiferal faunas of the
Cretaceous have then already disappeared in all sections,
showing the immediate response of the biosphere to the
consequences of the Chicxulub impact. Maybe these are not
the final extinctions, but at least there is the initial
dramatic reduction of foraminiferal populations.

Nature has played a number of post-depositional tricks on
the clastic deposits, often obscuring our view on the KT
events. Weathering of spherules to smectite and of fine
sandstone laminae to clay or zeolites has led to ambiguous
interpretations. In a number of outcrops in the US, where
the depositional depth of KT deposits is much less than in
Mexico, an early Danian sealevel fall has often removed
evidence of the tsunami deposits (Littig Pit and Walker
Creek Texas, Braggs Alabama). The lowstand deposits may
superficially resemble the tsunami deposits, because at the
lowstand the backwash sands of the tsunami waves are re-
reworked into the lowstand and succeeding transgressive
sands. Clastic KT deposits in sections like Brazos River and
Moscow landing barely survived the sealevel fall, and in
Moscow landing units 3 and 4 are stripped off. At even
shallower depths (above wavebase) like in the La Popa basin
in northeastern Mexico, where subsidence apparently keeps up
with the sealevel drop, all tsunami-wave deposits have been
destroyed and are incorporated in normal sediments with
hummocky cross bedding and other wave-ripple structures.

In conclusion analysis of the KT clastic beds in Gulf
coastal plains shows that the entire complex of clastic beds
has been deposited in one, pulsating event, lasting for
several days. Deposition by a series of giant tsunami or
seiche waves is the most likely explanation, and that
hypothesis has been confirmed by the bi-directional
paleocurrent directions and variable current strengths.

References: [1] Hildebrand A. R. et al. (1991) Geology, 19,
867-871. [2] Bourgeois J. et al. (1988) Science, 241, 567-
570. [3] Smit J. et al. (1992) Geology, 20, 99-103. [4] Smit
J. and Romein A. J. T. (1985) EPSL, 74, 155-170. [5] Mancini
E. A. et al. (1989) J. Res., 19, 93-104. [6] Stinnesbeck W.
et al. (1993) Geology, 21, 797-300.

Fig. 1, which appears here in the hard copy, shows Mimbral,
Mexico. Simplified cross section through the outcrop,
showing the different channellized units. Ir abundances from
[3]. The soft Mendez oozes were squeezed away by loading of
the unit 1 channels into diapiric structures between the
channels. The top ripples are more continuous than the lower
channels and drape over every outcrop in eastem Mexico, even
at those sites where the lower two units are not deposited,
like at the Mesa de Llera, 30 km NW of Mimbral. These
ripples could be due to contour currents generated by the
developing seiche in the Gulf of Mexico.

Fig. 2, which appears here in the hard copy, shows La
Lajilla, Mexico, 40 km E of ciudad Victoria. Measured
paleocurrent directions contain two dominant 180O different
directions: one toward and one away from the Chicxulub
crater. Those two directions alternate successively
stratigraphically upward, showing both up-surge and backwash
surges of at least 6 individual tsunami waves.


Keller G.*
   Global Biotic Effects of the KT Boundary Event: Mass Extinction Restricted
   to Low Latitudes?

One of the most important recent developments in
Cretaceous/Tertiary (KT) boundary studies is the growing
awareness that (1) the mass extinction associated with this
event is not the result of a single catastrophe, (2) that
extinctions occurred over an extended time period and were
selective rather than random within organismal groups as
well as between different groups, and (3) that the biotic
effects were most severe and sometimes limited to tropical-
subtropical regions while high latitude faunas and floras
escaped virtually unscathed. The growing global body of high
resolution faunal and floral data upon which these
observations are based now provide the empirical data base
and the opportunity to test various KT catastrophe scenarios
whether volcanic or bolide impact based. Moreover, specific
details of these scenarios can now be tested including the
dust cloud-darkness and shutoff of photosynthesis, acid
rain, nuclear winter, and global greenhouse scenarios all
presumably leading to global mass extinctions.
Unfortunately, there has been no serious test of these
scenarios based on empirical data largely because (a) many
catastrophists ignore paleontologists' data and
interpretations that widely disagree with their theories,
(b) many paleontologists ridicule catastrophists' theories
for which they see no basis in the fossil record, and (3)
experts disagree among themselves depending on which side of
this scientific chasm their sympathies lie. Despite these
seemingly insurmountable differences, there is hope for a
growing interdisciplinary discourse as both sides accumulate
and synthesize more observational data. All scientists agree
that a major environmental change occurred across the KT
boundary; they disagree on the cause, whether impact or
volcanism, and on the biotic effects of this environmental
change. Here I will address the latter. Ultimately it is the
biotic data that must determine how significant a bolide
impact or massive volcanism at KT boundary time was for life
on Earth.

The planktic foraminiferal record has played a unique role
in the KT boundary controversy. Near the base of the food
chain, these single celled marine plankton are both
sensitive to environmental changes and a critical food
source for higher, more complex organisms. Moreover, they
have long been known to undergo a virtually complete faunal
turnover between the end of the Cretaceous and early
Tertiary with deep-sea studies documenting a sudden mass
extinction at the boundary [1-3]. This prior interpretation
cannot be supported by subsequent studies for three major
reasons. First, this conclusion was originally based on
deep-sea sections that have been subsequently shown to
contain major hiatuses spanning from 60 kyr to 500 kyr of
the basal Tertiary [4,5]. In these sections extinctions and
originations, spanning as much as 0.5 m.y. across the KT
transition, are artificially concentrated at one horizon
giving the appearance of a sudden catastrophic mass
extinction. In contrast, shallow continental shelf sections
are generally more temporally complete and show a more
gradual transition. These sections are more complete
primarily due to the sea-level transgression that
characterizes the KT boundary and early Danian that shifted
the locus of sedimentation to continental shelf areas,
leading to sediment starvation, non-deposition and/or
erosion in the deep-sea [4,5].

Second, the presence of many Cretaceous taxa in lowermost
Tertiary sediments has been routinely ignored based on the
assumption that all Cretaceous species (except one) must
have become extinct at the KT boundary. Their presence in
Danian sediments is therefore assumed to be due to
reworking. However, about 1/3 of the Cretaceous fauna has
been observed to range well into the Tertiary worldwide and
isotopic measurements of their shell calcite provide
unequivocal evidence of their residence in Tertiary
environments [5-8].

Third, species extinctions are often assumed to have
occurred instantaneously due to a bolide impact. However, to
date, no sudden extinction horizon of all, or nearly all,
Cretaceous planktic foraminifera has been documented in any
chronostratigraphically complete KT boundary section [5,9-
13], although such an extinction horizon is sometimes
inferred [1,2,12].

Comparison of species extinction and evolution patterns of
planktic foraminifera across the KT boundary in the most
complete high and low latitude sections (El Kef, Agost,
Caravaca, Brazos River, El Mimbral, Nye Klov, and ODP Site
738) reveal major differences in extinction patterns. No
significant species extinction, or major faunal abundance
changes coincide with the KT boundary in high latitudes, and
nearly all Cretaceous species survive and thrive well into
the early Tertiary [5,11]. The same lack of species
extinctions has been observed in siliceous microfossils,
palynomorphs, and invertebrate faunas [14,15]. In contrast
major species extinctions occurred in low latitudes where
2/3 of the Cretaceous taxa disappeared at or below the KT
boundary. But, only tropical and subtropical, large, complex
and ornamented taxa disappeared. Stable isotopic data
indicate that these taxa were predominantly deeper water
dwellers living at or below the thermocline. Only surface
dwellers survived. All of the disappearing taxa were already
rare endangered species in the late Maastrichtian ocean
comprising only between 5% and 17% of the planktic
foraminiferal population [5-11,13]. Extinction of these rare
taxa could have been caused by relatively minor
environmental perturbations in temperature, salinity, O, or
nutrients. Although it is possible that the extinction of
some or all of these taxa was related to the climatic
changes associated with a KT boundary bolide impact, a
direct cause-and-effect relationship cannot be demonstrated.

In the KT boundary stratotype section of El Kef, all major
microfossil groups have been analyzed and their species
richness data are illustrated in Fig. 1. At this most
complete low latitude section, the biotic effects are highly
variable across the spectrum of microfossils. Planktic
foraminifera appear to be most severely affected with
gradual species extinctions across the KT boundary. Benthic
foraminifera and nannofossils also show pronounced biotic
effects beginning at the KT boundary [16,17]. But curiously,
ostracods and plants (pollen and spores) show little or no
effect [18,19], whereas dinoflagellates increase in
diversity [20]. Similar patterns in extinctions, or lack
thereof, have been observed in other low latitude sections
whereas in high latitudes no major species extinctions are
documented in any microfossil or macrofossil groups. These
variable extinction patterns across latitudes and between
fossil groups are difficult to explain by either volcanism
or bolide impact theories. Stable isotope data suggest that
the major difference between high and low latitudes across
the KT boundary is in surface water productivity. In low
latitudes, surface water productivity dropped significantly
beginning at the KT boundary and did not recover until about
300-500 kyr later, whereas in high latitudes surface
productivity remained nearly constant [5,11]. The cause of
this high vs. low latitude difference in surface
productivity remains an enigma.

None of the current impact, volcanism, or gradual
environmental change scenarios currently proposed can
explain these variable species extinction and isotope
records. Any successful scenario, however, will have to
account for the following low and high latitude differences.

Low Latitudes: (1) Mass extinctions limited to low latitudes
with only tropical-subtropical rare species extinct, (2)
selectivity in the extinction record within species groups
(extinction of deeper dwelling plankton), (3) gradual
extinction pattern beginning below and ending well above the
KT boundary, (4) survivorship of cosmopolitan (generalist)
taxa, (5) survivorship of surface dwelling plankton (implies
no shut-off of photosynthesis or acid rain), (6) absence of
extinctions in some low latitude faunal and floral groups,
(7) negative delta^13C shift beginning at the KT boundary
and gradually declining through Zone PO (~40 kyr), (only in
temporally incomplete KT sections is the delta^13C shift
sudden), (8) decreased surface productivity.

High Latitudes: (1) Absence of mass extinctions in any
fossil group, microfossils, invertebrates, or vertebrates,
(2) absence of delta^13C shift, (3) stable or increased
productivity.

These varied and complex extinction and survival patterns do
not support a single catastrophe as the sole causal agent,
but rather suggest a multi-causal scenario that has yet to
be defined.

References: [1] Premoli-Silva I. and McNulty C. L. (1984)
DSDP, 77, 547-584. [2] Smit J. (1982) GSA Spec. Paper, 190,
329-352. [3] Smit J. (1990) Geologie en Mijnbouw, 69, 178-
204. [4] MacLeod N. and Keller G. (1991) Geology, 19, 497-
501. [5] Keller G. et al. (1993) GSA Bull., 105, 979-997.
[6] MacLeod N. and Keller G. (in press) Paleobiology. [7]
MacLeod N. (this volume). [8] Barrera E. and Keller G.
(1990) Paleoceanography, 5, 867-890. [9] Canudo J. I. et al
(1991) Mar. Micropaleo., 17, 319-341. [10] Keller G. (1988a)
Mar. Micropaleo., 13, 239-263. [11] Keller G. (1993) Mar.
Micropaleo., 21, 1-45. [12] Liu C. and Olsson R. K. (1992)
JFR, 4, 328-346. [13] Keller G. et al. (in press) Palaios.
(J. G. Lopez-Oliva and G. Keller, eds.), (this volume). [14]
Askin R. et al. (this volume). [15] Zinsmeister W. J. et al.
(this volume). [16] Keller G. (1988b) Paleo., 3, 66, 153-
171. [17] Pospichal J. (in press) Geology. [18] Donce P. et
al. (1985) Actes du Premiere Congras Nationale des Sciences
de la Terre, Tunis, 161-169. [19] Meon H. (1990) Rev.
Paleobotany and Palynology, 65, 85-94. [20] Brinkhuis H. and
Zachariasse W. J. (1988) Mar. Micropaleo., 13, 153-191.

Fig. 1, which appears here in the hard copy, shows species
richness patterns in six microfossil groups across the KT
boundary at the El Kef KT boundary stratotype section.


Longoria J. F.*   Gamper M. A.
   The Planktonic Foraminiferal Event in the KT Boundary Succession of Mexico

No abstract available.



                              POSTER PRESENTATIONS

Wdowiak S. Y.   Wdowiak T. J.   Gillies D. C.   Dudley M.
   A Search for Shocked Grains in the KT Boundary at Braggs, Alabama

We have searched the KT boundary that is exposed at Braggs,
Alabama, for grains that exhibit evidence for having a shock
history. The transitional section has been described
previously by others [1-3] and shown to have small multiple
Ir anomalies in the tens of parts per trillion (ppt) range
[4]. The location of the Ir anomalies in the Braggs section
at the bottom of Anomaly 29 Normal paleomagnetic zone
(single Ir peak) and in Anomaly 29 Reversed paleomagnetic
zone (multiple Ir peaks) guided our search to Bed 10. The
combination of the Braggs, Alabama and Brazos River, Texas
[5] KT boundary sites is interesting from the standpoint of
their being approximately equidistant (~1000 km) from the
proposed KT impact feature at Chicxulub, Yucatan, but placed
on opposite sides of the inland sea that existed at that
period.

The Braggs' KT outcrop was located using Copeland and
Mancini's [2] paper. Beds were identified from the locations
and lithological descriptions that are stated in the
document. All the samples were collected using steel geology
picks, chisels, and spades. Descending vertical trenches
were excavated for the purpose of obtaining samples from
Beds 2, 4, 6, and 10. Trenching allowed the sampling of the
beds from top to bottom. The samples were stored in plastic
bags until they could be transferred into glass jars.
Depending upon the type and amount of available sample, 10-
300 g were extracted from the bulk mix of each specimen for
examination. Approximately 20 ml of HCl acid was added to
the samples to dissolve carbonate material. Afterwards the
samples were rinsed with de-ionized water and air dried. The
samples were wet sieved utilizing a 125-micrometer-sized
sieve (No. 230). The >125-micrometer particulate was air
dried and dry sieved with the following sieve sizes: 1
millimeter, 500 micrometers, 250 micrometers, and 125
micrometers.

During the first stage of grain selection, a
stereomicroscope was used to view the sample in a petri dish
that was placed between crossed polarizers. For samples 2-H
and 2-G and Beds 3 through 5, the stereoscope functioned
well. Quartz grains from the >1-millimeter and >500-
micrometer-sized samples were easily seen and easy to
remove. However, the increased amount of glauconite-filled
foraminifera that is present in higher beds such as Bed 10,
>250-micrometer samples, affected the locating and
extraction of quartz grains. An Olympus CH polarizing light
microscope was used to view the >250-micrometer-sized
particulates spread across a wide, flat glass slide. The 20X
objective was beneficial in observing details of small
grains as well as assisting in the removal of specific
grains. Even with the increased magnification, the amount of
glauconite, usually 50% of the processed sample, and the
particulate size prevented, easy observation and isolation
of quartz grains in the >125-micrometer material.

While selecting possible shocked grains, those exhibiting
obvious albite and polysynthetic twinning were avoided since
they are characteristics of feldspar. Grains containing
inclusions, suspected planar elements, and unusual features
were isolated and mounted in refractive index oil, 1.544.
They were examined with the Olympus CH, and petrological
characteristics such as Beckie lines, sign of elongation,
interference figure, and morphology were noted. Next,
details of the grains were observed with a video camera
attached to an optical microscope and coupled to a
television monitor. Use of a video system allowed contrast
to be manipulated when viewing the grains. A stage
micrometer was used to establish a scale for the 10X and 43X
microscope objectives. Grains were photographed from the
monitor using a Polaroid camera.

Optical signatures of individual grains having experienced a
shock environment were searched for. These signatures
included lamellar features, decorated planar elements, and
in particular, multiple occurrences of these features in a
grain. Those grains found by optical microscopy to have
characteristics suggesting candidacy for having been shocked
were rare (<0.1%) among all grains examined. None of these
candidate grains exhibited immediately evident features such
as those often pictured in the literature (e.g., Bohor,
Modreski, and Foord) [6]. Rather, they usually presented an
impression of decorated multiple planar elements and in one
case multiple lamellae in conjunction with decorated planar
elements (Fig. 1.). The paucity of readily evident shocked
grains in our samples from the Braggs, Alabama site is
similar to the experience of others who have searched for
such grains at the Brazos River, Texas locations [7].

We have utilized X-ray diffraction to examine some of the
most provocative grains for evidence of shock metamorphism
as has been done by others [6]. This includes Laue and
Debye-Scherrer techniques with a copper target source, and
Laue diffraction using a synchrotron source. Figure 2 shows
the Debye-Scherrer pattern, while Fig. 3 is a synchrotron
Laue pattern of the grain considered to be our best
candidate for exhibiting multiple lamellae most like those
seen in the literature. This is the grain also shown in Fig.
1. These diffraction patterns indicate the grain does not
have the ordered structure of a grain of quartz that has not
been shocked. In the Debye-Scherrer pattern there is
apparent asterism similar to that shown for KT grains by
others [6]. The synchrotron Laue pattern shows clearly what
the Debye-Scherrer pattern suggests, i.e., the asterism is
composed of a multiplicity of diffraction spots. Our X-ray
diffraction patterns of this grain as well as another
candidate grain indicate them to be highly disordered, which
is consistent with their optical appearance, and possible
shock metamorphism.

References: [1] Worsley T. (1974) in Soc. Econ Paleontol.
Mineral. Spec. Publ., 20, 94-125. [2] Copeland C. W. and
Mancini E. A. (1986) in GSA, Centennial Field Guide, 6, 369-
372. [3] Jones D. et al. (1987) Geology, 15, 311-315. [4]
Asaro F. (1991) personal communication. [5] Bourgeois J. et
al. (1988) Science, 241, 567-570. [6] Bohor B. F. et al.
(1987) Science, 236, 705-709. [7] Bohor B. F. (1992)
personal communication.

Fig. 1, which appears here in the hard copy, shows an
optical image using a video microscope of a grain from Bed
10 showing multiple (chevron) lamellae suggesting it has
been shocked. The long dimension ofthe image is 380
micrometers.

Fig. 2, which appears here in the hard copy, show the Debye-
Scherrer X-ray diffraction pattern of the grain shown in
Fig. 1. The asterism (streaking) of the pattern is
characteristic of shocked grains.

Fig. 3, which appears here in the hard copy, shows the
synchrotron Laue X-ray diffraction pattern of the grain
shown in Fig. 1. The asterism can be seen to be caused by a
multiplicity of diffraction spots that in turn are the
product of a disturbed lattice structure in a single grain.



Habib D.
   Biostratigraphic Evidence of the KT Boundary in the Eastern Gulf Coastal
   Plain North of the Chicxulub Crater

The Cretaceous Tertiary (KT) boundary in Alabama is
currently being restudied, in light of the proximity of this
area to the Chicxulub impact crater in the Yucatan Peninsula
of Mexico [1,2]. Previous studies have suggested that there
is an apparent discordance between the position of the KT
boundary and the stratigraphy based on the Ir anomaly
pattern [3], paleomagnetic reversals [4], and the major
biotic extinction event [5]. The purpose of this paper is to
present the biostratigraphic evidence from dinoflagellate
cysts [6], calcareous nannofossils [7], and planktonic
foraminifera [8] to show that the mass extinction datum is
situated precisely at the KT boundary in Alabama and that
the low values of Ir peaks in the sediments directly above
the boundary resulted from the effects of the rise of the
Ir-enriched ocean in the earliest Danian. The
biostratigraphic evidence indicates further that the hiatus
at the boundary was of short duration and could have
resulted from asteroid impact. The presence of the Micula
prinsii nannofossil zone in the paleomagnetically unstable
interval directly beneath the KT boundary indicates an
equivalence with the earliest part of Polarity Chron C29R.
The contact between C29R and the well-developed C30N at this
level supports the latest Maestrichtian age assignment
adjacent to the KT boundary.

In central and western Alabama, the latest Maestrichtian is
represented by a prograding succession of carbonaceous marls
and sandy mudstones. This interval is succeeded by a minor
disconformity at the KT boundary, cut by shallow channels
containing parallel-laminated crossbedded sand deposits
approximately 0.8m thickat Mussel Creek to pebbles and
boulders of reworked Maestrichtian marls in deposits
approximately 1.9m thick at Moscow Landing. The channel
sands contain nannofossils in the NP1 zone, foraminifera in
the PO zone, and dinocysts in the Senoniasphaera inornata
and Danea californica dinoflagellate zones which,
collectively, present convincing evidence of the earliest
Tertiary age directly above the disconformity. The top of
the sandy muds of latest Maestrichtian age represents the
severe and abrupt extinction of nannoplankton, foraminifera,
gastropods, bivalves, and cephalods, many of which revealed
high diversity of taxa with widespread and long-ranging
clades in the Maestrichtian [5,6]. Paleoshelf deposits lie
directly on the channels. In the Braggs area, the first
shelf deposit is a highly argillaceous limestone, which
contains a rubble zone at its base, suggesting a higher
energy depositional environment.
The Ir anomaly pattern [3] resulted from depositional
processes associated with the Ir-enriched ocean that covered
the area during the first Tertiary episode of sea level rise
[6]. Figure 1 illustrates the correlation between
dinoflagellate species richness, amorphous organic matter of
fecal origin, and Ir peaks. The Ir peaks occur in the tops
of limestone beds [3,5,9]. They are restricted to the
retrograding transgressive interval of the Transgressive
Systems Tract. The overlying condensed section and
prograding Highstand Systems Tract show no anomalous
occurrence of Ir. Dinoflagellate species peak in the
amorphous debris-rich mudstones directly above the Ir-
enriched limestones in the transgressive interval. This
organic matter may have played a role in concentrating the
Ir [9]. The shallow channel deposits may represent a short
episode of rapid erosion related to asteroid impact. The
enrichment of Ir in the world ocean at the KT boundary could
have deposited this material in neritic sediments during the
earliest Tertiary rise of sea level.
References: [1] Hildebrand A. et al. (1991) Geology, 19,
867-871. [2] Sharpton V. L. et al. (1993) Science, 261,
1564-1567. [3] Donovan A. D. et al. (1988) SEPM, Sp. Publ.
42, 299-307. [4] Channel J. E. T. and Dobson J. P. (1989)
Palaeogeogr. Palaeoclimatol. Palaeoecol., 69, 267-277. [5]
Bryan J. R. and Jones D. S. (1989) Palaeogeogr.
Palaeoclimatol. Palaeoecol., 69, 279-301. [6] Habib D. et
al. (1992) Geology, 20, 165-168. [7] Moshkovitz S. and Habib
D. (1993) Micropaleontol., 39, 167-191. [8] Olsson R. K. and
Liu C. (1993) Palaios., 8, 127-139. [9] Schmitz B. (1988)
Geology, 16, 1068-1072.

Fig. 1. Correlation between dinoflagellate species richness
peaks, Ir peaks, and amorphous debris (a) in the Braggs
roadcut section. Amorphous debris concentrated at the base
of each parasequence (ps). Ir peaks restricted to the
interval of beds[3-9] within the Transgressive Systems
Tract.


Beeson D.   Gartner S.   Keller G.   MacLeod N.   Medus J.   Rocchia R.
Robin E.
   The KT Boundary Along the Brazos River, Falls County, Texas:
   Multidisciplinary Stratigraphy and Depositional Environment

The recent suggestion that impact-generated megawave
deposits, turbidites, or gravity flows are present in the
Gulf of Mexico and northeastern Mexico [1-3] has renewed
interest in the Cretaceous/Tertiary (KT) boundary sections
along the Brazos River in Falls County, Texas. These
sections contain the best-preserved marine and terrestrial
microfossil assemblages within the region. These fossils
also reside within what some workers believe to be a nearly
continuous temporal record of the KT transition [4].
Therefore, it is important that the stratigraphic
controversies presently surrounding these Texas boundary
sections be fully addressed so that the data recorded at
Brazos River can be applied to regional KT boundary
problems.

Prior to this study placement of the KT boundary and
interpretation of the complex coarse clastic unit (herein
termed the "event deposit") within these sections has been a
subject of disagreement. For instance, in the principle
Brazos section (Brazos 1), the boundary has been
alternatively placed at the uncomformable base of the event
deposit [5,6], within the limestone at the top of the event
deposit [7], and 17 to 22 centimeters above the event
deposit [8-13; see Fig. 1]. In addition, several researchers
have argued that no uppermost Maastrichtian and lowermost
Danian sediments are present in these sequences [14,15,16].
The event deposit itself has also been variously interpreted
as an impact-generated tsunami deposit [2,17], an earlier
storm or sea-level lowstand deposit [10], and multiple storm
events [18].

Integrated biostratigraphic (planktic foraminifera,
nannofossils, and palynomorphs) and chemostratigraphic (Ir,
stable isotopes) studies of over 550 samples from the Brazos
River sections and cores place the KT boundary less than 22
centimeters above the top of the event deposit; in agreement
with the earlier studies of Maddocks [8], Jiang and Gartner
[9], and Keller [10]. A well-defined zone of Ir enrichment
is coincident with decreased in delta^13C values and with
the first appearance of Tertiary planktic foraminifera at
these localities, thus confirming their correlation with the
boundary stratotype at El Kef, Tunisia. Graphic correlation
of Brazos River micropaleontological data with corresponding
data from the El Kef stratotype section, and with a KT
composite standard reference section [4] suggests that
sediment accumulation was, within the present limits of
sampling, continuous across the boundary in these sequences.

These microfossil data provide no evidence that could
temporally link the underlying clastic event deposit with
the KT boundary bioevent. Distinct faunal and floral changes
suggest that this coarse clastic deposit represents an
independent Maastrichtian event. Benthic foraminiferal and
palynomorph data indicate that, during the latest
Maastrichtian and earliest Danian, deposition occurred in a
shallow, inner neritic environment. Palynomorph data
indicate that during the latest Maastrichtian sea-level
lowstand water depths may have been as little as 10 meters
in this area. This latest Maastrichtian lowstand is followed
by a rapidly rising sea-level across the KT boundary with
water depths becoming no greater than 70 meters by the
earliest Danian.

If the Brazos River event deposit is interpreted to be
impact-related [see 1,2,17] then the 17 to 22 centimeter
claystone interval between the top of this deposit and the
KT boundary must be regarded as part of this short-term
event (e.g., fine fraction sediments settling out of the
water column). But, neither paleontological nor
sedimentological data supports this interpretation. There is
no evidence of upward fining of these sediments or of the
foraminifera or palynomorphs that they enclose. Moreover,
the major planktic foraminiferal and calcareous
nannoplankton changes do not occur until well after the KT
boundary whereas changes in the benthic foraminiferal fauna
occur in pre-boundary Cretaceous sediments. Based on the
palynomorph evidence, terrestrial floras do exhibit some
periods of moderately rapid compositional change across the
KT boundary. But the most punitive change in palynomorph
composition occurs above the boundary; coeval with marine
plankton extinctions.

These data do not support a causal link between the local KT
boundary (defined by multiple criteria) and the underlying
event deposit. In addition, the Brazos River event deposit
contains no shocked quartz, microtektite-like glass
spherules, Ir, or Ni-rich spinels that would indicate a
genetic relationship between it and a bolide impact event.
The Ir enrichment that does suggest such a connection
coincides with KT boundary that lies 17 to 22 centimeters
above this clastic event deposit. The most likely
explanation for the event deposit in the Brazos River
sequences is that its deposition is related to the uppermost
Maastrichtian sea-level lowstand [19,20] with either local
or regional storm systems representing the generative
mechanism for the emplacement of coarse sediment. A local
sequence of hummocky cross-strata overlain by wave ripples
is repeated in some outcrops of the event deposit. Given the
independently-confirmed shallow water depth present in this
area during the uppermost Maastrichtian, such features are
much more readily compatible with storm-related depositional
patterns than with tsunami or megawave mechanisms that would
be expected to resulted in net erosion, rather than
deposition, in such nearshore settings.

References [1] Alvarez et al. (1992) Geology, 20, 697-700.
[2] Hidlebrand A. R. (1992) Unpublished Ph.D. thesis,
University of Arizona, 358. [3] Stinnesbeck W. et al. (1993)
Geology, 21, 797-800. [4] MacLeod N. and Keller G. (1991)
Bull. Geol. Soc. Am., 103, 1439-1457. [5] Hansen T. A.
(1984) AAPG-SEPM Annual Meeting Field Trip Guide, 21-36. [6]
Hansen T. A. (1987) Cretaceous Research, 8, 229-252. [7]
Wrenn J. H. et al. (1989) Palynology, 13, 288-289. [8]
Maddocks R. F. (1984) GSA, 16, A106. [9] Jiang M. J. and
Gartner S. (1986) Micropaleontology, 32, 232-255. [10]
Keller G. (1989) Paleoceanography, 4, 282-332. [11] Hansen
T. A. and Upshaw B. (1990) Extinction Events in Earth
History, Springer Verlag, 402-409. [12] Medus J. (1992)
Archs. Sci. Geneve, 45, 127-133. [13] Beeson D. (1992)
Unpublished Ph.D. thesis, Penn. State Univ., 341. [14] Smith
C. A. and Pessagno E. A. Jr. (1973) Cushman Found. Foram.
Res. Special Pub., 12, 68. [15] Pessagno E. A. Jr. et al.
(1990) GSA, 22, A627. [16] Montgomery H. et al. (1992) EPSL,
109, 593-600. [17] Bourgeois J. et al. (1990) Science, 241,
567-570. [18] Hansen T. A. and Kauffman E. G. (1986) GSA,
18, A627. [19] Donovan et al. (1988) in SEPM Special Pub. 42
(C. K. Wilgus et al. eds.), 299-307. [20] Habib et al.
(1992) Geology, 20, 165-168.

Fig. 1. Placement of the KT boundary in the Brazos 1 section
showing lithostratigraphic and chemostratigraphic data along
with biostratigraphic interpretations of various authors.
Hachured segments represent degree of biostratigraphic
uncertainty due to discontinuous sampling. Dashed "boundary"
interpretations are coincident with the unconformity at the
base of the event deposit.


Rocchia R.   Froget L.   Robin E.   Gayraud J.
   Stratigraphical Distributions of Cosmic Markers at the KT Boundary in the
   Caribbean Area

Introduction: The KT sites of the Caribbean area are
markedly distinct from the sites found in the rest of the
world. In this region, the KT sedimentary sequence is rather
complex and often contains lithological units which are
either absent or less pronounced elsewhere. These units,
located at the base of the KT sequence, are a bed of
spherules or coarse globules overlain by a massive sandstone
bed ending as fine grained ripple bed. Diverging
explanations have been proposed for this sedimentary
sequence. The sandstone layer could represent a series of
turbidites over 10^5 years or so without genetic link with
the KT event [1]. Another explanation is that the sandstone
was produced by a tsunami generated by a closeby impact [2].
In that hypothesis, the spherule bed is interpreted as the
remain of a tektite layer produced by the impact, which is
now supposed to have taken place in the Yucatan peninsula.
Then, the entire sequence, including the ripple beds and
marly layers overlying the sandstone, should represent a
deposition time not exceeding a few hours or at most a few
days. Clearly, the evaluation of the timing of the KT
transition sequence is critical. The position of the
chemical and mineralogical ingredients directly produced by
the impact [PGE overabundances, Ni-rich (meteoric) spinels
and shocked quartz] are powerful tools to evaluate with a
fine time resolution the formation of the sequence. We
report here about meteoric spinels and Ir. Spinels are very
important for timing considerations because they were
originally locked in rather big bodies (up to several
hundreds microns) which sedimented without delay. Therefore,
they are well adapted to record a rapid event.

Sites: We report about four very distant sites located
around, inside and outside the gulf of Mexico: Beloc
(Haiti), Membral (Mexico), Brazos River section (Texas) and
Hole 536 (Gulf of Mexico).

Iridium and Spinels Distributions: Beloc (Haiti). Results
about this section have already been published [3]. In this
section, formed under about 2000 m of water, the sandstone
is missing, probably because the site is out of the Gulf of
Mexico. The spherule bed (25-30 cm thick) seems to be nicely
size-graded. Ir and spinels appear significantly higher up
section, 46-63 cm above the base of the spherule bed and
separated from this bed by 20-30 cm of carbonated sediments.
At Beloc, the cosmic markers and the spherules are not
mixed: they are located in stratigraphically distinct
layers. Moreover, the glass remains contained in the
spherule bed differ significantly from tektites. Presently,
on stratigraphical and chemical grounds, there is no
evidence for a genetic link between spherules and the KT
cosmic markers. However, the interval of time separating the
sedimentation of spherules and Ir-spinels is not necessarily
long. The size grading observed in the spherule bed, likely
due to the size-dependant transit time through 2000 m of
water, suggests that this part of the sequence at least was
deposited rapidly. The fact that spinel-bearing spheroids of
the same size as the smallest spherules (0.2 mm) are found
in well separated layers implies a significant delay of
several tens of hours or days but not necessarily more.

Membral (Mexico). This section is typical of the KT sequence
observed in several outcrops around the Gulf of Mexico. The
sandstone is a 2-3 meter thick massive bed. Here again the
cosmic markers, Ir and spinels, are stratigraphically
separated from the spherule bed (contrarily to what was
published [2], spinels found in the spherule layer are not
meteoric spinels but terrestrial alumino- and titano-
chromites similar to those found all over the section). They
appear in the sedimentary sequence on top of the sandstone
bed in the 20-30 cm thick series of ripple beds. Both Ir and
spinel concentrations (maximum values: 0.5 ng/g and 300
spinel/g respectively) are low compared with other distant
sections (9-28 ng/g and 30000 spinels/g at Beloc for
instance). The integrated Ir (15 ng/cm^2) and spinel fluxes
(3 10^3 spinels/cm^2 compared with 10^5 spinels/cm^2 at
Caravaca) are also very low. The maximum spinel
concentration is found a few centimeters below the maximum
Ir abundance. These features show that part of the cosmic
infall is missing in Membral.

Brazos river (site 1-Texas). In this section, we have not
found any meteoric spinels. Ir overabundances are observed
only above the presumed tsunami deposit with a maximum
concentration of ng/g 25 cm above the ripple bed. In the
absence of meteoric spinels the KT boundary should be placed
at this level, consistent with paleontological data [4]. The
time represented by this 25 cm interval is uncertain.

ODP Hole 536 (Gulf of Mexico). In this core, the spherule
and sandstone beds are missing. Ir and spinels are found
mainly in a calcareous ooze containing cretaceous fossils
(section 536-9-5, 75 to 120 cm), the unit 4 of Alvarez et
al.[5]. Ir concentrations are low (max: 0.7 ng/g) but the
integrated flux is rather elevated: 55 ng/cm^2 and
comparable with the global average. Contrarily to what has
been published [6] spinel concentrations (15000 spinels/g)
and flux (10^5 spinels/cm^2) are also high. Alvarez et al.
[5] suggested that this unit represents the finest fraction
of cretaceous sediments stirred up by a KT impact wave. If
so, it means that most of the Ir and spinel bearing
particles sedimented at the very end of this redeposition
phase. However, this deposition mode is questioned by the
recent work of Keller et al. [6] who claim that both small-
and large-sized species are present in the  Whatever, the
exact explanation for this sequence could be, it remains
that Ir and spinels are found isolated in the sediment and
not mixed with globules. The most puzzling thing is that
sediments do not contain any globule or trace of tsunami-
turbidite when the record of the cosmic event (Ir and
spinels) is apparently complete in reworked sediments.

Discussion and Conclusion: The four sections studied in this
report, even if they differ on some points, have in common
the fact that the different markers of the KT boundary
transition (smectite-glass spherules, sandstone bed, cosmic
markers) are systematically found unmixed in very distinct
lithological units. In particular, the spherule-glass layer
is never found mixed with Ir and meteoric spinels. This is
rather strange if we consider the expected effects of giant
tsunami waves produced by an impact in the Yucatan
peninsula.

Two possibilities can be envisaged: (1) The glass globules
have a volcanic origin and are unrelated with the KT
boundary cosmic event. This would explain why the Fe3+/Fe2+
ratio of the glass remains is so high (0.7) compared with
the ratio measured in known tektites (<<0.1) [3] and why
they do not contain lechatelierite. (2)The glass remains are
relics of unusual tektites. Then, the non mixing of globules
and spinels can be explained by two distinct bolides
colliding the Earth over a rather short period of time. This
has already been proposed to explain the traces of erosion
that are observed at the interface between the boundary clay
(kaolinite due to a first impact) layer and the fireball
layer (second impact) in continental deposits of North
America [8]. Some paleobotanical data could also be
explained in that way [9]. However, the occurrence of two
exceptional and independant events is unlikely on
statistical grounds.

So, if we consider the case of a single impact, the non-
mixing of globules and spinels could result from the
fragmentation (not vaporization) of the bolide during the
collision with injection of big fragments on rather distant
orbits on which they can stay for a few days. This kind of
rebound, or ricochet, is expected to occur in oblique
collisions [10]. In addition, this multiple objects
dispersion mode could also account for the compositional
variations of spinels observed from sites to sites [7]. A
way to understand the KT sequence in the Caribbean region is
to study the distribution of shocked quartz with respect to
other KT markers.

References: [1] Stinnesbeck W. et al. (1993) Geology, 21,
797-800. [2] Smit J. et al. (1992) Geology, 20, 99-104. [3]
Jihanno C. et al. (1992) EPSL, 109, 229-241. [4] Beeson D.
et al. (1993). [5] Alvarez W. et al. (1992) Geology, 20,
697-700. [6] Keller G. et al. (1993) Geology, 21, 776-780.
[7] Robin E. et al., (1994), this volume. [8] Shoemaker et
al. (1992)  LPS XIII, 1293-1294. [9] Wolfe J. A. (1991)
Nature, 352, 420-422. [10] Schultz P. H. et al. (1990) GSA
Spec. Paper, 247, 239-261.


Smit J.   Roep T. B.   Alvarez W.   Montanari S.   Claeys P.
   Stratigraphy and Sedimentology of KT Clastic Beds in the Moscow Landing
   (Alabama) Outcrop: Evidence for Impact Related Earthquakes and Tsunamis

Around the Gulf of Mexico, over more than 2500 km from
Alabama to Guatamala, the KT boundary is marked by highly
unusual clastic beds. These beds have earlier been
interpreted as tsunami generated deposits [1,2], storm
deposits, turbidites and debris flows [3], fluvial
conglomerates, or sea-level low-stand deposits [4].

Sedimentologic analysis of the clastic beds at Moscow
Landing, Alabama, Brazos, Texas, and los Ramones, Rancho
Nuevo, Sierrita-Las Rusias, Penon, Porvenir, Mulato-
Bruselas, La Lajilla, Mesa de Llera, Mimbral, La Ceiba, and
possibly Bochil, all in eastern Mexico, show that the
buildup of the clastic beds follows a consistent pattern,
well recognizable all over the Gulf, but modified by local
variables, such as waterdepth, distance from shoreline, and
eustatic sealevel changes. The depositional pattern
comprises a first unit (1), consisting of ejecta reworked
with local material in shallow channels, which usually sink
down into the soft seafloor. Unit (2) is an upward grading
complex series of pulses of coarse sands, containing
distinctly more quartz sand, repeatedly backwashed from
nearshore areas. Unit (3) consists of fine sandstone
ripples, alternating with fine silts, grading into a
siltstone/mudstone top (unit 4). The grainsize decrease in
the top mudstone [2] at Brazos River, containing the Ir rich
dust, shows that the settling of the mudstone on the
seafloor takes a few hours to days, depending on the assumed
thickness of the suspension cloud.

The easternmost outcrop studied is Moscow landing, Alabama.
The KT clastic beds here are known as Clayton basal sands
and interpreted [4] as early Danian lowstand channel
deposits, topped by a transgressive surface in sequence
stratigraphy terms, as evidenced by a bored hardground.
However, the situation is clearly more complex. In some
sections the transgressive surface erodes down to the
Cretaceous Prairie bluff chalk (Braggs). It is possible that
a few lowstand channels are preserved (Mussel creek) and
filled with Clayton sands but most of the basal Clayton
channels should be reinterpreted as tsunami-generated
deposits, preceding the early Danian lowstand, because of
the presence of altered tektites identical to the tektites
at other Gulfcoast KT sections. The Moscow landing outcrops
combine all three (near) KT phenomena. Over most of the
outcrop, the bored transgressive surface lies disconformably
on the Prairie bluff chalk. We interpret most 'channels'
(actually triangular wedges) to be filled with tsunami
generated deposits. (Fig. 1). Large blocks (>800 m) of the
Prairie bluff chalk bounded by small faults are tilted to
the S. However most of these faults do not offset the early
Danian transgressive surface. The top of the Prairie bluff
chalk is liquified and slumped but only near the faults. The
fault movements also plastically deform bedding planes of
the basalmost Clayton sediments thus before they were
lithified. In some of the faults Clayton sands have been
injected. A second coarse (backwash) infilling cuts the
tilted bedding planes.

Thus fault movements (listric faulting) just precede and
continue through deposition of the basal part of the clastic
beds. Initially, a triangular seafloor depression is created
by the listric faulting on the downthrown side (the southern
end) of the tilted blocks. The first sediment to fill the
depression is a conglomeratic mixture of locally derived
chalk clasts, shells from the seafloor, tektite like
spherules, and relatively little coarse sand. These
conglomerates sink down into the soft seafloor oozes. We
interpret these basal sequences in the light of the arrival
of impact induced seismic waves causing sliding and slumping
(like in Guatamala) and simultaneous (or shortly followed
by) arrival of ejecta. A few hours later the first tsunami
waves arrive, eroding the seafloor and reworking the ejecta
and fossils from the seafloor into the triangular
depressions (unit 1). The next sediments to arrive are
pulses of fossiliferous conglomeratic and coarse sands,
still deformed near the faults. Similar sand pulses continue
and erode the (oversteepened) first sands. These (unit 2)
conglomeratic sands are probably backwashed from near-shore
areas and we think it even possible that all the sands from
the basal Clayton are repeatedly reworked backwash sands.

At Moscow landing the finer clastic topbeds (units 3-4), as
known from Brazos river and Mexico are lacking due to
truncation by the bored transgressive surface with the first
Danian fossils. This transgressive surface erodes and
truncates the Prairie bluff chalks, the faults, and the
above (unit 1,2) sequences in the channels. A few symmetric
channels may belong to the lowstand phase preceding the
transgressive surface. This awaits biostratigraphic testing
because they might contain Danian fossils, while tsunami
generated sediment should contain exclusively reworked
Cretaceous biota.

Fig. 1. Interpretation of sequence of events at KT boundary
time at Moscow landing, Alabama. The wrinkled line is a
phosphatic hardground, 1-2 m below the KT boundary, that has
not been liquified. Vertical depth exaggerated. Depth of sea
not to scale.

[1] Smit J. and Romein A. J. T.(1985) EPSL, 74, 155-170. [2]
Bourgeois et al. (1988) Science, 241, 567-570. [3]
Stinnesbeck W. et al. (1993) Geology, 21, 797-300. [4]
Mancini E. A. et al. (1989) J. of For. Res., 19, 93-104.


Smit J.   Alvarez W.   Claeys P.   Montanari S.   Roep Th. B.
   Misunderstandings Regarding the KT Boundary Deposits in the Gulf of Mexico

Some recent papers [1,2] suggest that (1) the KT boundary in
the Gulf of Mexico is not marked by unusual tsunami
generated clastic deposits, (2) clastic deposits, where
present, pre-date the KT boundary, and (3) are normal
debris-flow or turbidite deposits representing an extended
amount of time, not related to impact?

These suggestions however, are based on a few
misconceptions, which seriously hamper a clear view on the
KT boundary problem.

The definition of the KT boundary used in the literature [3]
is often not unequivocal. In [3] the KT boundary is said to
be at either (1) the first occurrence of Paleocene
(planktic) species or (2) at the mass-extinction of
Cretaceous biota. I will demonstrate below that these two
definitions are not synchronous and may differ by as much as
50,000 years. In practice, in non-extremely high resolution
studies, that is not a problem. However, in particular in KT
boundary sections around the Gulf coast, with thick, up to
12 m thick rapidly deposited clastic beds at the KT
boundary, the ambiguous definition leads to misplacement of
the KT boundary [2,4].

The KT boundary Stratotype and Point (GSSP) is located in
the Kef section in Tunisia and is defined at the base of the
so-called boundary clay, which includes a thin (red) clay
layer rich in Ir, spherules, and shocked quartz. This
decision of the KT working group of the International
Commission of Stratigraphy has formally been ratified by
IUGS [5]. The KT boundary in Kef is at a sharp lithologic
discontinuity between hemipelagic Upper Cretaceous marls and
a claylayer. The discontinuity coincides with an abrupt
decrease in abundance of planktic foraminifera and a
relative increase of survivor species (e.g., G. cretacea).
The KT boundary is however, not coincident with the first
appearance of new Paleocene species. These appear between 10
and 25 cm (representing > 5000 years) higher up in the
section at the transition of the so-called boundary clay and
hemipelagic marl. This clay interval is also coincident with
the P0 zone, a biozone that is defined [6] as the zone
between the mass-disappearance of Cretaceous foraminifera
and the first appearance of new Paleocene species.
Subsequent redefinition [7] of P0, including first
appearance of G. fringa and G. minutula (~G. conusa) in an
upper P0b zone, further confuses the stratigraphy and should
be rejected. Correlation of KT boundary strata from Kef to
the Gulf coast can be problematic when the sedimentology of
the KT (clastic) beds is not correctly interpreted.
Biostratigraphic correlation is also hampered by the
frequent use of relative abundance plots. These plots do not
discriminate between in situ and reworked species if they
are not corrected (normalized) for the abundance of
foraminifera and sedimentation rates [8]. These plots create
the illusion of large scale survivorship, the illusion of
thriving planktic populations surviving the KT event, while
in reality planktic populations decrease by orders of
magnitude.

Correlation can also be achieved by means of using the Ir
anomaly, spherules, and shocked quartz. All three of these
are present in both the Kef and the Gulf coast KT sections.
The red layer in Kef (and elsewhere) is easiest to explain
as the distal ejecta of a large impact event (presumably
Chicxulub). Using this assumption, everything in the red
clay should be equivalent to entire ejecta sequence in Gulf
coast sections, including the basal tektites in the clastic
beds, which have been found now in all the KT sections
around the Gulf coast. (Moscow landing, Snow creek, Lynn
creek in Alabama, in Darting minnows creek [9] in Brazos,
and all the clastic bed sequences in Mexico, including the
new KT section in La Ceiba, near Poza Rica). Because the KT
boundary is defined below the ejecta sequence in Kef, it
should be defined below the ejecta-rich clastic beds in the
Gulf coast.

The Brazos river sections do illustrate this dilemma best
because there is abundant literature on these sections.
Sometimes the KT boundary is placed above and sometimes at
the base of the clastic beds. Recently we found abundant
tektite-like spherules in the basal coarse grained channels
of the clastic beds in the Darting Minnows creek outcrop,
also containing glauconite and shell hash. The coarse basal
channels grade into fine sandstone with climbing ripples
(not wave ripples) with bidirectional current indications.
The grading may repeat 2-3 times. At Brazos and in all other
Gulf coast clastic KT beds these sequences were deposited in
one sedimentary event, estimated to last between several
hours to a few days. The top sandstone ripples are overlain
by 2 cm silt, 8 cm limestone, and 20 cm mudstone,
respectively. The first new Paleocene biota appear at the
top of the mudstone. We have analyzed the grain-size
distribution of the insoluble residue of the lime-mudstone
interval with a Fritsch A-22 high resolution laser particle
sizer and the gradual grainsize decrease (Fig. 1) clearly
shows that the lime-mudstone is the result of settling out
of silt and clay from a thick suspension cloud, apparently
settling together with the Ir from the vaporized bolide.
Assuming a suspension cloud thickness (Table 1) of 50 m,
this means that the first 20 cm of the mudstone has been
deposited in a few days at most and the remainder presumably
in a few weeks. The clastic bed and graded mudstone top
belong to one sedimentary event lasting between 1 and 4
days. This duration is compatible with tsunami wave travel
times from Chicxulub crater to Gulf coast outcrops (C =
square root of g x depth, or =3.13 x ~3000m, or ~600 km/hr)
and, including refraction and reflection, leading to
estimates from one to a few days for the duration of tsunami
wave and seiche activity in the Gulf. Estimates of prolonged
deposition [2] of the clastic beds of Gulf coast KT outcrops
are based on the assumption of hemipelagic beds within the
clastic deposits and burrowing in the clastic beds. However,
thin sections show that these so-called hemipelagic beds are
extremely silty/fine sandy, and fundamentally differ from
Mendez shales below and Velasco shales above the clastic
bed. Another argument [2] for prolonged deposition is the
occurrence of burrows.The Penon (Mexico) outcrop shows that
the burrows postdate the deposition of the latest rippled
sandstone. All foraminifera in the clastic beds and mudstone
tail are necessarily reworked and cannot be used for
biostratigraphic analysis. However, in the first
demonstrably hemipelagic strata above the clastic beds, the
abundance of Cretaceous foraminifera has already sharply
decreased, testifying to the direct relationship between the
Chicxulub impact and the extinctions at the KT boundary.

References: [1] Keller G. et al. (1993) Geology, 21, 776-
780. [2] Stinnesbeck W. et al. (1993) Geology, 21, 797-300.
[3] Keller G. (1989) GSA Bull., 101, 1408-1419. [4] Olsson
R. K. and Liu C. (1993) Palaios. [5] Cowie J. W. et al.
(1989) Episodes 112, 79-83. [6] Smit J. (1982) GSA Special
Paper, 190, 329-352. [7] Keller G. (1988) Marine
Micropaleontology, 13, 239-263. [8] Keller G. (1989) 101,
1408-1419. [9] Bourgeois J. et al. (1988) Science, 241, 567-
570.


Kruge M. A.   Crelling J. C.   Montanari A.   Stankiewicz B. A.   Bensley D. F.
   Characterization of Organic Material in KT Boundary Strata, Arroyo El
   Mimbral, Mexico

If the Cretaceous Period closed with a bolide impact [1],
evidence of the resulting catastrophic effect on standing
terrestrial vegetation should be detectable in the
sedimentary record. Around 25% of terrestrial biomass was
consumed in the conflagration, according to a recent
estimate [2] and indeed soot has been detected at KT
boundary sites throughout the world [3,4]. With the
Chicxulub site in northern Yucatan now recognized as the
likely point of impact [5,6] and the discovery of megawave
deposits in the vicinity [7-9], the search for evidence of
the impact aftermath in organic sediments can be
intensified. One site in particular (Arroyo el Mimbral,
Taumalipas, Mexico) is of special interest, as its KT
sequence contains apparent megascopic plant remains [10].
The Mimbral KT deposit consists of 3 m of coarse clastics,
interrupting a pelagic marl sequence [10]. Organic material
(OM) was isolated by acid digestion (HCl/HF) and floatation
from 6 Mimbral samples. While most samples yielded only
traces of OM, one sample contained 0.2% by weight, which was
sufficient for organic petrologic and geochemical analyses.

Petrographic examination in reflected white light revealed
an unusual organic assemblage containing 90% inertinite
(fossil charcoal). The inertinite particles are large,
approaching a millimeter in size in some cases, and show
extraordinary preservation of fine botanical detail. The
remaining 10% is vitrinite (diagenetically-altered, but
uncharred terrestrial plant remains). Flash pyrolysis-gas
chromatography/mass spectrometry of the OM concentrate
indicates a predominance of polyaromatic structures, often
unmethylated. These chemical features are indicative of
severe thermal alteration due to partial combustion and
therefore consistent with the petrographic recognition of
fossil charcoal.

While most sedimentary OM assemblages contain a few percent
of inertinite, it is most unusual to see inertinite as the
predominant component. It is even more unusual to find such
an occurrence within a pelagic sequence, as inertinite
originates from charring of land plant material. (Instead
fossil marine algal or bacterial remains would be expected.)
Thus the inertinite is "out of place", as are the coarse
clastic sediments. A case can be made for the transport of
the clastics by a tsunami of large magnitude, perhaps of
seismically induced. However, when the additional evidence
of the fossil charcoal (and the paucity of non-charred plant
remains) is considered, then a coincident firestorm must
also be evoked. A nearby bolide impact would provide the
mechanism to explain the origin of the Mimbral deposit.

References: [1] Alvarez L. W. et al. (1980) Science, 208,
1095-1108. [2] Ivany L. C. and Salawitch R. J. (1993)
Geology, 21, 487-490. [3] Wolbach W. S. et al. (1990) Geol.
Soc. Amer. Spec. Pap., 247, 391-400. [4] Venkatesan M. I.
and Dahl J. (1989) Nature, 338, 57-60. [5] Pope K. O. et al.
(1991) Nature, 351, 105. [6] Hildebrand A. R. et al. (1991)
Geology, 19, 867-871. [7] Bourgeois J. et al. (1988)
Science, 241, 567-570. [8] Hildebrand A. R. and Boynton W.
V. (1990) Science, 248, 843-846. [9] Alvarez W. et al.
(1992) Geology, 20, 697-700. [10] Smit J. et al. (1992)
Geology, 20, 99-103.


Bohor B. F.   Betterton W. J.
   Debris Flow/Turbidite Clastic Units at the KT Boundary, NE Mexico

Ten sites that contain clastic units enclosed in marine
calcareous rocks at the KT boundary have been discovered in
northeastern Mexico [1]; we visited two of these: Arroyo el
Mimbral [2,3] and La Lajilla [3]. The clastic sequences at
these sites consist of two distinct units; we interpret the
lower spherule-bearing bed as a debris flow, and the upper
sandstone unit as a turbidite. Both units are interpreted as
being caused by disturbances associated with the KT impact.

The lower spherule-bearing unit attains ~1 m in maximum
thickness and disconformably overlies Maastrichtian marls of
the Mendez Formation. It consists of poorly sorted and
poorly graded clayey spherules up to 3 mm in diameter in a
calcite matrix. Spherule-rich layers alternate with
spherule-poor, calcite-rich layers in almost horizontal
bedding. Numerous marl clasts of Mendez age [3] occur
throughout this bed. The basal surface of the spherule bed
marks a disconformity with the underlying Mendez marl.

Most of the clayey spherules consist of hollow
chlorite-smectite shells, representing alteration products
of palagonite rims that were observed on similar spherules
in Haiti [4]. Most of the original glass cores have been
dissolved and the central voids filled with secondary
calcite. We separated rare relict glass cores and fragments
from the spherule unit, at both Mimbral and La LaJilla, that
have compositions and colors similar to those from the Haiti
spherule beds, except that the Mexican sites also contain a
greenish-colored, hydrated glass [5]. Shocked quartz had
been reported previously from the Mimbral spherule bed [2],
but we did not find any. These beds contain pure albite
grains that are probably authigenic.

Disconformably overlying the spherule bed at Mimbral is a
massive to laminated, siliceous, calcite-cemented sandstone
unit that is ~2 m thick. The lowest part of this unit is
massive and normally graded, but it becomes horizontally
laminated upward. Wood fragments and other plant debris,
along with large mudstone rip-up clasts of Turonian age [3],
flute casts, and sole markings, are present at the base of
the massive sandstone. The laminated zone is succeeded by
wavy, rippled, and crossbedded strata; this zone is capped
by thin, parallel lamellae of siltstone and, finally, by
silty claystone (Fig. 1, left). An Ir anomaly of ~0.8-0.9
ppb has been reported from the top of the sandstone unit at
the Mimbral site [2,3]. We searched for shocked quartz in
several thousand grains from this zone at both sites, but
did not identify any. However, these quartz grains are
heavily overgrown with authigenic quartz, making recognition
of PDF difficult. We did find a single ~5 micrometer crystal
of high-Ni magnesioferrite spinel in the uppermost layer
from Mimbral. This sample also contains numerous grains of
pure albite.

Based on field and laboratory studies, we interpret the
clastic sequence at Mimbral and La Lajilla as a debris
flow/turbidite. The spherule bed was originally composed of
impact-glass spherules ejected from the putative KT impact
crater on the Yucatan peninsula. The impact nature of the
spherule bed was confirmed by the previous identification of
shocked quartz and relict impact glass. These melt-glass
spherules were deposited on a shallow shelf, probably
located NNW of the present sites in the Mendez basin [3].
Thick, unstable deposits of these spherules were mobilized
as debris flows at the mouths of canyons in the shelf/slope
by either tsunamis, earthquakes, or storms (associated with
the KT impact).

The debris flows either generated their own turbidity
currents involving unconsolidated shelf sands, as has been
calculated theoretically and observed experimentally [6], or
these sands were mobilized into turbulent flow
simultaneously with the spherules, unmixing from the latter
by a sieve mechanism operating on relative particle sizes
and flow densities. An alternative to this single-pulse
hypothesis is that of two separate episodes, wherein the
spherule-laden debris flows were separate events that
preceded the turbidity-flow units slightly in time. Arguing
against the latter dual-pulse hypothesis is the lack of
significant amounts of spherule-bearing material
incorporated in the base of the sandstone unit, which should
occur if this unit originated later as a separate turbidity
flow that scoured the top of the previously deposited debris
flow.

Either process resulted in debris flows consisting of glass
spherules that scoured and filled shallow channels in
basinal Mendez marls, followed closely by sand in turbid
suspension that was deposited on top of the spherule beds in
the neoformed channels. This model resembles the modern
example of upper Pleistocene chaotic silt beds transported
by debris flows (analogous to the spherule beds) and
directly overlain by turbidite sand beds in distal,
channelized, outer-fan lobes of the Mississippi delta in the
Gulf of Mexico [7]. As in this example, the presence of
relatively undeformed clasts in the Mexican spherule beds
suggests deposition in a non-turbulent, debris-flow regime.

Channel scouring by the debris flow is exemplified by the
rip-up clasts of underlying Mendez marl contained in this
bed. Also, the bed is deposited in a typical shallow-channel
profile with clearly defined edges. Both the spherule unit
and the sandstone unit contain benthic foraminifers of late
Maastrichtian age from a shallow neritic environment,
whereas benthic taxa are rare in the underlying, deep-water,
Mendez marl [3,8]. This strongly suggests that these clastic
units are allochthonous, having been transported from
shallow water to their present basinal setting.

The sandstone unit at Mimbral displays sedimentary features
similar to those that define a turbidite. For comparison,
these features have been marked as divisions of the Mimbral
sandstone unit in Fig. 1 (left), using the same nomenclature
(T(sub)a-e) that subdivides a classic turbidite sequence
(Fig. 1, right).

The final, fine-grained phase of turbidite deposition and
recommencement of the hemipelagic rain coincided with the
arrival of the last portion of the fireball layer at the
seafloor; components identifying this partial fireball layer
are an Ir anomaly and magnesioferrite spinel crystals [9].
The presence of a capping fireball layer limits the time of
formation of this clastic unit to a period of up to several
months following impact, according to our single-impact KT
model [9].

The almost instantaneous loading of a thick deposit of glass
spherules (up to 3 mm in diameter) onto a shallow shelf
constitutes a rather rare event that may, when mobilized,
produce unique deposits. For the reasons stated above, we
interpret the clastic sequences at Mimbral and La Lajilla as
spherule-bearing, debris-flow units overlain by sandstone
turbidite units. Furthermore, both of these clastic units
were mobilized on a distant shelf/slope soon after impact by
the effects of seismic or oceanic disturbances associated
with the KT impact event.

References: [l] Alvarez W. et al. (1992) GSA Absts., 24, 7,
A331. [2] Smit J. et al. (1992) Geology, 20, 99-103. [3]
Stinnesbeck W. et al. (1993) Geology, 21, 797-800. [4] Bohor
B. F. et al. (1993) LPSC XXIV, 145-146. [5] Bohor B. F. and
Betterton W. J. (1993) ibid., 143-144. [6] Hampton M. A.
(1972) J. Sed. Petrol., 42, 775-793. [7] Nelson C. H. et al.
(1992) Geology, 20, 693-696. [8] Keller G. et al. (1992) GSA
Absts., 24, A332. [9] Pollastro R. M. and Bohor B. F. (1993)
Clays, Clay Minerals, 41, 7-25. [10] Howell D. G. and
Normark W. R. (1982) in Sandstone depositional environments
(P. Scholle and D. Spearing, eds.), AAPG, 365-404.


Adatte T.   Stinnesbeck W.   Keller G.
   Mineralogical Correlations of Near KT Boundary Deposits in NE Mexico:
   Evidence for Long-term Deposition and Volcanoclastic Influence

Whole rock and clay mineral analyses of five near KT
boundary clastic deposits in NE Mexico indicate three
lithologically and mineralogically distinct and correlatable
units similar to those outlined based on lithological
correlations [1,2]. The examined sections span a NNW to SSE
transect and include: La Sierrita, El Penon, El Mulato, La
Lajilla, and El Mimbral (Fig. 1). Two sections have been
analyzed at Mimbral, the first in the channel center, the
second at its edge. Whole rock and clay-mineral compositions
have been determined by XRD, using the methods of Kubler
[3].

Unit 1, which disconformably overlies the Mendez Fm. is
lithologically defined as a poorly cemented spherule-rich
layer characterized by high calcite (50-70%, Fig. 2) and low
phyllosilicate contents (<20%), quartz and plagioclase
(albite, 5%). However, the composition of the sandy
limestone layer (S.L.L.) of unit 1 is significantly
different with lower calcite, higher quartz, and
plagioclase. This change in whole rock content indicates a
different detrital influx during deposition of these
spherule-rich sediments [1,2]. The massive laminated
sandstone of unit 2 is characterized by low calcite (20-40%)
and phyllosilicate contents and a significant increase in
quartz (up to 40%) and plagioclase (albite, 15%-25%). Whole
rock composition of unit 3 reflects the lithological
variations within this unit (including sandstone, sandy
limestone, silts and marly intervals) with high variations
in calcite (generally higher than in unit 2), quartz,
plagioclase, and phyllosilicates.

Zeolites (clinoptilolite) have been recognized near the base
and the top (below the rippled sandy limestone) of unit 3 in
all sections (Fig. 3). These zeolite occurrences are of
detrital origin and mainly present in the size fraction 2-16
micrometers. Zeolites, as well as Mg-smectites, are
generally considered an alteration product of volcanic
terranes and become abundant from Campanian to Eocene times
[4]. In the southeastern sections of El Mimbral, significant
enrichment in zeolites is also observed in the boundary
clay, the spherule-rich layer of unit 1, as well as in the
underlying marls of the Mendez Fm. [1,2]. These additional
layers enriched in zeolites, as compared to the northwestern
sections, implies an increase of volcanoclastic influx
towards the SSE, prior to and across the KT boundary.

Illite-smectite mixed layers characterized by low contents
of smectite (<30), are present in all lithologies in the La
Sierrita, El Penon, and El Mulato sections; however they
increase in the Mendez Fm., Velasco Fm., and in the silt-
marl intervals of unit 3. Random chlorite-smectite mixed
layers, characterized by higher contents in smectite layers
(>40%), are common in the spherule-rich unit 1 and in the
silt-marl intervals of unit 3; similar layers with lower
values are present in unit 2. The increase of mixed layers
in units 1 and 3 are similar to mixed layers in the Mendez
and Velasco Fms. and reflect periods of normal pelagic
sedimentation. Chlorite and mica increase in the massive
sandstone of unit 2, relative to units 1 and 2, indicating
an increase in detrital influx and probably more rapid
deposition. Chlorite and mica contents are variable in unit
3, but also increase in the sandstone layers. As already
implied by the mixed-layer distribution, unit 3 represents a
depositional environment that alternates between increased
detrital influx and rapid deposition (silt-sandstone layers)
and hemipelagic deposition (shale-marl layers). Based on
these observations we conclude:
(1) Mineralogically and lithologically distincts units,
which are correlatable over 300 km and represent variable
detrital influx alternating with normal hemipelagic
sedimentation, are incompatible with a Chicxulub impact-
generated tsunami deposit.
(2) Presence of distinct and correlatable layers enriched in
zeolites in the channel-fill clastic deposits as well as the
Mendez Fm. and Velasco Fm. indicates significant
volcanoclastic influx during these times.
(3) Dominant mixed/layer chlorite-mica assemblages, which
derive from direct and mechanical erosion of bed-rocks,
reflect tectonic instability and/or a sea-level lowstand
that increased active erosion of local bed-rock.
(4) There is no important change in clay-mineral
compositions across the KT boundary in the northeastern
Mexico sections. Kaolinite, which is abundant in the Mendez
Fm. (but absent in the KT boundary deposits) suggests a
change during the middle to Late Maastrichtian or active
erosion due to tectonic instability preventing the
development of soils enriched in kaolinite.

References: [1] Stinnesbeck W. et al. (1993) Geology, 21,
797-800. [2] Stinnesbeck W. et al. (this volume). [3] Kubler
B. (1987) Cahiers Inst. Geol. Neuchatel, Suisse Serie AX,
13. [4] Robert C. (1982) Ph.D. Thesis Fac. Science, Univ.
Marseille, France, 136.


Stinnesbeck W.   Adatte T.   Keller G.
   KT Boundary Sections in Southern Mexico (Chiapas): Implications for the
   Proposed Chicxulub Impact Site

It has been proposed that the circular gravity structure in
the subsurface of Yucatan, Mexico, represents the crater of
the KT boundary bolide impact. We studied the nearby surface
exposures of Bochil and Chilil in the Mexican state of
Chiapas in order to search for evidence, such as impact
ejecta or exotic sedimentation, that would support the
proposed Yucatan impact scenario.

The Chilil section is located 20 km northeast of San
Cristobal de las Casas and represents a transition between
the Angostura Formation of Late Cretaceous age and the
Soyalo Formation of the Paleocene. The Angostura Formation
consists of thin to medium bedded bioclastic limestones with
corals, echinoderms, large benthic foraminifera, and
rudists. This sequence is capped by 15cm of bioturbated
marly limestones with abundant early late Maastrichtian (G.
gansseri Zone) plankic foraminifera. Crusts and nodules of
Fe and Mn-oxides characterize the lithological contact to
the overlying rhythmically bedded pelagic marls and shales
of the Soyalo Formation, which contains abundant Late
Paleocene (Zone P2) foraminifera. Thus, both the lithology
and fossil assemblages indicate an extended hiatus spanning
the Upper Maastrichtian, the KT boundary and the Lower
Paleocene. This also appears to be the case in the KT
boundary sections of the Peten area in nearby Guatemala
where platform limestones with rudists discontinuously
underlie flysch-like sediments of the Paleocene. Moreover,
similar sedimentation and hiatus patterns are observed in
deep sea sections throughout the Caribbean and Gulf of
Mexico [1].

The Bochil section is located 30 km northeast of Tuxtla
Gutierrez and seems to present a record of the KT boundary
that is more complete. In this area, flysch deposition of
the Xochitlan Formation began in the Late Campanian and
continued into the Lower Tertiary, with many hundreds of
meters of rhythmically bedded shales, silts, and sandstones.
Debris flows of well-rounded conglomerates of possibly
fluviatile origin are intercalated in the lower part of the
sequence and indicate uplifting of the Chiapas Block and
erosion. Up-section occurrences of thick breccia beds of
platform carbonates and isolated olistolithic limestone
blocks suggest further tilting and collapse of the nearby
carbonate platforms. This "wildflysch" type of sedimentation
persists during most of the Maastrichtian and Paleocene.
Across the KT boundary, we observed no lithological or
sedimentological changes, other than this "wildflysch"
sedimentation, and no proximal impact ejecta or other
exotics.

Our field investigations of KT boundary sections in southern
Mexico have so far revealed no evidence of a tsunami deposit
or proximal impact ejecta related to the proposed bolide
impact on nearby Yucatan. Investigation of KT boundary age
deep-sea cores in the Caribbean and Gulf of Mexico has also
yielded no evidence of impact derived sediments [1].
Although such sediments have been earlier interpreted to be
present in DSDP sites 536 and 540 [2], these sites were
subsequently found to contain a hiatus spanning from the
Lower Danian (P. engubina Zone) through the Upper
Maastrichtian [1]. A KT boundary hiatus spanning from the
Danian through the upper Maastrichtian appears to be present
throughout the Caribbean and Gulf of Mexico and is probably
related to the latest Maastrichtian sea-level lowstand and
subsequent sediment starvation during rising sea levels. The
absence of KT boundary age deposits in deep-sea sections in
general [3] and in the Caribbean region in particular [1]
imdicates that the search for KT complete sections should
continue in shallow marine deposits.

References: [1] Keller G. et al. (1993) Geology, 21, 776-
780. [2] Alvarez W. et al. (1992) Geology, 20, 679-700. [3]
MacLeod N. and Keller G. (1991) Geology, 19, 497-501.


Montanari A.   Claeys P.   Asaro F.   Bermudez J.   Smit J.
   Preliminary Stratigraphy and Iridium and Other Geochemical Anomalies Across
   the KT Boundary in the Bochil Section (Chiapas, SE Mexico)

Coarse chaotic Cretaceous-Tertiary (KT) deposits produced by
large scale marine disturbances and megawaves have been
found in Texas [1], several localities in northeastern
Mexico [2], Haiti [3,4] and in deep sea cores in the Gulf of
Mexico [5]. All these sites surround the huge Chicxulub
crater in the State of Yucatan, Mexico. The chaotic
deposits, with a tektite-bearing layer at their base,
usually interrupt tranquil deep sea sequences of pelagic and
hemipelagic carbonate sediments. They are topped by layers
with anomalously high iridium (Ir) abundances, and
immediately underlie undisturbed sediments containing
microfossils of earliest Paleocene age and enhanced Ir
abundances tailing from the peaks. One of the best preserved
of these KT sequences is exposed in the Arroyo de Mimbral in
the State of Tamaulipas, Mexico and is located about 950 km
WNW of the center of the Chicxulub crater [2].

In the State of Chiapas (SE Mexico), a search was made for a
KT marine sequence as close as possible to the Chicxulub
crater, which might have preserved a complete, proximal
sedimentary record of a giant impact explosion. Such a
sequence was found near the town of Bochil, exposed along
the road leading to Pemex well "Soyalo 1A." This site is
located about 550 km SSW from the center of the Chicxulub
crater. As in other localities in the Gulf of Mexico-
Caribbean region, the KT sequence is characterized by a
thick, fining-up, clastic unit including thick heterogeneous
carbonate breccia and sandstone beds, followed by an
undisturbed sequence of lowermost Paleocene pelagic and
hemipelagic marls. The lithostratigraphy of the Bochil KT
sequence is shown in Fig. 1.

The Bochil clastic unit rests on top of a sequence of gray
pelagic and hemipelagic marls and limestones, interbedded
with pebbly mudstones and turbiditic calcarenites of
probable Cretaceous age. The contact between the clastic
unit and the underlying sediments is not clear due to poor
exposure along the road. It may represent an erosional
uncomformity but the possibility of a faulted contact cannot
be excluded. No spherule layer or tektites have been found
at the base of the clastic unit. The latter consists of
approximately 38 m of calcareous breccia composed of angular
poorly sorted clasts (ranging in size from less than 1 mm to
several m in diameter) of reefal and carbonate platform
rocks suspended in a finer calcareous matrix. The clast size
decreases upward. The proportion of the matrix varies
greatly, and locally the breccia is clast supported. Tests
of Murciella sp., an upper Cretaceous miliolid, are the
youngest microfossils identified in the calcareous matrix.

The breccia unit is discontinuously exposed along the road,
and is overlain by a 1 m thick, fining-, and thinning-up
sequence of yellowish sandstone, siltstone, and reddish clay
layers (Fig. 1). This sequence is topped by an irregular (10
+- 5 cm) micritic limestone layer underlying a thin
greenish-reddish, clayey arenite, and a whitish, laminated
chalk containing abundant Globigerina eugubina tests of
lowermost Paleocene age (Fig. 1). The rest of the Bochil
exposure comprises a 10 m thick sequence of gray,
homogeneous, pelagic and hemipelageic marls containing, in
the upper part, tests of Globorotalia pseudobulloides. A
more detailed paleontological analysis of the Bochil section
was still in progress at the time this abstract was
submitted.

Abundance measurements were made of the elements Ir, Co, Cs,
Eu, Fe, Hf, Ni, Sb, Sc, Se, and Ta with the Ir Coincidence
Spectrometer, following neutron activation on a suite of
samples collected in an expedition to Bochil in March, 1992.
Data for Ir are shown in Fig. 1, and selected abundances of
Cs and Fe in the 2 m interval across the major Ir peaks are
shown in Fig. 2.

A sample of limestone immediately underlying the breccia
unit contains abundant trace elements (suggesting a clay
component) and 36 +- 10 parts-per-trillion (ppt) Ir. This Ir
abundance is considerably higher than would be expected from
contributions of clay similar to Cretaceous black shales in
the Brazos 1 KT section in Texas, but comparable to
abundances in Paleocene marls of the same Bochil sequence.
Four samples from the breccia limestone are much more barren
in Ir with a best value of 5 +- 2 ppt, if the Ir content of
the samples is assumed to be constant.

There are 4 to 7 Ir peaks above background levels in the
sediments above the breccia unit. The largest peak has 1580
+- 72 ppt and is found in fine intensely-bioturbated silty
chalk about 6-10 cm below an irregular, 10 +- 5 cm thick,
micritic limestone layer. The upper tail of the Ir peak
extends into the micritic limestone (Fig. 1). There are
other considerably smaller Ir peaks extending as high as 7
and possibly 8 m above the main peak and 0.2 m and possibly
0.9 m below. Generally, the more distant the subsidiary
peaks are from the main peak, the smaller are their
abundances. The Ir abundance in a thin sandstone layer below
the main peak, however, was found to be lower than those in
underlying and overlying silt layers. If ratios are taken of
Ir abundances to those of Fe or Cs, anomalously high ratios
are found where the Ir abundance is anomalously high.

The element abundance patterns, which are similar to those
found in other marine KT sections in Haiti, Mexico, and
Texas in the U.S., are consistent with a fining-up debris
deposited from a tsunami triggered by a bolide impact on the
Yucatan Peninsula. Our preliminary study of the Bochil
section indicates the Ir began depositing (or redepositing)
immediately before the first appearance of the lowermost
Tertiary microfossils. The 38 m breccia unit may have
resulted from breakage of reefal sediments triggered by the
Chicxulub impact, but other explanations are possible.

References: [1] Bourgeois J. T. et al. (1988) Science, 241,
567-570. [2] Smit J. et al. (1992) Geology, 20, 99-103. [3]
Hildebrand A. R. and Boynton N. V. (1990) Science, 248, 843-
846. [4] Maurrasse F. J. M. R. et al. (1991) Science, 252,
1690-1693. [5] Alvarez W. et al. (1985) Geology, 20, 697-
700.


Ocampo A. C.   Pope K. O.
   A KT Boundary Section from Northern Belize

A KT boundary section recently discovered in northern Belize
provides new insights into cratering processes near the rim
of the Chicxulub KT crater in northwestern Yucatan. The
section is located in a quarry on Albion Island near the
Mexican border, which is only 2 to 3 crater-radii (~350 km)
from the center of Chicxulub. This is the most proximal
exposed KT section yet studied from the Chicxulub crater,
excluding material from deep drilling within the crater and
on the rim. The Albion Island section provides an example of
deposits intermediate between the crater rim and more
distal, possible impact-tsunami deposits in the region [1-
3].

Quarrying activity on Albion Island has exposed a 45 m thick
section in a region where deep weathering and dense
vegetation has obscured most bedrock exposures. The regional
stratigraphy has been established in two exploratory wells
drilled at near by Orange Walk and San Pablo [4]. The Orange
Walk 1 well records place the KT boundary between dolomites
of the Sand Hill Formation and undifferentiated Tertiary
limestones at a depth of about 1500 m. The KT boundary rises
to a depth of about 150 m in the San Pablo 1 well located 11
km north of Orange Walk 1. The Albion Island quarry is
located 15 km northwest of Orange Walk, near the crest of an
anticlinal fold and possible uplifted fault block that has
further elevated the KT boundary. A major erosional
unconformity separates the hard crystalline Cretaceous
dolomites and a chaotic, poorly sorted breccia at a depth of
about 15 m in the quarry. We propose that this unconformity
marks the KT boundary.

The unconformity is exposed for a lateral distance of over
100 m and is parallel with the bedding planes in the
underlying dolomites. It contains at least three erosional
troughs that cut into the dolomites, each approximately 15 m
wide and 2 m deep. The orientation of the troughs could not
be determined, but they indicate high-energy scouring. The
only sediments filling the scours are the chaotic breccias.

The maximum  breccia thickness  in the  quarry is 15 m,
however the  breccias continue  to the surface and the total
original thickness  of the  unit is unknown. Tertiary strata
are not  exposed in the quarry, but are found on the surface
in other  parts of  the  island,  presumably  overlying  the
breccia.  The   breccia  is   weakly  indurated  and  deeply
weathered. Clasts  range in  size from  30 cm to microscopic
grains, and  all sizes  are abundant. The clasts are angular
to sub-rounded  and supported  in a  dolomitic matrix.  Most
clasts, especially  the large ones, are dolomite, apparently
derived from  the underlying  beds. Other  carbonate clasts,
representing lithologies not found in the lower section, are
also  present.   Some  of   the   sub-rounded   clasts   are
grain-supported,   well   indurated,   and   partially   re-
crystallized  carbonate   breccias.  Highly  altered  clasts
consisting of  red, green, and yellow clay are also abundant
and give  the unit  a polychromatic appearance. The original
lithology of  these clasts  is unknown  (glass?),  but  they
occur in  a variety of shapes, including angular, round, and
elongated. Quartz grains are present, but rare.

We propose that the Albion Island breccia is the product of
ballistic sedimentation [5] from the Chicxulub impact. Such
deposits are created by secondary impacts of ejecta and for
large craters are composed mostly of brecciated local rock
[6]. Impact models [5,7] predict that ballistic
sedimentation would occur within 2-3 craterradii of
Chicxulub, consistent with the position of this KT section.
The breccia lithology and scoured contact are also
consistent with the ballistic model, however they do not
confirm an impact origin. The Albion Island breccia is
remarkably similar to the breccia/conglomerate unit found on
the rim of the Chicxulub crater, which we have also
interpreted as a product of ballistic sedimentation [8].
Petrographic analyses of the Albion Island breccias are
currently underway to determine if Chicxulub ejecta can be
recognized, thereby confirming an impact origin.

References: [1] Hildebrand A. R. and Boynton W. V. (1990)
Science, 248, 843-847. [2] Alvarez et al. (1992) Geology,
20, 697-700. [3] Smit J. et al. (1992) Geology, 20, 99-104.
[4] Geology and Petroleum Office (1986) Petroleum Prospect
and Potential of Concession Block-1 in Belize, Central
America. Belmopan: Geology and Petroleum Office. [5]
Oberbeck V. R. (1975) Rev. Geophys. Space Phys., 13,
337-362. [6] Horz F. et al. (1983) Rev. Geophys. Space
Phys., 21, 1667-1725. [7] Melosh H. J. Impact Cratering,
Oxford Press. [8] Pope K. O. et al. (in press) Earth, Moon,
and Planets.





                          Thursday, February 10, 1994
                      SESSION 4: KT RECORD: BIOTIC EVENTS
                                 2:30 - 6:00 PM

Chair(s):  R. A. Askin
           A. F. Gardner


Olsson R. K.*   Liu C.
   On the Reality of the KT Boundary

From the point of view of impact supporters the
Cretaceous/Tertiary boundary should contain a glassy
spherule layer on top of uppermost Maestrichtian strata
followed by a fallout layer containing an Ir anomaly. In the
Gulf of Mexico a third unit should be sandwiched between
these layers, a tsunami deposited clastic unit which may
contain several layers with various bedding characteristics.
Others regard the clastic unit as Maestrichtian in age.
However, it is clear that some of the clastic units
sandwiched at the boundary are Danian in age and, therefore,
they cannot have been deposited by a tsunami wave. At the
stratotype section at El Kef, Tunisia only the spherule
layer and the fallout layer are present. At Deep Sea
Drilling Sites in the Caribbean the completeness of the KT
boundary is in dispute with many sites apparently containing
hiatuses either in the uppermost Maestrichtian or in the
Danian. Outside of the Caribbean at Deep Sea Drilling and
Ocean Drilling Sites it is a fact that recovery of the KT
boundary has been disappointing and only a few sites are
regarded as containing a continuous section.

Identifying the KT boundary by means of planktonic
foraminifera has not been straightforward. If one uses the
stratotype section (GSSP) at El Kef as the standard
reference, the KT boundary is identified on the basis of the
boundary clay layer with the Ir anomaly, which is associated
with Zone PO, the Guembelitria cretacea Zone. This zone was
originally interpreted as containing only the nominate
species with other planktonic taxa (i.e., Cretaceous taxa)
regarded as reworked. Newly evolved Danian species do not
appear until the upper part of this zone. In this view the
oceans had been sterilized by the boundary event, which
caused a mass extinction of planktonic foraminifera so that,
initially, there was only one survivor, Guembelitria
cretacea. The second view identifies the KT boundary on the
first occurrence of newly evolved Danian species. This view
apparently assumes that Danian taxa evolved immediately
after the KT event so that evolution must have been
instantaneous. The other view allows a short period of
geologic time to pass before the first Danian species
evolve. This view has been fortified by the recognition
oftwo additional survivors in Zone PO, Hedbergella
holmdelensis and H. monmouthensis. The origins of all
evolved Danian species can now be traced to the three
survivor species. In any case, the KT boundary cannot be the
same as it is applied by these two opposing views.

There is a certain amount of consistency in the various
opposing views of the geology and micropaleontology at the
KT boundary. There are hiatuses, there are clastic units at
the boundary in the Gulf of Mexico, there are spherule
layers, there is an Ir anomaly, there are specimens of
Cretaceous taxa above the boundary, and there are newly
evolved Danian taxa above the KT boundary. But few of these
criteria are present at all KT boundary sections. We offer
the hypothesis that the K/T transition should be taken at
face value. A KT section that contains a gap in criteria may
in fact, be complete in the sense that it contains a record
of the KT event. The true record of the KT boundary is not
at any one section (e.g., there is no clastic unit at El
Kef) nor does it lie with the hunt for biostratigraphically
continuous sections. We illustrate this concept with the
record at Millers Ferry and its comparison with other well
known KT boundary sections.

At Millers Ferry photographs of the excavation pit for the
construction of the powerhouse for the Millers Ferry Dam on
the Alabama River offer an unusual opportunity to view the
top of the latest Maestrichtian Prairie Bluff Chalk. The
surface of the Prairie Bluff Chalk is irregular and
undulating without any interconnections, which would
indicate fluvial erosion during a period of subaerial
exposure. Furthermore, there is no evidence of weathering in
the photographs nor in two cores taken from the site. On the
contrary, the surface is marked by scour features, flute
markings, groove markings, and striations that have a north-
south orientation. Direction of flow is from south to north.
In addition, synthetic normal faults with a north-south
orientation and offsets of 10s of centimeters break the
Prairie Bluff surface. They are buried by the overlying
Danian Basal Clayton Sands. The faults and the surface
markings indicate an instantaneous event for their origin.
We interpret that these features formed as a result of a
tsunami wave and earthquakes created by a bolide impact at
Chixulub Crater in Yucatan.

The overlying Clayton Basal Sands fill in the topographic
lows on the Prairie Bluff Chalk and thin over the
topographic highs. As a consequence, the basal Danian Zone
PO is found where the Clayton Basal Sands are thickest and
is absent where the sands thin on topographic highs.
Calcareous microfossil biostratigraphy shows that the
Prairie Bluff Chalk lies within the uppermost Maestrichtian
calcareous nannofossil Micula Prinsii Zone, which is
equivalent to the upper part of the planktonic foraminiferal
Abathomphalus Mayaroensis Zone. Paleomagnetic data show that
the Prairie Bluff Chalk/Clayton Basal Sands (KT) boundary
lies in the middle part of paleomagnetic chron 29R in the
cores taken at Millers Ferry. The last occurrence (LO)
within the Prairie Bluff Chalk of the planktonic foraminifer
Gansserina gansseri is a datum from which to measure the
degree of topographic irregularity on the top of the Prairie
Bluff Chalk. This indicates about a meter or so of relief.
The relief could be due entirely to the erosive effect of a
tsunami wave or it could represent a modified original
depositional surface. In either case, it is clear that the
KT boundary at Millers Ferry is biostratigraphically
continuous but it is not contained in one section.

We interpret that the abrupt change from the Prairie Bluff
Chalk to the Clayton Basal Sands is due to the mass
extinction of the Prairie Bluff Chalk biota, which includes
planktonic foraminifera, calcareous nannoplankton, and the
molluscan megafauna. Benthic foraminifera, which were little
affected by the extinction event indicate that there was
little, if any, change in sea level across the boundary.
Minor changes in foraminiferal taxon abundances across the
boundary are due to substrate change from chalk to sand.

At Brazos River, Texas the "tsunami bed" has been placed in
the Cretaceous [7,3] and in the Danian [8]. A Danian age
would indicate a similar origin for this unit as the Clayton
Basal Sands. A Cretaceous age does not necessarily exclude a
bolide impact-generated tsunami wave origin. At Mimbral,
Mexico the sand unit at the boundary interpreted as a
tsunami deposit, whereas an alternate hypothesis regards it
as a possible incised valley deposit of Cretaceous age.
These different interpretations are not necessarily
incompatible since they center around the age of the sand
unit and the interpretation of the origin of the spherules
at the base of the unit. The use of different
biostratigraphic criteria for placing the KT boundary may
give a different age assignment to the sand unit [2]. It is
difficult to ignore the fact that thin discontinuous sands
occur in very close proximity to the KT boundary around the
Gulf Coastal Plain for a distance of about 2000 kilometers
in different paleodepth settings (upper bathyal at Mimbral
to shallow shelf at Millers Ferry). The origin of these
sands could well be different and still be related to a KT
terminal event, which dispersed enormous energy. Some of the
sands could be tsunami deposits whereas others must
represent a brief period of increased clastic deposition
that can be explained by a fireball-denuded, erosion-
enhanced landscape. The scoured surface upon which these
sands lie can also be regarded as evidence of the passage of
a powerful tsunami wave.

At the Caribbean deep sea drill sites the apparent record
[6] of hiatuses on either side of the KT boundary can be
interpreted as evidence for signicant disturbance of the sea
bottom. Enormous earthquakes generated by a bolide impact
would cause displacement of sediments and would lead to the
development of localized hiatuses. At greater distances away
from the impact, such as El Kef in a deep margin setting,
the KT boundary records the mass extinction event of
calcareous marine plankton, the spherule layer, and the Ir
anomaly in direct succession.

In our view KT boundary sections should be taken at face
value because they each record various aspects of the KT
event. In highly disturbed areas, such as the Gulf of Mexico
and the Caribbean the regional picture of the terminal event
needs to be developed in order to appreciate the reality of
the KT event.

References: [1] Smit J. et al. (1992) Geology, 20, 99-103.
[2] Olsson R. K. and Liu C. (1993) PALAIOS, 8, 127-139. [3]
Keller G. (1989) Paleocean., 4, 287-332. [4] Stinnebeck W.
et al. (1993) Geology, 9, 797-800. [5] Smit J. (1990) Geol.
Mijnbouw, 69, 187-204. [6] Keller G. et al. (1993) Geology,
9, 776-780. [7] Jiang M. J. and Gartner S. (1986)
Micropaleon., 32, 232-255. [8] Montgomery H. et al. (1992)
EPSL, 109, 593-600.


MacLeod N.*
   An Evaluation of Criteria That May be Used to Identify Species Surviving a
   Mass Extinction

One of the most difficult obstacles to establishing a causal
connection between mass extinctions and large body impacts
is the existence of what appear to be many more Cretaceous-
Tertiary (KT) survivor species than previously suspected.
Though interpretations of "Cretaceous" faunal elements in
lowermost Danian sediments differ, this enigmatic fauna has
now been recovered from every biozone-complete boundary
section, including the El Kef stratotype. In terms of their
potential for providing constraints on scenarios seeking to
account for the KT extinction event, the significance of
such observations cannot be overstated.

Owing to the consistency with which these observations have
been made over the last several years, the possibility of
widespread trans-KT biotic survivorship can no longer be
dismissed. Rather, the survivorship hypothesis must be
tested alongside its alternative (the reworking hypothesis)
to determine which explains the available data in the most
complete yet parsimonious manner. Moreover, valid tests for
survivorship cannot be based on negative evidence or on the
assumption that only a small cohort of species could have
survived the KT boundary event. Several authors have
recently proposed various criteria that might be used to
test alternative interpretations for this aspect lowermost
Danian biotic record (Table 1). These are evaluated below.

Preservational State: The most commonly applied test for
survivor identification is based on the principle that
physically reworked specimens should exhibit abrasions,
breaks, recrystallized surface texture, or other signs of
degraded preservation. Although it is possible to collect
and quantify evidence bearing on preservational state, all
reports of degraded physical condition of Cretaceous species
in lowermost Danian sediments offered thus far have been
qualitative and anecdotal. Nevertheless, several authors
have noted that when specimens of "Cretaceous" species
collected from Danian sediments are compared side-by-side
with conspecific uppermost Cretaceous specimens, predicted
differences in preservational state oftentimes fail to be
observed. In addition, recent statistical analyses [1]
indicate that the planktic foraminiferal survivor fauna is
dominantly composed of species that are susceptible, rather
than resistant, to abrasion and diagenesis.

Relative Abundance/Population Ratio: Olsson and Liu [2]
explain this test as follows: "If one species survived (the
KT boundary) its population size in the lower Paleocene
would be composed of both the reworked and indigenous
surviving fractions while that of the extinct species
consists only of the reworked fraction. As a result, the
relative abundance of the survivor taxon to that of the
extinct species would significantly increase after the
extinction of the other species." (p. 136). Applying the
relative abundance/population ratio test to depositional
rate-normalized KT planktic foraminiferal data of [2] and
[3] shows that patterns of abrupt relative abundance decline
in Danian occurrences of "Cretaceous" planktic foraminiferal
species are not consistently observed for either established
or putative KT survivor species. Nevertheless, patterns of
post-boundary relative abundance variation exhibited by many
"Cretaceous" species are indistinguishable from patterns
present in widely-accepted survivor species. Overall, these
data support recognition of a expanded survivor fauna and a
progressive faunal turnover.

Stable Isotope Geochemistry: This test involves determining
whether the stable isotopic signature of post-boundary
populations differs from that of pre-boundary populations.
Recovery of different isotopic signatures from these
populations constitutes a direct species-level test of
survivorship. However, adequate controls must be maintained
to insure that the observed difference cannot be accounted
for through diagenetic or species-specific metabolic
factors. Comparative analyses of planktic and benthic
foraminiferal species from Brazos River (Texas), Nye Klov
(Denmark), and ODP Site 738 (S. Indian Ocean) suggest that
several survivor species are present in these faunas.
Changes in the isotopic signatures of these survivor
populations also begin in the upper Cretaceous; well below
the appearance of any putative impact debris.

Quantitative Morphology: Like the stable isotope test,
consistent changes in test size and shape parameters provide
another means of identifying survivor species. MacLeod and
Keller [4] demonstrated that lowermost Danian samples of
Heterohelix globulosa and H. navarroensis from Brazos Core
undergo statistically significant test size reductions,
while at the same time preserving the ontogenetic
trajectories of comparably-sized uppermost Maastrichtian
populations. Once again, this size reduction does not
coincide with the KT boundary but rather takes place well
below this horizon, within the uppermost Maastrichtian.
Since this initial study, similar size/shape patterns have
been documented for several other "Cretaceous" survivor
species in other boundary sections/cores.

Comparative Biogeography: Occurrence of Danian populations
of "Cretaceous" species in areas in which they were not
observed during the upper Cretaceous is consistent with
trans-KT survivorship. Also comparison of changes in the
biogeographic structures of survivor and indigenous Danian
faunas can be used to determine whether the spatial
organization of these faunas are similar (suggesting
survivorship) or different (suggesting reworking). MacLeod
and Keller [1] and MacLeod [5] have carried out an extensive
series of analyses designed to implement this test. Results
show that: (1) many "Cretaceous" taxa occurring in the
Danian portions of various boundary sections are not present
in underlying Cretaceous strata, (2) a close and predictive
association exists between "Cretaceous" and indigenous
Danian species richness values throughout the lower Danian,
(3) a close geographic correspondence is present between
centers of Danian planktic foraminiferal speciation and the
disappearance of "Cretaceous" species from the Danian fossil
record, and (4) a pronounced similarity exists between
changes in the general biogeographic structures of
"Cretaceous" and indigenous Tertiary faunas throughout the
lowermost Danian. Furthermore, these data show that the KT
planktic foraminiferal extinction event exhibited a marked
geographic structure with low and middle latitude faunas
experiencing differentially high extinction rates in the
lowermost Danian zones PO and P1a and high latitude survivor
faunas persisting relatively unchanged into the overlying
Danian.

Association with Elevated Ir Abundances: This test argues
that Cretaceous faunas occurring in lower Danian sediments
in which elevated (= non-background) levels of Ir also occur
should be regarded as reworked. There are several problems
with the use of this test. Association of "Cretaceous"
morphotypes with elevated abundances of Ir in lowermost
Danian sediments is indeterminate with respect to the
question of survivorship. Bolide-introduced Ir can diffuse
through undisturbed sediments via interstitial fluids [6]
while fossils require physical disruption of the sediment to
be reworked. Moreover, many lines of evidence indicate that
the KT Ir anomaly reflects input of Ir into the environment
from multiple sources over a considerable time interval.
Assuming that stratigraphic thickness of the Ir anomaly
might place an upper limit on the reworking depth, though,
continued occurrence of "Cretaceous" morphotypes above the
Ir anomaly constitutes evidence for survivorship. Using this
test almost all Danian occurrences of "Cretaceous" taxa,
regardless of taxonomic group, are identified as survivors.

Synchronous Last Appearance Datums (LAD's): This test
suggests that true survivors should have synchronous LAD's.
Both the International Stratigraphic Guide and the North
American Stratigraphic Code agree that the boundaries of
most biozones, unlike the boundaries of chronozones, are
characteristically and conceptually diachronous. Thus, there
is no justification for the idea that synchrony of local
LAD's, or lack thereof, can play any role in deciding the
question of survivorship. This conceptual distinction
between biostratigraphic and chronostratigraphic units is
also borne out by the large number of high-resolution
biostratigraphic studies that routinely document substantial
diachrony in paleontological datums.

Phylogenetic Ancestry: This proposed test is based on the
principle that incoming species must be derived from
survivor species. Of course, the phylogeny test is
irrelevant to determining the survivorship of Cretaceous
species that may have existed to in the lower Danian but
failed to give rise to any fully Danian descendants. A more
serious problem, however, lies in the fact that phylogeny
can only provide a relative ordering, rather than the
absolute temporal location, of speciation events. These
problems, along with present levels of indeterminacy with
respect to the species-level phylogenies of virtually all
trans-KT clades invalidate this application of phylogeny to
the question of survivor identification.

The unambiguous identification of KT survivor species is
extremely difficult. But, as outlined above, criteria are
available to test the survivorship/reworking alternatives at
both the individual species and faunal levels, though not
all tests that have been proposed are adequate on
theoretical or phenomenological grounds. While much more
work needs to be done on this question, the data presented
above demonstrates that there is a large body of evidence
consistent recognition of a greatly expanded survivor fauna
that dominated both marine and terrestrial ecosystems during
the earliest Tertiary.

References: [1] MacLeod N. and Keller G. (in press)
Paleobiology, 20. [2] Olsson R. K. and Liu C. (1993)
PALAIOS, 8, 127-139. [3] Keller G. et al. (in press)
PALAIOS, 9. [4] MacLeod N. and Keller G. (1990) GSA, 22,
A106. [5] MacLeod N. (in press) Historical Biology, 8. [6]
Colodner, et al. (1992) Nature, 358, 402-404. [7] Keller G.
(1988) Marine Micropaleontology, 13, 239-263. [8] Barrera E.
and Keller G. (1990) Paleoceanography, 5, 867-870. [9] Liu
C. and Olsson R. K. (1992) GSA, 24, A29. [10] Pospichal J.
J. (1993) GSA, 25.


Pospichal J. J.*
   Calcarious Nannofossils and the KT Boundary Crisis

Calcareous nannofossil assemblages in closely-spaced
samples, which span the Cretaceous/Tertiary (KT) boundary of
several sections around the globe, were quantitatively
analyzed to assess the nature of nannoplankton extinctions.
The results of this investigation are summarized as follows.
(1) Climate-sensitive nannoplankton populations are
relatively stable during the latest Maastrichtian, which
indicates that in low-latitude sections, such as El Kef,
Tunisia and Agost, Spain, no major ecologic stresses such as
temperature change foreshadowed the KT boundary event. At
mid-latitude South Atlantic Site 527, cooling near the end
of the Maastrichtian is suggested by a slight drop in
abundance of warm-water nannofossil taxa and rise in
abundance of cooler water forms. However, no extinctions are
associated with these changes and abundance fluctuations are
within limits of error. (2) No stepwise or gradual
extinctions are noted across the KT boundary at any of the
sites examined. Nannoplankton extinctions are abrupt and
synchronous, and correlate well with reported shifts in C
isotopes, carbonate sedimentation decline, and anomalous Ir
abundance.

(3) In the sections examined, no noticeable difference
exists between the low latitude and the high austral
latitude sites in terms of the tempo and mode of
nannoplankton extinctions. Cosmopolitan Cretaceous species
exhibit the same extinction pattern across latitude. (4) The
early Danian is characterized by sequential blooms of
surviving and new nannoplankton species. No two localities
are the same in terms of the species composition of these
blooms. This reflects the unstable conditions around the
globe following the KT boundary crisis.

(5) There is no unequivocal evidence for the survival of
additional Cretaceous species into the Tertiary (other than
the traditionally accepted survivor taxa). Cretaceous
specimens present in variable amounts above the KT boundary
are the result of reworking by bioturbation and/or current
activity. This is suggested by overall abundance patterns in
each section and correlations with short term sea-level
lowstands at El Kef. In addition, correlation of Cretaceous
nannofossil abundance and Ir abundance above the KT boundary
at Indian Ocean Site 752 strongly suggests reworking of
these specimens.


Fastovsky D. E.*   Sheehan P. M.
   Habitat vs. Asteroid Fragmentation in Vertebrate Extinctions at the KT
   Boundary: The Good, the Bad, and the Untested

The vertebrate fossil record is a touchstone in popular and
scientific perceptions of Cretaceous-Tertiary (KT) events.
However, the record actually is limited largely to the upper
Great Plains of the United States. Within these confines,
nevertheless, sufficient data are available to test
vertebrate extinction scenarios in a limited way. Among
competing theories, extinction by asteroid impact has been
contrasted with extinction due to marine regression and
concomitant habitat fragmentation. Here, we address the
evidence for these competing hypotheses, considering first
the sedimentary environments represented by the relevant
stratigraphic units, then the timing of the marine
regression, and finally the evidence for habitat
fragmentation and partitioning. It is our contention that
(1) the marine regression maximum probably preceded KT
boundary, (2) geological evidence for habitat fragmentation
is completely lacking, and (3) patterns of vertebrate
survivorship are fully concordant with extinction by
asteroid impact.

The KT boundary in the upper Great Plains of the United
States is represented by a conformable sequence spanning the
last 2.2-3.5 m.y. of the Cretaceous (Lancian) through early
Paleocene (Puercan). The Cretaceous is represented by the
Hell Creek Formation; the Tertiary is represented by the
Tullock Formation. These units are part of a clastic wedge
that developed as the Laramide pulse of the Cordilleran
orogeny began. The progradation of the wedge was
approximately concurrent with the latest Cretaceous marine
regression, whose timing we consider below.

The Hell Creek Formation represents an aggradational
meandering fluvial depositional system along a coastal
plain. It is composed largely of sandstones and mudstones,
representing, respectively, river channel and floodplain
deposits [1]. Because of the nature of the depositional
system and the presence of successive stacked paleosols, the
Hell Creek is riddled with hiatuses, each probably measured
in 100s to 1000s of years [2]. By contrast, the base of the
Tullock Formation primarily consists of widespread
suspension settling deposits interbedded with coals and some
river channel deposits, representing a broadly flooded
landscape during earliest Paleocene time [1,3]. The KT
boundary itself is commonly--although not exclusively--
represented by a coal, and a hiatus is indicated during
which time peat deposition took place. The KT boundary in
this region is thus marked by a facies change, however, in
the context of local, short-duration hiatuses, sedimentation
was continuous to a first-order approximation.

Although it is widely known that the KT boundary in the
Great Plains was associated with a marine regression, the
precise timing of the boundary vis-a-vis the regression
remains unclear. What is clear is that the Hell Creek
clastic wedge is temporally sandwiched between the
Campanian-Maastrichtian retreat of the Bearpaw Sea and the
early Paleocene transgression of the Cannonball Sea,
represented by the Cannonball Formation. In Montana, the
Cannonball Formation is stratigraphically approximately 20 m
above the KT boundary; in Alberta the unit is
stratigraphically less than 3 m above the boundary [4]. This
is reflective of the fact that during KT time, Alberta was
closer than Montana to the marine strand line, which must
have been located in northern Canada. Transgression, when it
took place, would be expected to raise riverine base levels,
and subsequently impede Cordilleran drainages. This is in
fact what is observed at and above the Hell Creek/Tullock
lithostratigraphic contact, suggesting that by the time of
this lithostratigraphic feature (approximately coincident
with the KT chronostratigraphic boundary), the Cannonball
transgression had proceeded sufficiently to modify stream
base levels.

J. D. Archibald and colleagues [5-10], have repeatedly
linked the KT extinctions and the marine regression,
suggesting that the retreating seaway caused habitat
fragmentation that led to the extinctions:

Although total area increased during marine regression, the
size and variety of low coastal plain habitats...declined
drastically. Shallow coastal marine habitats showed a
similar decline. Not only did habitat size in these two
realms decrease, but habitat fragmentation...further
stressed living populations [6].

As discussed earlier, the regression resulted in the loss of
tens of thousands of square miles of coastal habitats in
which these dinosaurs lived [7].

As the seaways bisecting North America receded some 65
million years ago, it [sic] reduced and fragmented the low
coastal plains where the dinosaurs lived [8]...and dinosaurs
disappeared as habitats in low coastal plains were severely
reduced and fragmented during regression [9].

Neither geomorphology, nor the sedimentary geology of the
Hell Creek and Tullock, nor the patterns of extinction of
the biota provides any support for this premise. It is
counterintuitive to posit that an increase in land surface
area (as occurs by definition as the result of a marine
regression) will be accompanied by habitat fragmentation;
why should the terrestrial realm be "fragmented" by
increasing the habitable area? An increase in land should
provide opportunities and space previously not available to
land-dwelling organisms.

Hell Creek sedimentation appears to have been constant
through time, and there is no evidence for fragmentation or
deteriorating habitats. Channel size, shape, and lateral
relations with floodplain sediments do not change through
the thickness of the formation. It is conceivable that a
lowering of base level could increase incision by stream
channels. This should not be equated with habitat
fragmentation; the system was clearly aggradational and not
erosional (as the preservation of sediments recording the KT
time interval attests). Moreover, the KT interval in Montana
(the source of the vertebrate faunas) is stratigraphically,
and thus by inference geographically quite distant from the
shore line. This means that the faunas from which KT events
are being reconstructed were living in a setting 100s of km
from marine influences. It is significant that the two
previous great transgressive and regressive pulses of the
western interior seas failed to cause anything close to the
magnitude of the KT extinction.

It is generally agreed that in the terrestrial realm
patterns of survivorship across the KT boundary strongly
favored land-dwelling vertebrates over aquatic ones [6,11].
This provides clues to the nature of the causal agent in the
KT extinction. Land-dwelling vertebrates underwent an 88%
extinction; aquatic vertebrates underwent only a 10%
extinction. Sheehan and Fastovsky [11] explained this
disparity by differences in trophic structures: aquatic
vertebrates exist within detritus-based or detritus-enhanced
food chains, while land-dwelling vertebrates are directly
dependent upon primary productivity. A hypothesis that calls
for a cessation in primary productivity, such as via the
loss of sunlight due to an asteroid impact, can explain this
marked pattern of survivorship and extinction.

The pattern of vertebrate survivorship in eastern Montana
takes its place among the mounting evidence that global
primary productivity was severely but temporarily curtailed
at the KT boundary [12-14]. This should be contrasted with
the idea of habitat fragmentation, for which there is no
geological evidence, and which has no context or relevance
beyond the terrestrial vertebrates of the upper Great Plains
of the United States. In this sense, habitat fragmentation
is no hypothesis at all; it is only an ad hoc explanation
for a local pattern of survivorship and extinction. It does
not take into account the wealth of geochemical, geological,
and biological events that are known to have characterized
the KT boundary around the world. The asteroid impact
hypothesis provides a global context in which to consider
the extinctions in eastern Montana. Considered thus, the
eastern Montana record is fully concordant with virtually
all that is known of KT events.

References: [1] Fastovsky D. E. (1987) Palaios., 2, 282-295.
[2] Fastovsky D. E. (1990) GSA Spec. Paper, 247, 541-548.
[3] Belt E. S. et al. (1984) SEPM Spec. Publ., 7, 317-335.
[4] Lerbekmo J. F. and Coulter K. C. (1984) Can. Soc. Pet.
Geol. Mem., 9, 313-317. [5] Archibald J. D. (1991) GSA
Abstr. with Prog., 23, A-359. [6] Archibald J. D. (1993)
Geology, 21, 90-92. [7] Archibald J. D. (1993) in The
Ultimate Dinosaur, (B. Preiss et al., eds.), Byron Preiss
Visual Publications, 276-288. [8] Archibald J. D. (1993) New
Scientist 13/2/93, 24-27. [9] Archibald J. D. (1993) GSA
Abstr. with Prog., 24, A-297. [10] Archibald J. D. and
Bryant L. J. (1990) GSA Spec. Paper, 247, 549-562. [11]
Sheehan P. M. and Fastovsky D. E. (1992) Geology, 20, 556-
560. [12] Sheehan P. M. and Hansen T. A. (1986) Geology, 14,
868-870. [13] Wolfe J. A. and Upchurch G. A. (1986) Nature,
324, 148-151. [14] Arthur M. A. et al. (1987) Cret. Res., 8,
43-54.



                              POSTER PRESENTATIONS

Lopez-Olivia J. G.   Keller G.
   Biotic Effects of the KT Boundary Event in NE Mexico

A large Cretaceous-Tertiary (KT) boundary bolide impact near
Chicxulub on the Yucatan Peninsula is predicted to have
produced catastrophic destruction of marine and terrestrial
organisms. These catastrophic biotic effects should be
evident in nearby areas of the Gulf of Mexico and
northeastern Mexico. Here we evaluate the biotic effects in
planktic foraminifera in eight KT boundary sections spanning
over 300km in north-eastern Mexico (Fig. 1). Planktic
foraminifera are the group of marine microplankton long
believed to have suffered most severely as a result of a
bolide impact at KT boundary time.

All eight sections examined reveal similar depositional
sequences, although of variable thickness, spanning from the
Maastrichtian Mendez Fm. to a 0.2m to 7m thick clastic
deposit and to the overlying Tertiary Velasco Fm. We
interpret the clastic deposits to represent channelized
sediments that thin out towards the channel edges and may
locally disappear [1]. In sections where the clastic deposit
is very thin (10-20cm) or absent, the Mendez marls grade
into the shales and marls of the Velasco Fm.

Stratigraphically, the upper Mendez Fm. is of late
Maastrichtian Abathomphalus mayaroensis Zone age and the
lower Velasco Fm. is of earliest Tertiary (Danian) age. The
clastic deposit is also of A. mayaroensis Zone age as
indicated by the presence of the A. mayaroensis Zone fauna,
including the index taxon, and absence of Tertiary species.
Four of the eight sections examined (El Mimbral, La Lajilla,
El Mulato, and La Parida) span Cretaceous to Tertiary
sediments. In four of the sections (El Penon, La Sierrita,
Rancho Nuevo, and Los Ramones) Tertiary sediments are
eroded. At La Lajilla and El Mulato a thin layer of A.
mayaroensis Zone age marl overlies the top of the clastic
deposits clearly marking clastic deposition as of
Maastrichtian age. In addition, abundant Chondrites,
Zoophycos, and y-shaped Thalassinoides burrow networks
present in the top of the clastic deposit are infilled with
Maastrichtian sediments that also indicates that deposition
occurred in the Maastrichtian. The basal Tertiary Zone PO
has only been recognized at El Mimbral near the edge of the
channel-fill in a 4cm thick dark grey clay layer [1,2]. In
all other sections, Zone PO is missing as well as part of
the overlying Zone Pla (P. eugeubina). The absence of the
lowermost Tertiary sediments in these sections is due either
to non-deposition or erosion.

In order to observe the biotic effects of the KT boundary
event in these northeastern Mexico sections, we have
qualitatively analyzed the upper Cretaceous to lower
Tertiary planktic foraminiferal faunas of each section in
closely spaced (5cm intervals) samples. Planktic
foraminifera, although recrystallized are abundantly present
in the Mendez and Velasco Fms. In the channel-fill deposits
they are common in the spherule-rich layer of unit 1, the
mudclasts of unit 1 and unit 2, in hemipelagic layers of
unit 3 and in the rippled sandy limestone at the top of unit
3. Since much of the channel-fill represents transported
sediments, an evaluation of the biotic effects must be based
upon faunas from the Mendez and Velasco Fms. Thus, the
faunal data from the channel-fill deposits must be
disregarded or, ideally, a trans-KT boundary sequence
without the clastic deposit must be used. At El Mimbral at
the edge of the channel, a nearly continuous Mendez to
Velasco marl-shale sequence is present interrupted only by
the top 20cm (rippled sandy limestone) of the clastic
deposit. A similar sequence has been found at La Parida. In
all sections that contain the clastic channel-fill, an
undulating erosional surface marks the contact between the
Mendez Fm. and the base of the clastic deposit. This
indicates that the topmost Maastrichtian sediment has been
eroded within the channel. Despite this erosional
disconformity all trans-KT boundary sections show similar
faunal turnovers as illustrated here for El Mimbral.

Figure 2 shows that about 2/3 (28 species) of all Cretaceous
taxa disappeared at the base or top of the rippled sandy
limestone (R.S.L. top of clastic deposit). Isolated
specimens of some of these taxa are also found in the lower
Tertiary and are assumed to be reworked (dots). Since
preservation in the R.S.L. is poor, we tentatively assume
that all 28 species disappeared at the KT boundary. About
1/3 (13 species) of the Cretaceous species are continuously
present well into the Tertiary Velasco Fm. and these are
considered as survivors for the reasons outlined in several
studies [3-5]. The relative species abundances of survivor
taxa and early Tertiary evolving species are illustrated in
Fig. 3. It is notable that Cretaceous survivors are all
small surface dwellers, of simple morphology, little
ornamented, and cosmopolitan taxa. They dominate the late
Maastrichtian assemblage and show little or no adverse
effects at the KT boundary. Their relative abundances
decline in Zone Pla (P. eugubina) about 100 kyr after the KT
boundary, coincident with the evolutionary diversification
and increased competition of the early Tertiary fauna. The
decline and eventual extinction of the Cretaceous survivor
fauna in Zone Pla thus seems to be a direct consequence of
increased competition, rather than the belated effects of a
KT boundary bolide impact.

In contrast to the survivor fauna, the 2/3 of the Cretaceous
species that disappeared at or below the KT boundary are
tropical-subtropical large, deep dwellers (living at or
below the thermocline) of complex morphology and well
ornamented. They are generally rare with a combined relative
abundance of less than 17% of the population. Thus, their
disappearance had little effect on the overall foraminiferal
population. Similar high rates of species extinctions, but
low combined relative abundance in the foraminiferal
population were earlier observed at El Kef, Tunisia (KT
boundary stratotype) and Agost and Caravaca in southern
Spain [6,7].

Our study indicates that the biotic effects of the KT
boundary event on planktic foraminifera in the northeastern
Mexico sections were not as catastrophic as predicted from a
large bolide impact on Yucatan. Although 2/3 of the
Cretaceous species disappeared at or below the KT boundary,
the effect on the overall foraminiferal population was small
(<17%) because only rare, already endangered taxa
disappeared. These taxa were specialized tropical and
subtropical forms living at or below thermoclinal depth;
this group is historically the least tolerant of
environmental changes. The limited negative biotic effects
observed across the KT boundary in northeastern Mexico are
consistent with other low latitude sections. They indicate
that if a bolide impact occurred on Yucatan, the biologic
consequences were not as catastrophic as generally assumed
even within a radius of 2000 miles.

References: [1] Stinnesbeck W. et al. (1993) Geology, 21,
797-800. [2] Keller G. et al. (in press) Palaios. [3]
MacLeod N. and Keller G. (in press) Paleobiology. [4] Keller
G. (1993) Mar. Micropaleo., 21, 1-45. [5] MacLeod N. (this
volume). [6] Keller G. (1988) Mar. Micropaleo., 13, 239-263.
[7] Canudo J. I. et al. (1991) Mar. Micropaleo., 17, 319-
341.


Liu C.   Olsson R. K.
   Patterns of Planktonic Foraminifer Extinction at the End of Cretaceous,
   Stepwise, Gradual, Foreshadowed/Extended, Latitudinal-controlled, or
   Instantaneous?

Various gradual patterns including intensified background,
foreshadowed followed by prolonged [1], gradual and stepwise
[2-3], as well as variations with climatic belts [4-5] have
been proposed for the KT extinction of planktonic
foraminifer in contrast to the observation of a single
instantaneous global event [6-8]. Despite the contrasting
hypotheses, the datasets of various workers are fairly
compatible. A large part of the dispute must come from
contrasting interpretations on the observed fossil record.
These include equivocal placement of the K/T boundary,
differential treatment of Cretaceous taxa occurring in the
basal Danian, taxonomic inconsistencies, and the logic used
in separating local events from a global change.

Placement of the KT Boundary. The placement of the KT
boundary above the KT GSSP at El Kef led workers to propose
a foreshadowed extinction pattern [1]. Application of
different criteria for defining the KT boundary in
individual sections may result in correlating the KT event
in a different temporal framework and to view a single
global event as several local events. For instance, the KT
boundary in the sections at El Kef (Tunisia) and Agost,
Caravaca (Spain) are placed at the mass disappearance of
Cretaceous taxa by Smit [6], Keller and co-workers [2-5,9].
However in other sections, Keller and co-authors [2-5,9]
defined the KT boundary as the FO (first occurrence) of
Tertiary species. This change in criterion obviously leads
to correlation of diachronous layers as the geochronologic
boundary because Tertiary taxa have never been found in the
basalmost Danian in the stratotype and in other sections
where the boundary is unequivocally placed. It is the mass
disappearance of Late Cretaceous taxa marked by an abrupt
decline in abundance that coincides with the KT boundary.
Because diversity measurement alone may cause a problem in
the accurate placement of the KT boundary due to reworking
of Cretaceous taxa into the Danian, the initial abundance
increase of the survivor species Guembelitria cretacea,
which is the direct result of the mass disappearance of
Cretaceous taxa, was used to define the KT boundary in shelf
margin sections [8].

Obviously only the magnitude, but not the time of this
relative abundance increase is affected by reworking of
Cretaceous taxa. However, recently Keller and co-authors [4-
5] defended the use of the FO of Tertiary taxa and
misinterpreted the initial increase in relative abundance as
the peak abundance of G. cretacea. They explain the absence
of Tertiary taxa in some KT boundary layer as due to
dissolution of Tertiary species. This ad hominem explanation
misses the fact that the more fragile G. cretacea occurs
abundantly in the basal Danian while Tertiary species are
absent. The explanation of selective dissolution of Tertiary
taxa without supportive taphonomic data is subjective.

Reworking. Reworking of microfossils may significantly
distort the KT boundary fossil record. Paleoenvironment is
one of the major factors affecting the magnitude of
reworking of Cretaceous taxa. Reworking of Late Cretaceous
planktonic foraminifera into Zone P3b and 45 meters above
the KT boundary in Sao Paulo Plateau off Brazil coast due to
down-slope transportation in the paleoslope is an example.
Unfortunately, there is no satisfactory means for separating
reworked specimens from indigenous species although up to 9
criteria have been utilized [10]. Stable isotope and
phylogeny are perhaps the most reliable and positive
methods. Stable isotopic comparisons show that reworked
Cretaceous specimens have significantly lower delta^18O and
higher delta^13C signature than indigenous survivor species
and evolving Paleocene taxa [11]. However, stable isotopic
analysis can only be applied to well preserved specimens.
Both Tertiary and Cretaceous taxa may display similar stable
isotope signatures due to diagenetic replacement of elements
in the tests.

A comparison of diversity vs. abundance change is indicative
of the origin of Cretaceous taxa in the basal Danian because
abundance will sharply decline while reworking may still
bring specimens of extinct species up. The result is that
diversity drops less abruptly than abundance. The abrupt
abundance decline in contrast to slower diversity change at
El Kef and Millers Ferry obviously indicates a reworked
origin of the Cretaceous taxa in basal Danian. Since
reworked specimens of extinct taxa gradually diminish
following the abrupt decline at the KT boundary, with
smaller sized and more abundant Cretaceous taxa reworked
higher than the larger sized and rarer species, the apparent
diversity (the number of reworked taxa) will decrease sample
by sample. If these reworked taxa are regarded as surviving
indigenous species [2-5], a gradual, or stepwise extinction
pattern would be concluded.

Application of phylogenetic criterion led us [7-8,10-11] to
recognize three species from two separate stocks as
survivors. However, phylogenetic criterion will not
recognize survivorship of species without descendants (if
they did indeed survive). The exact number of survivor
species is also affected by taxonomy. For instance, we [7-8]
recognize three survivor species with Guembelitria cretacea
as the only triserial species. When Keller and co-authors
[2-5] split Guembelitria into 4 species (irregularis,
trifolia, danica in addition to cretacea), survivorship of
this triserial morphotype quadrupled by this taxonomic
treatment.

Local Events and Global Change. To solve the problem whether
the KT mass extinction is an accumulation of a series of
events or a single event, one important aspect to
investigate is the diversity change across the KT boundary
in a variety of environments. Recent studies by Keller [3-4]
concluded that gradual environmental perturbations for
~200,000-300,000 years before and after the KT led to
"gradual mass extinction." They also suggested that the KT
extinction is mainly a low latitude phenomenon while no
major extinction coincides with the KT boundary in high
latitudes.

In order to separate local environmental perturbations from
a global event, we compare the Late Cretaceous diversity
change of planktonic foraminifera in various
paleoenvironments in high and low latitudes. The data are
from tropical site El Kef, low latitude Gulf Coast of
Mexico, southern high latitude ODP Site 738C, and northern
high latitude Danish section (Nye Klov) [2-8]. The diversity
in four of the five sections are nearly constant although
they show contrasting background diversity in the Upper
Cretaceous. The only section that displays significant
decline in diversity in the Upper Cretaceous is at Braggs,
Alabama. The number of species declined from 28 at 50 cm to
21 at 20 cm, 22 at 10 cm, and further to 17-18 species at 2-
5 cm below the KT boundary. These changes cannot be due to
gradual global extinctions because during the same interval
the diversity basically remains constant (40-43) at Millers
Ferry, which is only 34 miles west of Braggs. The reason for
the diversity decline at Braggs is a change in paleodepth.
In the sampled interval, the planktonic-benthic foraminifer
ratio (higher P/B ratio indicates deeper paleodepth) also
drops from 53% to less than 8%, i.e., from an outer shelf to
an inner shelf environment. The Nye Klov section, as at
Braggs, was deposited in a marginal marine environment
characterized by a low diversity planktonic fauna (5-10
species) if all triserial guembelitriids are regarded as
Guembelitria cretacea. The two "stepwise" declines (16 to 14
species and 14 to 12 species at 103 and 4 cm below the KT
boundary respectively) at ODP Site 738C are not compatible
with either the stepwise or the gradual extinction
hypotheses if the quality of preservation and poor core
recovery (8-10%) are considered. To the contrary, no matter
what the diversity was in the Cretaceous, an abrupt decline
to approximately 3 species occurred right at the KT boundary
in all sections regardless of the latitudial position or
paleoenvironment of the section. This dramatic change
occurred in an interval of less than ~3000 years and
therefore is considered a geologically instantaneous event
[11].

In conclusion, an equivocal temporal framework, an illogical
interpretation of the observed data, and taking local events
for global phenomenon are three major sources of artifacts
in KT related studies. If the change in planktonic
foraminifer biota is viewed globally, in the evolutionary as
well as stratigraphic context, and geologic processes are
fully understood, one would conclude that extinction of
planktonic foraminifera at the end of Cretaceous is a
single, instantaneous, global event.

References:n[1] Brinkhuis H. and Zachariasse W. J. (1988)
Marine Micropal., 13, 153-191.[2] Keller G. (1988) Marine
Micropal., 13, 239-263. [3] (1989) Paleocean., 4, 287-332.
[4] (1993) Marine Micropal., 21, 1-45. [5] et al. (1993) GSA
Bull., 105, 979-997. [6] Smit J. (1982) GSA Special Publ.,
190, 329-352. [7] Liu C. and Olsson R. K. (1992) JFR, 22,
328-346. [8] Olsson R. K. and Liu C. (1993) PALAIOS, 8, 127-
139. [9] Canudo J. I. et al. (1991) Marine Micropal., 17,
319-341. [10] Liu C. and Olsson R. K. (1992) GSA, A29. [11]
Liu C. (1993) Ph.D. Thesis, Rutgers, 189.


Gamper M. A.   Longoria J. F.
   Periodicy in Planktonic Formanifera Evolutionary Trends: Facts and Causes

No abstract available.


Barrera E.   Huber B. T.
   Stable Carbon Isotopic Evidence for Cretaceous Planktic Species Survivorship
   and Reworking

The pronounced turnover in planktic foraminiferal faunas at
the time of the Cretaceous/Tertiary (KT) boundary is well
documented (e.g., [1,2]). Large and complex forms such as
the globotruncanids, which are abundant in sediments
deposited in the last several million years of the
Cretaceous, were replaced by relatively small and primitive
forms in the early Paleocene. Yet, the sedimentary record of
the lowermost Danian shows the presence of several
Cretaceous taxa, mostly cosmopolitan species, suggesting
that their extinction may have been gradual (lasting up to
several hundreds of thousand years) rather than abrupt or
instantaneous as a consequence of the KT boundary event [3-
5]. Analysis of the delta^13C composition of the tests of
these species provides the opportunity of testing their
survivorship. This is because the KT boundary event is
characterized in low and mid latitudes by a pronounced
decline in near-surface water carbonate delta^13C values of
up to several permil with a virtual elimination of the
surface to bottom delta^13C gradient, reflecting a decrease
in productivity in the lowermost Danian [6-8]. In the high
latitudes the KT boundary event appears to have had a less
pronounced environmental effect since near-surface water
carbonate delta^13C appears to remain constant at the KT
boundary. However, there is a significant delta^13C decrease
in the upper portion of Nannofossil Zone NP1 (lower portion
of Magnetochron C29N), which is then followed by an increase
in delta^13C values in lowermost Zone NP2 [9].

In the low latitudes (Brazos River, Texas), stable C
isotopic evidence has been used to demonstrate survivorship
of the Cretaceous planktic foraminiferal species Heterohelix
globulosa and Guembelitria cretacea up to the top of
Magnetochron C29R and in C28R, respectively [8]. This was
established by delta^13C values that are 2.5 permil higher
in tests recovered from Cretaceous sediments than tests from
the Danian sequence (125-200 micrometer-size fraction). In
contrast, delta^13C values of these taxa from the northern
boreal section in Nye Klov, Denmark [9] (63-120 micrometer-
size fraction) exhibit no significant change across the KT
boundary, and have similar values in uppermost Maastrichtian
and lowermost Danian sediments. In southern high latitude
deep-sea Hole 738C, delta^13C values of the Cretaceous
survivor planktic species Zeauvigerina waiparaensis also
remains constant at the boundary and decrease subsequently
at the top of Zone NP1. A similar pattern is observed at
this site in the delta^13C record of the Tertiary planktic
species Eoglobigerina eobulloides and Subbotina
pseudobulloides, indicating that the NP2 negative delta^13C
excursion is not an artifact of sediment reworking across
the KT boundary at this site.

No isotopic record of a surviving surface-dwelling taxa is
yet available to document Danian surface water conditions.
The delta^13C values of these surviving species in
Maastrichtian sediments indicate a deep habitat within the
water column. For example, the similarity in delta^13C
values of the Cretaceous species Globigerinelloides
multispina in uppermost Maastrichtian and Danian sediments
through the base of NP2 at Site 738 (this study), Site 750
[10], and Site 690 [11] suggests that this surface-dwelling
species did not survive the KT boundary event. Stable
isotopic analyses of additional Cretaceous planktonic
foraminiferal species that occur in Danian sediments at Site
should reveal whether these survived the terminal Cretaceous
extinction event or were reworked.

References: [1] Luterbacher H. P. and Premoli Silva I.
(1964) Riv. Ital. Paleont., 70, 67-128. [2] Smit J. (1982)
GSA Spec. Paper, 190, 329-352. [3] Keller G. (1988) Mar.
Micropaleontol., 13, 239-263. [4] Keller G. (1989)
Paleoceaogr., 4, 287-332. [5] Huber B. T. (1991) in Proc.
ODP Sci. Res., 119 (J. Barron et al. eds.), 451-466. [6]
Zachos J. C. and Arthur M. A. (1986) Paleoceanogr., 1, 5-26.
[7] Hsu K. J. et al. (1982) GSA Spec. Paper, 190, 317-328.
[8] Barrera E. and Keller G. (1990) Paleoceanogr., 5, 867-
890. [9] Barrera E. and Keller G. (in press) Bull. Geol.
Soc. Amer. [10] Zachos J. C. et al. (1992) in Proc. ODP Sci.
Res., 120, R. Schlich et al. eds.), 961-977. [11] Stott L.
D. and Kennett J. P. (1990) in Proc. ODP Sci. Res., 113, (P.
F. Baker et al. eds.), 829-848.


Gartner S.   Alcala J.   Grossman E.
   Coccolithophore Extinction at the KT Boundary: Gradual or Abrupt

The past fifteen years have witnessed a staggering
expenditure of effort in an attempt to understand the events
that surround the KT boundary. Yet with all that effort some
very fundamental issues remain unresolved, issues that go to
the heart of the problem. One unanswered question remaining
is what precisely was the pattern of the extinction of the
various groups of organisms, the very issue that first
defined the problem and, indeed, continues to frame it to
this day. Few deny that an extensive faunal and floral
turnover occurred near the KT boundary. But even the closest
scrutiny of the record has failed to yield unambiguous
evidence that the demise of a particular group of organisms
was massive and near instantaneous, as would be expected,
indeed required, from a catastrophe such as a large impact.
No group of organisms, it seems, has left a record that does
not lend itself to contradictory interpretations. The
principal problem is the inherent lack of resolution to
allow discrimination of events that may have occurred over a
time span of perhaps less than a decade to a century, but
certainly not more than a millennium. The problem is
complicated further by ubiquitous redeposition, which seems
to be more prevalent at the KT boundary than in any other
interval of the geological record (except perhaps the
Pleistocene).

One of the first groups to have its remarkable turnover at
the KT boundary documented in detail were the coccoliths
(calcareous nannofossils), a group of marine planktonic
algae that, like terrestrial reptiles, thrived during the
Cretaceous as never before and probably not ever thereafter.
When catastrophic extinction mechanisms were proposed the
coccolith record was again scrutinized, this time more
closely than before. It became clear that the species
abundance pattern alone could not demonstrate conclusively
whether extinction of Cretaceous species was geologically
instantaneous (on a time scale of decades to centuries),
which would require that all of the many Cretaceous species
occurring in Danian sediments were redeposited, or whether
some Cretaceous species actually survived into earliest
Danian time. At some boundary sections stable C isotope
ratios in the abundant calcite of the sediments seem to
indicate survival of some Cretaceous coccolithophores into
Danian time: delta^13C values obtained from early Danian
samples in which presumed redeposited Cretaceous species
dominated, were so different from a delta^13C values of
average latest Maastrichtian (latest Cretaceous) samples
from immediately below the boundary at the same location as
to require unrealistic values for the indigenous Danian
species that contributed only a small proportion of the
biogenic carbonate and, hence, of the delta^13C signal in
the early Danian sample [1].

It is important to note that coccoliths constitute much of
the fine fraction of the carbonate in these sediments, and
because it is very difficult to isolate coccoliths from
other fine sediments, all C isotope determinations were made
on whole rock samples. The assumption made in these
interpretations is that a delta^13C values of the whole rock
samples are in all cases an adequate approximation of the a
delta^13C values of the coccoliths in the samples. A further
assumption is that the whole rock delta^13C value is a
composite of the contribution from the "Cretaceous"
coccolith and Danian coccolith delta^13C values, with each
contributing to the C isotope signal in the proportion in
which the coccolith calcite occurs in the sample. Thus the
delta^13C data lead to the conclusion that some Cretaceous
coccolithophores survived into early Danian time and
actually formed coccoliths with a typical Danian a delta^13C
signature.

In order to test this conclusion a study was undertaken on
two deep sea KT boundary sections, one at DSDP Site 527, the
other at DSDP Site 529, both from Walvis Ridge in the
eastern South Atlantic. For this study C isotope
determinations were not made on whole rock samples. Instead
the samples were repeatedly processed by settling from
suspension in a water column, thereby incrementally removing
all particles that were larger or smaller than coccoliths.
Slides were prepared at each successive step and compared to
slides prepared from the previous step, to make certain that
no undue number of coccoliths was lost from either end of
the size range of the various species. In no case was it
possible to remove all non-coccolith debris from a sample,
but the final concentrate of coccoliths contained
significantly less non-coccolith particles than did the
initial unprocessed material. The residue remaining at the
end of the processing was used to determine O and C isotope
ratios for that sample.

From the measured C isotopic composition of the sample was
calculated the delta^13C of the earliest Danian indigenous
coccoliths, using the same mathematical expression used by
Perch-Nielsen [1]

delta^13C(sub)m = f(sub)K(delta^13C(sub)K) +
f(sub)T(delta^13C(sub)T)

where delta^13C(sub)m is the measured C isotope ratio of the
sample; delta^13C(sub)K is the average C isotope ratio of
the latest Maastrichtian samples (coccoliths) immediately
below the KT boundary; and delta^13C(sub)T is the
(calculated) C isotope ratio of earliest Danian coccoliths.
The two values f(sub)K and f(sub)T represent the proportion
of Cretaceous and Tertiary coccoliths, determined for each
sample from the proportion of area covered by Cretaceous and
Danian coccoliths in a representative number of fields in
the light microscope. (For the purpose of the calculations
it is assumed that all Cretaceous coccoliths occurring above
the KT boundary have been redeposited). The resulting data
are plotted on Fig. 1. The largest excursion of the
calculated delta^13C(sub)T from the measured delta^13C(sub)m
at both sites is from the sample at or closest to, but
above, the boundary; at Site 527 the excursion is -3.63
permil, at Site 529 it is O.85 permil. At both sites there
is a distinct shift of the C isotope ratios in the negative
direction but at neither site do the calculated Danian
values seem so unreasonable as to require that the
Cretaceous species that dilute the signal must have lived in
Danian seas. Our preferred interpretation, therefore, is
that Maastrichtian species probably did not survive into
early Danian time and, where they occur within earliest
Danian sediments, their occurrence should be ascribed to
redeposition.

If this conclusion is correct, it raises two other difficult
questions. The first is that if the epiplanktonic Cretaceous
coccolithophores became extinct essentially in a single
event at the boundary, then why were some Cretaceous
planktonic foraminifera living in the same environment able
to survive into early Danian time? There seems little doubt
that some planktonic foraminifera did indeed survive. The
second question is what component of unprocessed early
Danian sediment is contributing the intensely negative
isotope signal that other investigators have found, but
which clearly does not seem to be associated with the
calcareous nannoplankton. It seems there are still more
questions than there are answers.

References: [1] Perch-Nielsen K. et al. (1982) GSA Spec.
Paper, 190, 353-371.

Fig. 1, which appears here in the hard copy, shows C isotope
ratios in lates Maastrichtian and earliest Danian calcareous
nannofossil concentrates at DSDP sites 527 and 529, Walvis
Ridge, eastern South Atlantic.


Keller G.   Li L.   MacLeod N.
   The KT Boundary Stratotype Section at El Kef, Tunisia: How Catastrophic Was
   the Mass Extinction?

The El Kef section of Tunisia was officially designated the
Cretaceous-Tertiary (KT) boundary global stratotype section
and point (GSSP) at the XXVIII International Geological
Congress in Washington in 1989. Chosen for its apparently
continuous and expanded sedimentary record, excellent
preservation of microfossils, and geochemical and
mineralogical marker horizons, the stratotype provides an
ideal sequence by which other sections can be compared and
correlated worldwide. As such the El Kef stratotype has
become the single most important KT boundary section and the
standard by which the completeness of the faunal, floral,
and sedimentary records are judged worldwide. Such a
comparison was recently published by MacLeod and Keller [1]
based on graphic correlation of over 30 KT boundary sections
worldwide. Results show that the El Kef section indeed
represents the most expanded and stratigraphically complete
KT boundary transition know to date. Similar nearly complete
sections also occur at Brazos River (Texas), Agost and
Caravaca (Spain), Nye Klov (Denmark), Mimbral (Mexico), and
ODP Site 738C.

Placement of the KT Boundary: The KT boundary is easily
identified in the El Kef stratotype or elsewhere based on
the following criteria: (1) A lithologic break from chalk or
marl deposition of the Cretaceous to a thin layer of dark
organic-rich and CaCO3 poor clay, known as the boundary
clay. At El Kef this layer is 55cm thick and represents the
most expanded boundary clay observed to date in any KT
boundary sections. More frequently, the boundary clay is
only 4 to 6cm thick (e.g., Agost, Caravaca, Stevns Klint,
Nye Klov, El Mimbral [3,4]. (2) A 2-3mm oxidized red-layer
at the base of the boundary clay. This red layer is present
at El Kef as well as in all complete KT boundary sections.
(3) Maximum Ir concentrations generally concentrated in the
red layer and boundary clay although they may trail several
tens of centimeters above the boundary clay. At El Kef
maximum Ir concentrations are found in the red layer and
values decrease through the boundary clay [5]. (4) Ni-rich
spinels frequently present in the red clay or base of the
boundary clay as also observed in El Kef [5]. (5) A negative
delta^13C shift in marine plankton of low and middle
latitudes as also present at El Kef [6]. (6) The first
appearance of Tertiary planktic foraminifera at the base or
within a few cm of the boundary clay, red layer, Ir anomaly,
and Ni-rich spinels. At El Kef, the first Tertiary species
(G. conusa) appears at the base of the boundary clay and
three other new species appear within the basal 15cm of the
boundary clay [10,17].

In the stratotype section at El Kef all of these criteria
are met and most of them are present in all the best and
most complete KT boundary sequences (e.g. Agost, Caravaca,
Nye Klov, Brazos River, El Mimbral, ODP Site 738C). The
coincidence of these lithological, geochemical, and
paleontogical criteria is unique in the geological record
and virtually insures that the stratigraphic placement of
the KT boundary is uniform and coeval in marine sequences
across latitudes. Any of these criteria used in isolation,
however, diminish the stratigraphic resolution of the KT
boundary.

Selectivity and Gradual Mass Extinction Pattern at El Kef:
The El Kef stratotype has been extensively studied based on
planktic foraminifera [7-10], benthic foraminifera [9],
ostracods [11], nannoplankton [12], dinoflagellates [8],
palynoflora [13], isotope geochemistry [6], and Ir and Ni-
rich spinels [5]. With the exception of Smit's [7] report,
none of these microfossil studies record a catastrophic mass
extinction horizon at the KT boundary. Instead, some
microfossil groups record gradual species extinctions across
the KT boundary whereas others show no significant species
extinctions. For instance, Meon [13] reports that there is
"no drastic modification at the KT boundary and hence no
phenomenon of catastrophic dimension." Similar observations
have been made in palynofloral studies from northern and
southern high latitudes [14] and from Brazos River, Texas
[15]. Only in the U.S. western interior (Raton Basin) are
sharp floral changes, including a fern spore spike, observed
coincident with KT boundary [16]. Dinoflagellates have long
been known to have survived the KT boundary event virtually
unscathed and studies at El Kef confirm this observation.
Brinkhuis and Leereveld [8] report "No sharp qualitative
changes in dinoflagellate cysts associations coincide with
that boundary." Nannofossil diversity (Shannon-Wiener
diversity index) shows a gradual decline into the earliest
Tertiary (Danian) with most Cretaceous species present well
into the early Tertiary [12]. This record has proved
difficult to interpret because nannofossils are easily
reworked due to their small size and there is no unequivocal
criterion that can be used to distinguish reworked from in
situ specimens.

Planktic foraminifera have been considered as the
microplankton group that suffered virtually total extinction
due to a bolide impact at KT boundary time at El Kef and
elsewhere [3,7]. This interpretation could not be supported
by subsequent studies for three reasons. First, this
conclusion was originally based on deep-sea sections, all of
which have been subsequently shown to contain major
hiatuses. MacLeod and Keller [1] demonstrated that boundary
hiatuses spanning from 50kyr to 500 kyr of the basal
Tertiary are present in nearly all deep-sea sections
deposited at depths below 1000m. In these sections
extinctions and originations, spanning as much as 0.5 my
across the KT transition, are artificially concentrated at
one horizon giving the appearance of a sudden catastrophic
mass extinction. In contrast, shallow continental shelf
sections, such as El Kef, are generally more complete and
show a more gradual transition.

Second, the presence of many Cretaceous taxa in lowermost
Tertiary sediments has been routinely ignored based on the
assumption that all Cretaceous species (except one) must
have become extinct at the KT boundary. Their presence in
Danian sediments is therefore assumed to be due to
reworking. For instance, at El Kef Smit [7] reported that
"The mass extinction event at the KT boundary...exterminated
all but one species of the planktonic foraminifera
(Guembelitria Cretacea Cushman)." In contrast, Keller [2,17]
found that about 1/3 of the Cretaceous taxa are consistently
present in early Tertiary sediments at El Kef and elsewhere
and their presence could not be considered due to reworking,
but indicated survivorship. Moreover, the simple morphology
and small size of survivor taxa imply that these species
were generalists able to tolerate a wide range of
environmental conditions. Although Smit [3,7] also observed
these Cretaceous species in early Tertiary sediments at El
Kef as well as Agost and Caravaca, he chose to ignore them
(no indication of their presence in range charts) in the
belief that they were reworked. Survivorship of cosmopolitan
taxa has since been documented in KT boundary sections
worldwide with isotopic measurements of shell calcite of
several taxa providing unequivocal evidence of their
residence in Tertiary environments [4].

Third, species extinctions are often assumed to have
occurred instantaneously due to a bolide impact. Smit [7]
stated that "the mass extinction is inferred to have
occurred within 50 years and a new stable planktonic fauna
re-established within 35,000 years." Biostratigraphic data
from El Kef and Agost provided by Smit [3,7] in support of
this interpretation show the extinction of all Cretaceous
taxa (except G. cretacea) precisely at the KT boundary even
though many of these same species are commonly present in
overlying Danian sediments [2,4,10,17]. Moreover, Smit's El
Kef study identified only about a dozen Cretaceous taxa and
most of these are not identified to species level. In
contrast, Keller [2,17] and Liu and Olsson [18] identified
48 and 54 Cretaceous species respectively in the same
samples of the same section. We have also observed that
species extinctions began before the KT boundary and
continued well into the Tertiary. Moreover, species
extinctions appear sequential with complex, large, ornate
deep water dwelling forms disappearing first and smaller,
less ornate forms surviving longer. All deep dwelling
species disappeared by KT boundary time and only surface
dwellers survived into the Tertiary.

How catastrophic was the mass extinction at El Kef? The
severity of a mass extinction is generally measured by the
number of species extinct giving equal weight to extinction
of species that are very rare as to those that dominate the
assemblage while ignoring dramatic changes in relative
abundances of species that survived. A more reliable measure
of the catastrophic nature of mass extinctions would include
measurement of the relative abundance of species extinct and
the relative abundance changes of survivors because the
extinction of rare taxa may be caused by minor environmental
perturbations. By using this measure, the combined relative
abundances of all 72% species of planktic foraminifera
extinct between 50cm below to 10cm above the KT boundary at
El Kel, account for an average of less than 20% of the
planktic foraminiferal population (Fig. 1). While this
certainly represents a major mass extinction in low
latitudes, it is not of the magnitude generally quoted. If
we also consider the fact that no significant species
extinctions occurred in high latitudes [4], we must
seriously consider a killing mechanism that was not only
selective across organismal groups and species selective
within organismal groups, but also restricted to low
latitudes.

References: [1] MacLeod N. and Keller G. (1991) Geology, 19,
497-501; MacLeod N. and Keller G. (1991) GSA Bull., 103,
1439-1457. [2] Keller G. (1988a) Mar. Micropaleo., 13, 239-
263. [3] Smit J. (1990) Geol. Mijnbouw, 69, 187-204; Smit J.
personal communication, (1992). [4] Canudo J. I. et al.
(1991) Mar Micropaleo., 17, 319-341; Keller G. (1989)
Paleoceanography, 4, 287-332; Keller G. (1993) Mar.
Micropalea., 21, 1-45; Keller G. et al. (in press) Palaios;
MacLeod N. and Keller G. (in press) Paleobiology. [5] Robin
E. et al (1991) EPSL, 107, 715-721. [6] Smit J. et al.
(1982) Cret. Res., 3, 307-332; Kuslys M. and Krahenbuhl U.
(1983) Radiochim. Acta., 34, 139-141; Keller G. and
Lindinger M. (1989) Paleo., 3, 73, 243-265. [7] Smit J.
(1982) GSA Spec. Paper, 190, 291-296. [8] Brinkhuis H. and
Leereveld H. (1988) Rev. Paleobot. Palynol., 56, 5-19;
Brinkhuis H. and Zachariasse W. J. (1988) Mar. Micropale.,
13, 153-191. [9] Keller G.(1988b) Paleo., 3, 66, 153-171;
Keller G. (1992) Tokai Univ. Press, Tokyo, 77-91. [10] Ben
Abdelkader O. B. (1992) Geol. Survey Tunisia, 9-10. [11]
Donce P. et al. (1982) Bull. Cent., Rech, Explor. Prod, Elf-
Aquitane, 6, 307-335; Peypouquet J. P. et al. (1986) Geol.
Rundschan, 75/1; 159-174. [12] Perch Nielsen K. (1981) Cah.
Micropaleont., 3, 7-23; Jiang and Gartner (1986) Micropaleo
32, 232-255; Pospichal J. (in press) Geology. [13] Meon H.
(1990) Rev. Paleobot. Palynol., 65, 85-94. [14] Sweet A. R.
et al. (1992) GSA Spec. Paper, 190, 457-470; Askin R. A.
(1988) GSA Mem., 169, 131-156; Askin R. A. (1992) Antartic
Journal of the U.S., 25, 42-44. [15] Beeson D. et al. (in
press) Palaios. [16] Tschudy R. H. et al (1984) Science,
225, 1030-1032. [17] Keller G. et al., Paleo., 3.
(submitted). [18] Liu C. and Olsson R. K. (1992) J. Foram.
Res., 22, 328-346.


Donze P.   Meon H.   Rocchia R.   Robin E.   Froget L.
   Biological Changes at the KT Stratotype of El Kef (Tunisia)

Introduction: KT boundary sediments are characterized by a
strong enrichment in Platinum Group Elements (PGEs) and the
presence of exotic minerals (shocked quartz and meteoric
spinels). A cosmic collisional origin for all these markers
is now widely accepted. However, the biological consequences
of this event are still a subject of debate. The site of El
Kef (Tunisia) offers exceptional conditions permitting the
precise stratigraphical study of the development of the
biological crisis.

Site description: The paleontological boundary is
exceptionally well defined by a sharp lithological
discontinuity. The top of the A. Mayaroensis zone is marked
by a thin millimetric brown-reddish layer containing Ir and
meteoric spinels: the signature of the cosmic event [1,2].
Above that layer there is a 10-12 cm bed of dark grey marl
overlain by 5 cm of nearly pure and azoic black clay. Above
that, we observe a slow increase of the carbonate fraction
indicating the recovery of the biological activity. Stable
sedimentation conditions are found again about 2 meters
above the boundary.

Floral and faunal populations: The upper Maastrichtian is
extremelly rich in pelagic and benthic microorganisms:
Foraminiferas, Ostracods, Coccoliths, Dinoflagellates, etc.
It also contain pollens and spores from the nearby continent
(Kasserine island).

Evolution of the marine environment: In the upper
Maastrichtian, one can identify at least, 30 species of
Foraminifera, 15 species of calcareous Nannofossils [2], 30
species of Dinoflagellates, and 15 species of Ostracods [3].
No significant change is observed till the appearance of
meteoric spinels in the stratigraphic column. Immediately
above the millimeter thick layer containing spinels and
maximum Ir concentration, we note that 90-95 % of the
populations disappear. The surviving specimens of
Foraminifera, Coccoliths, and Ostracods are characterized by
reduced sizes, unhealthy aspect and badly calcified and
fragile tests. In the 10-12 cm of marls above, the plankton
gets progressively extinct. Some species seem more resistant
(Gumbeletria cretacea). The reaction of benthos is
different: only simple and resisting organism survived
(Lenticulina sp.). This progressive degradation ends in the
black clay layer where we have found only small benthic
arenaceous tests.

As far as Dinoflagellates and other palynomorphs are
concerned they do not exhibit such a dramatic extinction
pattern. Counting reveal (after correction for the
variations of the sedimentation rate) a reduction by only a
factor of two of the number of specimens [4]. This moderate
decrease is strikingly different from the collapse of
productivity characterizing microorganisms with calcareous
tests. Dinoflagellates are known to survive in water with
low nutrient concentrations. This is a possible explanation
for their survival in an environment which could not be
accepted by other species.

Evolution of the terrestrial environment: Because of the
proximity of emerged lands (Kasserine island) the site of El
Kef is extremelly rich in land plant remains, pollen grains,
and spores. We do not observe at El Kef any sharp change in
continental plant populations. Early extinctions of species
are observed well inside the Maastrichtian and continue
across the KT boundary and the Danian. These gradual
extinctions appear to be related to long duration climatic
changes and not to the cosmic boundary event.

Conclusion: The coincidence, now well established, of a
cosmic material infall with the sharp decrease of the marine
productivity implies a causal link between the two
phenomena. The occurrence of extinctions is obvious but the
way the extinctions occurred is far from being understood.
Some points, however, appear rather clear. Faunal and
geochemical analyses show that the environment became
suddenly highly reducing right at the time of the cosmic
event. This change is likely due to the phytoplankton whose
development is strongly modulated by the intensity of solar
radiations. It has been proposed that the dust blanket
produced by the alleged KT impact might have resulted in a
strong reduction of the solar radiation income and,
consequently, in a drastic reduction of photosynthesis [5].
This hypothesis is attractive and widely accepted. However,
at El Kef it raises some difficulties. First, the decrease
of solar light should have affected not only planktonic
species but also land plants. This is not what we observe:
sporomorph populations do not significantly change across
the boundary. Second, it does not explain the very different
behavior of calcareous planktonic species and
Dinoflagellates, which have chitinous tests. Planktonic
species got nearly totally extinct when most
Dinoflagellates, which also need light for their
photosynthesis, survived. In fact, in most sections (with
the possible exception of the sections in the Caribbean
area) we have analyzed so far, the observed amount of ejecta
is always very low. In El Kef, for instance, the total
ejected material is represented by the 1-2 millimeter thick
layer containing spinels. This is far below the amount
predicted by models (10-100 times the impactor mass).

The cause of this difference could be, instead of a decrease
of the solar flux, a disturbance of chemical conditions in
the ocean. A significant infall of some elements (Ni for
instance) could have prevented or reduced the action of
usual oligoelements (Fe, Cu, Mn) which are involved in life
processes and, in particular, in the fixation of carbonates.
This could explain the extinction of calcareous planktonic
species and, as a consequence, the decrease of oxygen which
is mainly provided by these species in warm oceanic waters.

The survival of Dinoflagellates, which at high latitudes are
efficient oxygen providers, could explain why the biological
crisis was less severe in northern seas than in tropical
regions [6].

References: [1] Robin E. et al. (1991) EPSL, 107, 715-721.
[2] Perch-Nielsen K. (1981) Cahiers de Micropaleontologie,
3, 25-36. [3] Donze P. (1992) Int. Workshop on KT
Transitions (El Kef section), April 1992 Tunis, abstracts,
2-3. [4] Mion H. (1991) Palaeontographica, Abt. B, 223, 107-
168. [5] Alvarez et al. (1980) Science, 208, 1095-1108. [6]
Keller G. et al. (1993) GSA, 105, 979-997.


Askin R. A.   Elliot D. H.   Jacobson S. R.   Kyte F. T.   Li X.
Zinsmeister W. J.
   Seymour Island: A Southern High-Latitude Record Across the KT Boundary

Exposures on Seymour Island, Antarctica, provide an
apparently continuous, shallow marine, siliciclastic record
across the Cretaceous Tertiary (KT) boundary. At the end of
the Cretaceous, Seymour Island was located at nearly the
same latitude as today (64 degrees S), and thus at a mid to
high southern latitude for this geologically important
interval. Exposures of Campanian to Tertiary rocks on this
island form part of the James Ross Basin [1,2], a back-arc
basin of Early Cretaceous (Barremian) through Eocene age
lying east of the northern Antarctic Peninsula. The KT
boundary occurs near the top of the Lopez de Bertodano
Formation, a monotonous, richly fossiliferous, mud to silty
sand unit deposited under low energy, shallow marine
conditions. Ten members are recognized in the Lopez de
Bertodano Formation; the KT boundary is in a several meters
thick glauconitic interval at the base of member 10.

The glauconitic interval crops out for 5.5 km along strike
on southern Seymour Island.The highest ammonites are found
within the lower part of the glauconitic interval, which is
above the level of the highest preserved Cretaceous
planktonic foraminifera. Invertebrate faunas [3] exhibit a
long term, gradual turnover from upper member 9 into member
10. The glauconitic interval is only sparsely fossiliferous,
except for dinoflagellate cyst assemblages, spores and
pollen, and particulate organic material (kerogen).
Calcareous nannofossils and planktonic foraminifera are not
preserved in the glauconitic interval; only non-age
diagnostic benthonic forms remain [4]. The absence of the
more fragile calcareous microfossils may be caused by
dissolution during diagenesis, by depositional chemistry, or
by modern weathering processes. Siliceous microfossils [5]
are too sparse over the KT transition to be
stratigraphically useful.

Environmental conditions and dinoflagellate cyst assemblages
remained relatively stable throughout most of the
Maastrichtian on the Seymour shelf, with dinocyst assemblage
change beginning in the latest Maastrichtian (in member 9)
and continuing into the Danian [6]. Spread over several tens
of centimeters, there is a major compositional change from
Manumiella dominated assemblages to Senegalinium obscurum-
dominated asssemblages [7] that are followed by more varied
Paleocene assemblages. However, palynomorph remnants of the
adjacent land vegetation show no evidence of major
environmental upheaval. Dominant constituents of the
terrestrial palynoflora (spores and pollen) continue
unchanged from upper Maastrichtian into Danian sediments
[8]. Minor elements of the terrestrial flora change
gradually from the upper Maastrichtian to lower Danian,
possibly in response to climate shifts.

No single studied horizon exhibits evidence of sudden
catastropic change at or near the KT boundary on Seymour
Island. Furthermore, the gradual floral and faunal turnover
makes it difficult to pinpoint the boundary. The ubiquitous
dinoflagellate cyst assemblages indicate a narrow KT
boundary transitional zone (~30 cm at "Filo Negro" section).
A sharp Ir anomaly within this narrow stratigraphic interval
at two separate sites identifies the best candidate horizon
for the KT boundary.

Following reconnaisance analysis of a stratigraphic section
collected at 0.5 m intervals (e.g., [3,7]), a short (16 m),
well-exposed section (Filo Negro) was collected at 10 cm
intervals for more detailed analysis [9]. This section
comprises unconsolidated, glauconitic silty sands, with a
richer glauconitic interval between 7.7 m and 10.9 m. There
is no observable lithologic expression of the KT boundary,
which occurs in the middle of this glauconite-rich unit.
Sedimentation is believed to have been continuous, albeit
slow, through this interval. Total Organic Carbon (TOC)
values remain low (most below 0.4 wt%) throughout the
glauconite unit and exhibit no major perturbations in trend.
Kerogen analyses indicate the organic assemblages are
dominated by inertinite (oxidized, inert matter), with
lesser amounts of woody material (some degraded), and
relatively small proportions of palynomorphs and plant
cuticle. The depositional environment for the KT transition
on Seymour Island was below wave base, in low energy,
somewhat oxidizing conditions, on a very wide (tens of
kilometers?) shallow shelf.

"Typically Paleocene" dinocyst species show gradual first
appearances, from strata of definite Maastrichtian age
beneath the glauconite unit, to within the glauconite unit.
Some of these species also appear first in uppermost
Maastrichtian strata in the New Zealand KT section at
Waipara [10]. Their first appearances elsewhere in the world
are in Danian beds. However, one species, Danea californica,
is not currently known below the KT boundary anywhere. It is
found at levels 9.6 and 9.7 m in the Seymour Filo Negro
section. The 9.7 m horizon is notable because it is the
level of highest marine dinocyst diversity, lowest "marine
dominance" (an offshore indicator [11]), and highest
relative abundances of offshore chorate dinocysts. This
horizon with its diagnostic palynomorph suite is found at
several localities along strike and is believed to record a
marine transgressive pulse. We consider it a valuable,
easily identified chronostratigraphic marker, and correlate
it with the flooding event found in earliest Danian sections
elsewhere ([e.g., [12]).

A single peak in Ir concentration was found [9] at the 9.4 m
level (1.6 ppb Ir, 40x background). This sharp peak, which
occurs 30 cm below the earliest Danian flooding horizon and
in the uppermost Manumiella dinocyst zone, is presumed to
represent the main KT "iridium event." We consider it a
chronostratigraphic horizon marking the KT boundary.
Although the Ir peak is commonly assumed to reflect a bolide
impact, there is no discernible direct effect on the floras
and faunas at this location. The processes responsible for
the systematic changes in latest Maastrichtian-earliest
Danian Seymour Island floras and faunas may be related to
one or more noninstantaneous changes in sea level, in water
chemistry, and in climate.

References: [1] Elliot D. H. (1988) GSA Memoir, 169, 541-
555. [2] Del Valle R. A. et al. (1992) Antarc. Sci., 4, 477-
478. [3] Zinsmeister W. J. et al. (1989) J. Paleont., 63,
731-738. [4] Huber B. T. (1988) GSA Memoir, 169, 163-252.
[5] Harwood D. M. (1988) GSA Memoir, 169, 55-129. [6] Askin
R. A. (1988) GSA Memoir, 169, 131-153. [7] Askin R. A.
(1988) GSA Memoir, 169, 155-162. [8] Askin R. A. (1990) Rev.
Palaeobot. Palyn., 65, 105-113. [9] Elliot D. H. et al. (in
prep. for Geology). [10] Wilson G. J. (1987) NZGS Record,
20, 8-15. [11] Goodman D. K. (1979) Palyn., 3, 169-190. [12]
Brinkhuis H. and Zachariasse W. J. (1988) Mar. Micropal.,
13, 153-190.



Zinsmeister W. J.*   Feldmann R. M.
   Antarctica, the Forgotten Stepchild: A View of KT Extinction from the High
   Southern Latitudes

Considerable hyperbole has been published during the last
decade about the great terminal Cretaceous extinction event.
With the publication of the Impact Hypothesis in 1980, a
vocal segment of the scientific community and the media have
accepted as factual the impact scenario as the primary cause
for the extinction at the end of the Cretaceous, and for the
dinosaurs, in particular. It is quite evident that a major
biotic crisis occurred 64 mya; but, for many in the geologic
community, the cause is far from clear. The discovery of an
impact crater that may have formed at the end of the
Cretaceous proves only that there was am impact at the end
of the Cretaceous. The crater in itself does not provide the
final piece of evidence to explain the demise of the
dinosaurs or the wholesale loss of life at the end of the
Cretaceous.

If the K/T extinction event resulted from a cataclysmic
impact, the fossil record provides little positive evidence
to support this mode of extinction. Virtually all the
paleontologic arguments used by proponents of the impact
hypothesis are based on either negative evidence, apparent
abrupt species disappearance, or untestable models. The
scientific method demands that an hypothesis be testable.
Thus far, impact proponents have provided little direct
evidence that can be tested to support an impact driven
extinction. Where is the layer of burnt and twisted
dinosaurs bones? Not one burnt stump has been offered in
support of the global forest fires that are alleged to have
swept across the Earth, consuming the last of the dinosaurs.
For that matter, if a faint IR enrichment layer can be
preserved in marine rocks, where is the associated
extirpation layer of ammonite shells and marine reptile
bones? If the impact was as catastrophic and as pervasive as
portrayed by the impact community, where are the bodies?

Another paleontologic observation that demands to be
addressed, but has been ignored, is the well-documented
decline in biotic diversity during the Campanian and
Maastrichtian. The extinction event has been portrayed by
some as though the world simply had a "very bad day." The
long decline in diversity during the Campanian/Maastrichtian
has been considered to be unimportant or has been ignored.
If biotic diversity had remained unchanged until the K/T
extinction event, the impact scenario would be very
appealing; however, the Earth's biota did decline during the
Late Cretaceous. To suggest that the decline in diversity is
distinct and unrelated to the extinction event verges on
special-pleading. Was the Late Cretaceous diversity decline
a harbinger of impending global catastrophe? If we are to
resolve the cause of one of nature's great catastrophes, we
must integrate all available physical and biological data.

The K/T section on Seymour Island, Antarctica, has one of
the best, if not the best, marine K/T boundary sections
known in terms of exposures as well as the occurrence,
distribution, and preservation of megafossil faunas and
microfloras. The Cretaceous/Tertiary sequence exposed in the
Seymour Island region provides the only record of shelf and
nearshore environments from the continent of Antarctica and
data from these deposits provides a unique insight into the
effect of events at the end of the Cretaceous on the biota
from the high southern latitudes.

Seymour Island is one of several islands in the James Ross
Island region on the north east tip of the Antarctic
Peninsula. The sequence on Seymour Island consists of
continuous shallow shelf sandstones and sandy siltstones of
the Lopez de Bertodano Formation and ranges in age from late
Campanian through the Maastrichtian into the Paleocene. Well
exposed sections on the nearby James Ross and Vega islands
have increased the Late Cretaceous record in Antarctica back
through the Campanian into the Santonian. Although the
Seymour Island region was first studied by Otto
Nordenskjold's Swedish South Polar Expedition at the
beginning of the century, it was not until the mid 1970's
that the region began to be systematically studied by
scientists from several nations. Although geologists from
Argentina and the United Kingdom have been active in the
James Ross Island region, only the Americans have focused
their efforts exclusively on Seymour Island. Four major
stratigraphic and paleontologic field programs were
conducted during the 1980's. In 1989, the results
documenting the stratigraphic and faunal changes across the
K/T boundary were published (1). Although there is a marked
change in diversity and composition across the K/T boundary,
the change is not abrupt, but occurred over a stratigraphic
interval of approximately 30 m and is referred to as the
"K/T transitional interval." While a number of taxa with
stratigraphic ranges extending back to the Campanian
disappeared within this interval, other species passed
through completely unaffected. No single horizon within the
measure section could be termed an extirpation horizon.
Microfloral studies from Seymour Island also show a
comparable transition across the boundary. Although these
data are well documented and important to the understanding
of events at the end of the Cretaceous in the southern
hemisphere, they have not been addressed.

Of equal interest to the biotic changes at the K/T boundary
are the longer term faunal changes during the Late
Cretaceous from the high southern latitudes. The Earth's
high-latitudes regions are very sensitive to global climatic
perturbations. Changes in the diversity and composition of
high-latitude faunas and floras may be viewed as the
harbinger of impending global events. The Late Cretaceous
record of several major widespread groups of marine
invertebrates from the James Ross Island region differs
significantly from the lower latitudes. The bivalve
inoceramids, which were common elements of the late Mesozoic
marine community up to the very end of the Cretaceous in the
lower latitudes, disappeared in Antarctica during the late
Campanian. Belemnites also show a similar pattern,
disappearing in the latest Campanian or earliest
Maastrichtian in Antarctica while surviving until the end of
the Cretaceous in the lower latitudes. The ammonite faunas
display what can only be term a "diversity crash" in the
Late Cretaceous of Antarctica. The ammonite fauna is
particularly diverse in the middle Campanian with 35 species
being reported from the middle part of the Santa Marta
Formation. By the Late Campanian the diversity fell to fewer
than five species in the lower part of the Lopez de
Bertodano Formation followed with a slight increased in
diversity to about ten species by the middle Maastrichtian,
only to decline to five species just prior to the K/T
transition interval. It is important to note that a number
of contemporaneous Campanian/Maastrichtian benthic
invertebrates do not show comparable diversity changes
suggesting that the ammonite diversity decline was not a
result of local environmental changes in the James Ross
Island region.

Although the Late Cretaceous paleontologic record from
Antarctica is one of the most complete records known, it has
been largely ignored. The significant differences in the
record from southern high latitudes, such as the
transitional nature of the faunas at the K/T boundary and
absence of an extirpation horizon, indicates that the
extinction event at the end of the Cretaceous was not a
globally instantaneous event, but occurred over a finite
period of time. The diachronous disappearance of several
major marine invertebrate groups in the high southern
latitudes suggests that the terminal Cretaceous biotic
crisis may represent the culmination of a terrestrial event,
such as a longer term period of volcanism, and not to an
impact event. Although a major impact event at the end of
the Cretaceous was not the primary cause of the Terminal
Cretaceous Extinction Event, it may have had a significant
effect regionally.

References: [1] Zinsmeister et al. (1989) JP, 64, 731-738.


Florentin J.-M.   Maurrasse R.   Geier L. J.
   Biozonation of the Beloc Formation and its Relation to the Chronology of the
   KT Boundary Event

Biostratigraphy of the Beloc Formation from the Southern
Peninsula of Haiti compared to various locations worldwide
indicates that, in addition to problems related to taxonomic
identification, discrepancies associated with the
biostratigraphic succession that characterizes the KT
boundary are related to various superimposing regional and
local factors: (1) faunal provincialism controlled by global
ecological constraints such as temperature gradient; (2)
local exclusions influenced by paleogeographic factors; (3)
physical disturbance of varying magnitude coeval with a
major impact event, which occurred toward the close of the
Maastrichtian. This event left a distinct horizon in the
circum-Caribbean area where it includes splash ejecta melt
(tektites) and other cosmically derived residues.

The biostratigraphy across the marker horizon implies that
the initial impact event occurred toward the close of the
Maastrichtian, in the foraminiferal biozone that can be
correlated with the Trinitella scotti Zone. This zone
suggested by Masters (1977) is a viable concurrent
alternative zone to the traditional Abathomphalus
mayaroensis zone whose specimens are often missing.

An apparent second major physical disturbance affected the
ocean in the earlymost Tertiary and caused further mixing of
early Danian and Cretaceous taxa.

Thus, the main discrepancies that exist worldwide concerning
the biostratigraphic continuity, or the exact
biostratigraphlc are of the KT boundary, are seen as related
to complex interacting factors further intensified by
reworking caused by deep-water megawaves during the
impact(s), as postulated by Sonnet et al. (1991) and Gault
et al., (1979), not by turbidites. Subsequent subaqueous
flow events also enhanced mixing with more local
characteristics.

Despite the fact that the same physical evidence (tektite
layer, subaqueous flow deposits, etc.) will be synchronous
everywhere, the actual biochronologic continuity at
different KT sites is often very different, an artifact due
to physical disturbances.

References: [1] Gault D. E. and Sonett C. P. (1982) GSA,
Spec. Paper, 190, 69-92. [2] Masters B. A. (1977) in Oceanic
Micropaleontology (A. T. S. Ramsay, ed.), Academic Press,
301-731. [3] Schett C. P. et al. (1991) Adv. Space Res., 11,
6, 77-86.


Ward P. D.
   Dissecting the KT Extinction: Components and Comparisons With Permo-Triassic
   and `Modern' Mass Extinctions

The KT extinction can now be differentiated into a number of
separate components, or phases, equivalent in some ways to
the steps of Earl Kauffman (1988). The initial phase
commenced in the marine realm at the base of the A.
mayaroensis Zone in equatorial regions, and perhaps later in
high latitudes. The major taxa affected were inoceramid
bivalves, the reef facies, and other benthic mollusks. This
phase coincided with (and was probably caused by)
oceanographic changes including differences in ocean water
oxygenation, and a sea level drop of great magnitude. The
second pulse occurred at the KT boundary, and was far
greater in magnitude, producing the extinction of all
ammonites and a majority of planktonic foram and nannofossil
species. The final phase was a benthonic foram extinction
during the Paleocene. On land, the initial pulse was
evidenced by floral and perhaps terrestrial vertebrate
extinctions, with a greater pulse coinciding with the KT
boundary. It is not known if the precursor pulses on land in
the oceans were synchronous.

The Permo-Triassic extinctions also show pulses of
extinction. New work in the Karoo Series of South Africa
shows three major pulses of extinction among land
vertebrates, which, like KT, may or may not coincide with
extinctions in the marine realm.

Finally, many workers now believe that we have entered a new
period of mass extinction. This extinction began 2 million
years ago with marine mollusk extinctions. Its second phase
occurred during the last 50,000 years with mammalian and
avian extinctions. Its major pulse can be expected to occur
within the next 500 years, coincident with human population
maxima.



Cutler A. H.   Behrensmeyer A. K.
   Bonebeds at the Boundary: Are they a Realistic Expectation?

Over the past decade and a half an impressive array of
evidence has been amassed to support the hypothesis that the
Mesozoic Era was brought to a close by the impact of one or
more extraterrestrial bodies on the Earth [1-3]. The most
convincing lines of evidence have been mineralogic,
geologic, and geochemical rather than paleontologic. Indeed,
the fossil record remains ambiguous, and paleontologists
disagree as to whether the putative impact was in fact
responsible for abrupt and widespread extinction of species
[4]. The problem is especially acute for the terrestrial
record where KT boundary sections are not extensive and the
fossil record tends to be fragmentary and facies-dependent
[5].

One source of confusion is the absence of a clear picture of
what the fossil record of a global catastrophe should look
like. Signor and Lipps [6] made an early contribution along
these lines by pointing out that the vagaries of
preservation would tend to truncate species apparent ranges
below their true level of extinction, with the result that
truly abrupt extinction events would appear gradual in the
fossil record. In their analysis, however, Signor and Lipps
assumed that taphonomic processes remained constant up to
and across the extinction boundary, and at least one author
[7] has suggested that the scale of mortality implied by an
impact should be reflected in the record by widespread bone
beds and mass mortality horizons.

Mass mortality horizons associated with local or regional
events such as droughts [8], fires [9], and volcanic
ashfalls [10] can be documented in the vertebrate fossil
record. Is it reasonable to expect that mass mortality of
large vertebrates on a global scale would leave a
recognizable signal in the record? It is obviously too much
to expect to find vast horizons of dinosaur carcasses in
their final death postures dusted with a layer of Ir, but it
is less obvious whether such an event would produce a
detectable elevation in the frequency or scale of bone
concentrations (a "bone spike") at the extinction horizon.

Based on calculations of mortality (turnover) rates of
dinosaurs from known relationships of body size to life span
[11,12], estimates of the dinosaur carrying capacity of the
late Cretaceous ecosystems (animals/km^2), and faunal lists
indicating the number of co-existing species of animals
>20kg [13], we infer a range of probable densities of live
animals and carcasses on the landscape of the latest
Cretaceous. From this we infer the differences between a
bone assemblage formed under normal turnover (attritional
input of bones) and from a catastrophe that wiped out all
large tetrapods over a period of hours to days. In addition
to the issue of attritional vs. single-pulse availability of
carcasses, longer-term effects of burial and preservation of
bone-bearing strata must be taken into account in assessing
whether a mass die-off of dinosaurs would leave a detectable
signal in the fossil record at the KT Boundary. We conclude
that it is unlikely that a single event such as a bolide
impact would leave dense, multispecies dinosaur bonebeds in
the area of greatest mass mortality.

It is possible that ecological stress farther from the
impact site, which allowed some species time to congregate
around specific areas prior to death (waterholes, remaining
patches of forage), would have created more concentrated
bone assemblages for particular taxa. Whether any of these
were preserved in the long-run would depend on whether such
zones of variable, "less catastrophic" mortality intersected
active areas of deposition at the time. Recovery of a fossil
record from these same areas would depend also on later
tectonic processes that created outcrops, which we can
search today. The complexity of the combined requirements
for there to be a record of any major mass mortality event
for land animals at the KT boundary makes it unlikely that
this will be detectable. The absence of bone beds at the
boundary should not be used for or against hypotheses
regarding causes of the extinction event.

References: [1] Sharpton V. L. and Ward P. D. (1990) GSA
Spec. Paper, 247, 631. [2] Hildebrand A. R. et al (1991)
Geology, 19, 867-871. [3] Swisher C. C. (1992) Science, 257,
954-958. [4] Flessa K. W. (1990) GSA Spec. Paper, 247, 1-7.
[5] Fastovsky D. E. (1990) GSA Spec. Paper, 247, 541-548.
[6] Signor P. W. and Lipps J. H. (1982) GSA Spec. Paper,
190, 291-296. [7] Carpenter K. (1988) LPI Contribution 673,
24-25. [8] Rogers R. R. (1990) Palaios., 5, 394-413. [9]
Sander P. M. (1987) Palaeogeogr. Palaeoclimatol.
Palaeoecol., 61, 221-236. [10] Voorhies M. R. (1985) Natl.
Geog. Soc. Res. Rep., 19, 671-688. [11] Farlow J. O. (1976)
Ecology, 57, 841-857. [12] Western D. (1980) in Fossils in
the Making (A. K. Behrensmeyer and A. P. Hill eds.), 41-54.
[13] Beland P. and Russell D. A. (1978) Can. J. Earth Sci.,
15, 1012-1024.


Pillmore C. L.   Lockley M. G.   Fleming R. F.   Johnson K. r.
   Footprints in the Rocks--New Evidence from the Raton Basin that Dinosaurs
   Flourished on Land Until the Terminal Cretaceous Impact Event

In recent months, we have found convincing evidence that
dinosaurs were present and probably flourished in the Raton
Basin, southern Colorado and northern New Mexico, possibly
until, or only a very short time before, the asteroid impact
at the end of the Cretaceous Period. This evidence is in the
form of impressions and natural casts of dinosaur footprints
that occur in the lower part of the continental Raton
Formation of Late Cretaceous and Paleocene age within one
meter below the Cretaceous-Tertiary (KT) boundary claystone
layer. Tracks are also present at several other horizons
farther below the boundary. Curiously, Raton Basin rocks
have never produced dinosaur skeletons. Though rare
fragmentary remains of fish, crocodiles, and turtles have
been identified in samples from the Cretaceous portion of
the Raton Formation, only one bone attributed to a dinosaur
has ever been found.

In the southern part of the Raton Basin, the KT boundary
occurs at the top of the lower coal zone of the Raton
Formation. This zone consists of sequences of rocks
comprising beds of mudstone, carbonaceous shale, and thin
beds of coal with interbeds of sandstone; sandstone beds at
the base of the lower coal zone are commonly conglomeratic.
The depositional environment is fluvial, consisting of broad
flood plains characterized by mudflats and swamps that were
periodically flooded by crevasse splays from meandering
channelways. Crevasse splays occur when natural levees are
breached along river channels and sediment-laden water pours
out over the floodplain. Footprints and other impressions in
silty muds of the mudflats were quickly buried by sand as
crevasse splays prograded across the floodplain. Where the
splay sand deposits were not subsequently eroded by crevasse
channels and associated meandering rivers, the animal tracks
and traces on the buried floodplain surface may be preserved
as sand-filled depressions at the base of the deposits. This
might be considered analogous the making of huge plaster of
Paris casts of the floodplain surface or, in a sense, to
"photographing" an instant in time. Traces identified to
date include: hadrosaurid and ceratopsid tracks; an
unidentified trace, possibly a turtle; bird tracks; and a
well-preserved tyrannosaur footprint. The tyrannosaur print
is the first convincing record ever found.

Natural casts of the hadrosaur footprints are preserved in
trackways at several horizons in the lower coal zone. One
horizon is only 37 cm below the KT boundary clay bed about
15 km northwest of Trinidad, Colorado at the Berwind
locality. Ceratopsid and hadrosaur tracks also occur only a
short distance (1 to 2 m) below the boundary at the Clear
Creek north site 5 km south of Trinidad. The tyrannosaur
footprint was found about 75 km to the southwest near
Cimarron, New Mexico. This track as well as other traces are
preserved as natural casts at the base of loose sandstone
blocks derived from a ledge 20 meters below the level of the
KT boundary. The position of the track-containing ledge with
respect to the boundary was defined recently by
palynological analyses. The Cretaceous pollen and spores
recovered from a carbonaceous mudstone unit immediately
beneath the tyrannosaur track-bearing sandstone ledge
suggest that palms and ferns typical of a subtropical
environment dominated the plant community on the floodplain
around the track site.



Archibald J. D.*
   Testing KT Extinction Hypotheses Using the Vertebrate Fossil Record

Although we do not have geographically extensive data for KT
vertebrate turnover, available data from the Western
Interior of North America can be used to examine proposed
extinction scenarios. The most recently published
compilation [1] included 111 species of vertebrates within
the uppermost Cretaceous Hell Creek Formation of Montana and
found that species survival at the KT boundary was between
53 and 64% when three artifacts were considered. These
artifacts were as follows: (1) When a species survived
elsewhere but disappeared locally in eastern Montana, this
obviously was a survival rather than an extinction. Clearly,
when one is attempting to ask global questions of
extinction, one must not count local disappearances as true
extinctions. (2) Pseudoextinction occurred when a species
disappeared in the process of forming a new species through
cladogenesis or anagenesis, and thus its disappearance is
not true extinction. If pseudoextinction is not recognized,
extinction rates can be greatly exaggerated. In a study of
early Tertiary mammals it was found that in about 25% of the
cases, species disappearances were the result of speciation
events not extinction events [2]. We do not have detailed
phylogenetic analyses for most latest Cretaceous mammals,
but common sense tells us that their apparent 97%
disappearance is not all true extinction. There is no
question that mammals were beginning a tremendous radiation
that occurred after the disappearance of non-avian
dinosaurs. Such pseudoextinction is undoubtedly masking as
true extinction in other vertebrate groups. (3) Finally, the
third artifact is rarity of taxa. Archibald and Bryant [1]
found a much higher level of disappearance of rare species
(91% for rare vs. 54% for common), suggesting an artifact of
preservation.

When all three artifacts were considered, including the
exclusion of rare disappearances, a 64% species survival was
calculated. The 28 rare species in this study were more
fully examined to estimate if rarity is an issue in
estimating survival. Eight rare species that were either
extremely poorly known or were based on extremely
questionable taxonomy were excluded leaving a revised data
set of 104 species (including 20 rare species). Including
all 104 species or species lineages, a 51% (53 of 104
species) survival was found for vertebrates across the K/T
boundary in the Western Interior. If all of the extinctions
of the rare species are actually artifactual, survival would
rise to 70% (73 of 104). This gives a range of 51-70%
species survival. The lower estimate is probably more
accurate, but some of the extinctions of very rare species
are certainly artifactual. For vertebrates the highest
percent extinctions are among sharks and relatives, lizards,
bird and reptile-hipped dinosaurs, and marsupials. These
five groups account for 75% of all extinctions of
vertebrates at the KT boundary in the Western Interior. In
comparison to other intervals in the Late Cretaceous and
early Paleocene, the KT vertebrate turnover is only about
10% higher at the generic level [1]. These tabulations are
useful in examining the validity of various extinction
scenarios.

Starting with the bolide impact theory, the basic premise is
that on a global scale, a dust cloud shrouded the Sun,
cutting off photosynthesis, killing herbivores, finally
killing carnivores. This scenario is so broad spectrum in
its implications because it tries to explain everything and
thus explains nothing. Major extinctions (>50%) in only 5 of
12 major taxa are correctly predicted by this hypothesis.

There are some interesting corollaries to the bolide
impact:--a short, sharp decrease in temperature, highly
acidic rains, and a global wildfire. Based on what we know
of extant vertebrates, if a short, sharp decrease in
temperature occurred, the vertebrates most likely to be
affected are ectothermic tetrapods, that is, reptiles and
amphibians. Endotherms such as birds and mammals and
ectotherms such fish would be much less severely affected.
Some amphibians and reptiles inhabiting areas with low
winter temperatures or of severe drought, evolved methods of
torpor to survive. Most Late Cretaceous ectothermic
tetrapods in sub-tropical to tropical climates probably did
not have such methods of torpor. Further, torpor is most
often preceded by decreases in ambient temperature, changes
in light regimes, and decrease in food supply. Ectotherms
could not have anticipated a short, sharp decrease in
temperature. Except for a decline in lizards, the idea of
temperature decrease does not accord well with the
vertebrate data at the KT boundary because only 4 of 12
major taxa show extinction or survival patterns in
agreeement with this scenario.

Related to this, Clemens and Nelms [3] discussed the lack of
ectothermic tetrapods at Late Cretaceous sites in northern
Alaska. Endothermic tetrapods, dinosaurs and mammals, and
fishes could deal with the lower temperatures and thus are
present. If lowered temperatures were a factor for
ectothermic tetrapods in Alaska, then a severe drop in
temperature would devastate such animals at mid-latitudes at
the KT boundary. This is clearly not the case.

The next corollary of a bolide impact is acid rain. Today in
regions of decreased pH rainfall lakes are basically
stripped of life and some plants succumb to changes in soil
chemistry. Also, we see diminished amphibian, especially
frog populations. We thus have a very clear picture of how
modern vertebrate biotas react to acid rain. When we compare
these modern patterns to those observed at the KT boundary,
we find almost no correlation. Only 3 of 12 major taxa have
extinction/survival patterns matching this corollary of
bolide impact.

The final corollary of bolide impact is global wildfire.
During a global wildfire, terrestrial vertebrates would be
baked or fried and the aquatic environment choked with
debris, killing most life. Like the global bolide impact
scenario, the global wildfire is so broad in its killing
effects that it explains everything and nothing. Localized
wildfires, however, agree better with extinction patterns
for both vertebrates and lower latitude plants. Local
wildfires agree with with 9 of 12 extinction/survival
pattrens for major taxa while global wildfire only agrees in
5 of 12 cases.

Next I turn to global marine regression and habitat
fragmentation. Near the end of the Cretaceous, maximum
transgression divided NA into two continents. As regression
begin and continued across the KT boundary coastal plains
decreased in size and became fragmented, stream systems
multiplied and lengthened, and as sea level fell, land
connections were established or re-established. The largest
land vertebrates, the non-avian dinosaurs, were most
affected by loss and fragmentation of habitats. This is
similar to what we see happening today as various habitats
are fragmented, especially in modern rainforests. As the
stream systems increased in size, freshwater vertebrates did
well except for those with close marine ties such as sharks
and some bony fishes. Although one cannot predict in general
which species experienced increased competition through the
establishment of land bridges, the appearance of archaic
ungulates in N.A. as marsupials decline, fits this pattern
well. In total, the above scenario agrees with 11 of 12
extinction/survival patterns seen for major vertebrate taxa.

In general, regression was hardest on terrestrial species,
especially of larger size, while aquatic forms fared better
by comparison. Would the opposite be true during a
transgressive phase? We know of four major transgressive-
regressive cycles in N.A. during the Late Cretaceous into
the earliest Tertiary. Unfortunately, we currently only have
decent vertebrate records near the KT boundary. There are,
however, two examples that bear further scrutiny.

Horner and colleagues [4] documented anagenetic appearances
within four separate lineages of dinosaurs during the last
transgressive phase of the Late Cretaceous. During the
deposition of the preceding Judith River Formation that
extended for five million years, much of which was during a
lowered sea-level still stand, they argue little change
occurred in dinosaurian and mammalian lineages.

Eaton and Kirkland [5,6] reported that across the
Cenomanian-Turonian boundary in southern Utah, which
encompasses the first maximum transgression in the Late
Cretaceous of North America, the riverine-riparian
vertebrate fauna shows notable extinction, while groups of
terrestrial vertebrates continued to radiate.

These two earlier transgressions, combined with the KT
marine regression, suggest that regressions are hardest on
terrestrial species, but favor freshwater forms, while
transgressions are hardest on freshwater species but favor
terrestrial forms. Thus, within the cycles of transgression
and regression we may have processes that more completely
explain the patterns of vertebrate turnover at the KT
boundary. Single cause theories of extinction, such as a
bolide impact and its corollaries fail to explain the
pattern of vertebrate extinction at the KT boundary. [1]

References: [1] Archibald J. D. and Bryant L. (1990) GSA
Spec. Paper, 247, 549-562. [2] Archibald J. D. (1993)
Paleobiology, 19, 1-27. [3] Clemens W. A. and Nelms L. G.
(1993) Geology, 21, 503-506. [4] Horner J. et al. (1992)
Nature, 358, 59-61. [5] Eaton J. and Kirkland, J. (1993) GSA
Abst., 25, [6] Kirkland J. (1987) Hunteria, 2, 1-16.

Table 1, which appears here in the hard copy, shows Late
Cretaceous vertebrate species from eastern Montana compared
to survival across the KT boundary. Levels of observed
extinction (>50% = Y; <50% = N) vs. predicted for various
extinction scenarios. Capital letter indicates agreement
with observed and small case indicates disagreement. Numbers
on right indicate for how many of twleve major taxa
predicted agrees with observed. See text for discussion.


Coccioni R.   Galeotti S.
   The KT Boundary Extinction: A Geologically Instantaneous or Gradual Event?
   Evidence from Deep-Sea Benthic Foraminifera

The Cretaceous/Tertiary (KT) boundary event, which 65 Ma ago
led to a well-known global mass extinction, is one of the
most discussed subjects in the field of the Earth Sciences.
A lively debate developed in the past decade between those
who favor a gradual disappearance of late Cretaceous fauna
and flora [1] and those who argue for a catastrophic
extinction event [2]. The changes in the benthic
foraminiferal community structure across the KT boundary
have been intepreted differently including multiple
mechanisms. According to previous studies the deep-sea
benthic foraminifera were not severely affected by the KT
boundary event [3]. However, most data on the rate of
benthic foraminiferal extinction and distribution across the
KT boundary cannot be compared because different methods and
taxonomic concepts were followed by different authors [4,5].
In particular, concerning the methods, a very high-
resolution (mm scale) study on the benthic foraminiferal
record at the KT boundary transition had not been done. The
present study is therefore concerned with the development of
such a very high-resolution investigation on deep-sea
benthic foraminiferal distribution across the KT boundary at
Caravaca (SE Spain) (Fig. 1). Thirty-nine samples were
collected from 80 cm below to 120 cm above the KT boundary
clay layer, encompassing the last 20 kyr of the Cretaceous
and the early 60 kyr of the Paleocene. The 7 cm-thick KT
boundary clay layer, which spans 5-6 kyr [6] was sliced up,
every seven mm to obtain eleven samples. All the specimens
from the fraction greater than 125 micrometers were picked,
classified at generic level and counted. More than 100
species were identified belonging to 59 genera among which
32 are calcareous-hyaline genera and 27 are agglutinated
genera. Based on benthic foraminifera, the terminal
Maastrichtian-basal Danian sediments at Caravaca were
deposited in a middle bathyal environment (paleodepth
600-1000 m).

Our data show a complex history of deep-water benthic
foraminiferal decimation, survivorship and recovery, and a
close coupling between the surface- and deep-water
ecosystems, the collapse of the planktonic environment
deeply disturbing the deep-oceanic biota. In terms of biotic
decimation, survivorship, and recovery, it was recognized a
distinct four-step pattern (Fig. 2). The influence of the KT
boundary event on deep-water benthic foraminifera was
dramatic, consisting of a sudden and marked decrease in
diversity that is equivalent to a taxonomic decimation
associated with a concurrent bloom of infaunal deposit
feeder, low-O tolerant forms. Such a dramatic faunal change
points to an exceptionally large nutrient influx, which
suddenly reached the sea-floor as a consequence of the rapid
accumulation of the "nutrient soup" resulting from the mass
mortality at the KT boundary. The stepped pattern recognized
in the recolonization fauna is here regarded as the response
to return to (1) more oxygenated bottom-water conditions,
(2) a wider spectrum of utilizable nutrient resources on and
in the sediment, and (3) a larger environmental stability.
The complete recovery of the benthic foraminiferal ecosystem
was much faster than that of the planktonic foraminifera
ecosystem, the adverse environmental conditions lasting for
about 7 kyr on a so-called "stressed sea-floor." During the
20 kyr before and the 50 kyr after this time interval the
sea-floor was characterized by environmental stability and
normal conditions of oxygenation and food supply.

The sudden and dramatic collapse on the sea-floor at the end
of the Cretaceous as evidenced by deep-water benthic
foraminiferal record can only be explained by a geologically
instantaneous, catastrophic event.

Acknowledgments: This study was supported by the Italian
Ministery of University, Scientific Research and Technology
(M.U.R.S.T. 60% to R.C.).

References [1] Clemens W. A. et al. (1981) Paleobiology, 7,
293-298. [2] Alvarez L. W. et al. (1980) Science, 208,
1095-1108. [3] Thomas E. (1990) GSA Spec. Paper, 247, 481-
495. [4] Coccioni R. et al. (1993) Terra abstracts, 5, 694.
[5] Coccioni R. et al. (in press) Paleopelagos. [6] Smit J.
(1990) Geologie en Mijnbouw, 69, 187-204. [7] Corliss B. H.
and Chen C. (1988) Geology, 16, 716-719. [8] Kahio K. (1991)
Palaeogeogr., Palaeoclimatol., Palaeoecol., 83, 65-85. [9]
Smit J. and Hertogen A. J. T. (1985) Nature, 285, 198-200.

Fig. 1. Faunal parameter changes in benthic foraminiferal
assemblages across the KT boundary at Caravaca. In order to
document changes in benthic foraminiferal assemblages across
the KT boundary and to determine how the deep-sea benthic
foraminifera responded to the KT boundary event, the faunal
density expressed as number of specimens per gram of
sediment, the diversity expressed as number of genera, and
the agglutinated to agglutinated and calcareoushyaline ratio
percent have been taken into consideration. Moreover, all
the calcareous-hyaline taxa were allocated into seven
morphogroups [7] and assessed as aerobic, intermediate, and
anaerobic forms [8]. All these parameters, together with the
calcareous-hyaline morphogroup distribution and the record
of the Dissolved Oxygen Index (DOI), expressed as the
aerobic to aerobic and anaerobic forms ratio percent, have
been plotted against the chronostratigraphy,
lithostratigraphy, planktonic foraminiferal biostratigraphy
[9], and carbonate content. Note the marked fluctuation of
all the parameters at the KT boundary.

Fig. 2. Idealized paleoenvironmental reconstructions of the
sea-bottom across the KT boundary at Caravaca. The roman
numbers indicate the different steps (not in scale) of
decimation, survivorship, and recovery of deep-water benthic
foraminifera. IDF = infaunal deposit feeder; EDF = epifaunal
deposit feeder; ESF = epifaunal suspension feeder. Both the
nutrient levels and the biofacies were inferred through the
calcareous-hyaline morphotype distribution together with the
faunal density and diversity records.


Colombo-Pinol F.
   The KT Boundary in the Southeastern Pyrenees, Ager Basin, N. E. Spain
   (Lleida Province)

Introduction: The materials that constitute the transition
between the Mesozoic and the Cenozoic in the area studied
were accumulated in different depositional environments in
continental areas as evidenced by the abundant non-marine
fossils.

The aim of this paper is to describe the exact limits
between the Mesozoic and the Tertiary in the Ager basin, NE
Spain. It has been confirmed that in the KT transition the
limits are not coincidental with respect to the different
fossil groups considered (Feist and Colombo, 1983; Ashraf
and Erben, 1986).

The KT Boundary: A number of studies have been carried out
to determine the location of this boundary (Fig. 1). The
results have enabled us to confirm a lack of unanimity on
the location of this boundary. Thus, there is still a lack
of precision concerning the vertical palynoflora
distribution (Medus et al., 1988; Medus and Colombo, 1991;
Medus, Colombo and Durand, 1992). However, it seems that
some forms considered typical of the Tertiary have been
found in the maastrichtian levels whereas characteristic
forms from the Cretaceous have been found in materials that
could be attributed to the base of the Tertiary. The present
palynofloristic associations, do not allow us to distinguish
the floras of the upper part of the Maastrichtian from the
ones at the base of the Tertiary (Thanetian).

The charophyta group poses the problem that the KT boundary
does not appear to be exactly related to any type of
specific association. Thus, there was an interval between
the upper Cretaceous and the lower Paleocene that could
correspond approximately to 5 million years and is comprised
between the latest cretaceous floras (upper Maastrichtian)
and the upper part of the Danian or Danian-Montian [13].
Although we have managed to define five (Fig. 2) overimposed
biozones that include the KT boundary, this could be located
within one local zone [7]. For this reason, forms clearly
from the upper Maastrichtian coexist with forms that
corresponds to the Thanetian ancestors. Thus, current
knowledge of the paleontological type does not allow us to
know with sufficient exactitude, the position of the KT
boundary in the Ager area.

As for the Ir layer, many samples were systematically
collected in the lithostratigraphic intervals where, from
the paleontological point of view, the possible location of
the KT boundary is known. The samples were treated by means
of neutronic bombardment with instrumentation that permitted
the detection of a smaller concentrations than that ranging
between 50-100 pg/g. Thirty-five samples were treated with
the result that significant concentrations greater than 100
pg/g were not found. It could therefore be concluded that
there is no layer with an Ir enrichment in the Fontllonga
section.

Finally, a paleomagnetic study of the same levels was
carried out. We collected 99 samples by means of a manual
perforator, which was treated in a cryogenic magnetometer
[7]. When the paleomagnetic data were incorporated into the
stratigraphic log (Fig. 2), it was possible to propose a
general sequence of magnetic polarity.

Discussion and Conclusions: From paleomagnetic studies we
know that the KT boundary is placed within the Cron 29R
[2,4,12] but its precise position cannot be determined since
a sedimentary uniformity in the studied outcrops does not
exist. Thus, the standard magnetic sequences that are very
well correlated with marine biozonations on a global scale
[8] are utilized.

Data from the study of different groups of fossil organisms
together with the paleomagnetic data of the Fontllonga
section allows correlation with the standard paleomagnetic
sequence. The section includes the Crons 32R to 26R, from
the lower Maastrichtian to the lower Thanetian. Despite some
discontinuities in the data, the most continuous succession
ranges from the Crons 31 N to 28R. Thus, the Cron 29R where
the KT boundary exists has been clearly identified in the
USA [15] but not so clearly in the SE of France [14,6] this
being one of our most significant findings.

The extinction in this area of the cretaceous charophytes
and dinosaurs (last appearance of egg remains,
footprints,...) seems to have occurred some two million
years before the beginning of the Tertiary. Thus, these
extinctions would have taken place before and independently
of the crisis associated with the KT boundary, as a
consequence of considerable decrease in climatic temperature
during the Campanian and the Maastrichtian.

As for the absence of the Ir anomaly, despite its interest
is not advisable to use this as an important argument in
favor of the worldwide presence of the catastrophic effects
related to the crisis associated with the KT boundary [1.2].
In the Fontllonga section all the Cron 29R is contained in
some lutitic and sandy materials deposited in a fluvial
depositional environment of the braided type. Its
sedimentologic characteristics suggest elevated ratios of
synsedimentary reworking of the materials by means of
autocyclic processes. Therefore, the possibilities of the
preservation of the KT boundary (Ir, Ni rich spinels, impact
minerals,...) markers that yield evidence of a catastrophic
event on a global scale are very scarce.

Acknowledgments: This work was partially supported by the
DIGICYT Project PB91-0805.

References: [1] Alvarez W. et al. (1982) GSA Spec. Paper,
190, 305-315. [2] Alvarez W. et al. (1977) GSA Bull., 88,
367-389. [3] Ashraf A. R. and Erben H. K. (1986)
Palaeontogr., 200, 111-163. [4] Chave A. D. (1984) Deep Sea
Drilling Project Leg 74, Init. Rep. DSDP, 74, 525-531. [5]
Feist M. and Colombo F. (1983) Geol. Medit., 20, 3-4,
303-326. [6] Galbrun B. et al. (1991) C. R. Acad. Sci.
Paris, 312, 1467-1473. [7] Galbrun B. et al. (1993)
Paleogeo. Paleoclim. Paleoec., 102, 41-52. [8] Haq B. U. et
al. (1987) Science, 235, 1156-1167. [9] Medus J. and Colombo
F. (1991) S. Acta. Geol. Hisp., 26, 27-35. [10] Medus J. et
al. (1992) Cret. Res., 13, 119-132. [11] Medus J. et al.
(1988) Newsl. Statigr., 18, 123-138. [12] Petersen N. P. et
al. (1984) Rep. DSDP, 73, 657-661. [13] Riveline J. (1986)
Cah. Paleont., 1-227. [14] Westphal M. and Durand J. P.
(1990) Bull. Soc. Geol. Fr., 6, 4, 609-620. [15] Wolbach W.
S. et al. (1985) Science, 230, 167-170.

Fig. 1, which appears here in the hard copy, shows a studied
stratigraphic log in the Fontllonga section.

Fig. 2, which appears here in the hard copy, shows a
Fontllonga stratigraphic log. The magnetic polarity log is
referred to the Haq et al., 1987 standard. The charophyte
local biozones and the imprecision tract, located between
the Upper Maastrichtian and the Danian (modified after
Galbrun et al., 1993), are noteworthy.





                           Friday, February 11, 1994
                     SESSION 5:  DISTURBANCE AND RESPONSE
                                8:30 - 12:30 PM

Chair(s):  B. Schuraytz


Kauffman E.*
   To Be Determined

No abstract available.


Hills J. G.*
   Consequences of the Impact of Comets and Asteroids with Earth

The fragmentation of a small asteroid in the atmosphere
greatly increases its cross sections for aerodynamic braking
and energy dissipation. At a typical impact velocity of 22
km/s, the atmosphere absorbs more than half the kinetic
energy of stony meteoroids with diameters, D(sub)M < 220
meters and iron meteoroids with D(sub)M < 80 meters. The
corresponding diameter for comets with impact velocity 50
km/s is D(sub)M < 1600 meters. Most of the atmospheric
energy dissipation occurs in a fraction of a scale height,
so large meteors appear to "explode" or "flare" at the end
of their visible paths. This dissipation of energy in the
atmosphere protects the Earth from direct impact damage
(e.g., craters), but it produces a blast wave that can do
considerable damage. The area of destruction around the
impact point in which the over pressure in the blast wave
exceeds 4 pounds/inch^2 = 2.8 x 10^5 dynes/cm^3, which is
enough to knock over trees and destroy buildings, increases
rapidly from zero for chondritic meteoroids less than 56
meters in diameter (15 megatons) to about 2000 square km for
those 80 meters in diameter (48 megatons). (The probable
diameter of the Tunguska impactor of 1908 is about 80
meters).

Crater formation and earthquakes are not significant in land
impacts by stony asteroids less than about 200 m in diameter
because of the air protection. A tsunami is probably the
most devastating type of damage for asteroids 200 meters to
1 km in diameter. An impact by an asteroid this size
anywhere in the Atlantic would devastate coastal areas on
both sides of the ocean. An asteroid a few kilometers across
would produce a tsunami that would reach the foothills of
the Appalachian Mountains in the upper half of the East
Coast of the United States. Most of Florida is protected
from a tsunami by the gradual slope of the ocean off its
coast, which causes most of the a tsunami energy to be
reflected back into the Atlantic. The atmosphere plume
produced by asteroids with diameters exceeding about 120
meters cannot be contained by the atmosphere, so this bubble
of high-temperature gas forms a new layer on top of the
atmosphere. The dust entraped in this hot gas is likely to
have optical depths exceeding tau = 10 for asteroids with
diameters exceeding about 0.5-1 km. The optical flux from
asteroids 60 meters or more in diameter is enough to ignite
pine forests. However, the blast wave from an impacting
asteroid goes beyond the radius in which the fire starts.

The blast wave tends to blow out the fire, so it is likely
that the impact will char the forest (as at Tunguska), but
the impact will not produce a sustained fire. Because comets
dissipate their energy much higher in the atmosphere than
asteroids, they illuminate a much larger region and their
blast wave is weaker, so they are much more effective in
producing large fires. This suggests that the KT impactor
was a comet rather than an asteroid.


Turco R. P.*   Toon O. B.   Zahnle K.
   Global Environmental Effects of Large Impacts: A Review

Gilmour I.*   Wolbach W. S.
   Diamonds, Soot, and Molecules: The Geochemistry of Carbon at the KT Boundary

The C geochemistry of the KT boundary, in particular the
isotope systematics, have provided clues to both the nature
of the postulated impact and its associated effects. To date
5 C components have been identified that are associated with
the fireball and ejecta layers. (1) Soot/elemental C [1,2].
(2) Diamonds-occurring as nanometer-size cubic crystals
[3,4]. (3) Carbon released from impact melt glassesprimarily
as CO2 inclusions [5]. (4) Organic C-in the form of
insoluble kerogen and extractable organic molecules [6-8].
(5) Amorphous C particles [9]. Marine KT boundary sequences
also display carbonate C and O isotope shifts [10].

Soot and Elemental Carbon: A total of 12 KT boundary sites
worldwide that were examined for C were found to be enriched
in soot and charcoal [1,2,11,12]. The enrichment parallels
that for Ir and apparently comes from major fires triggered
by the impact. Evidence that at least part of the fuel
source was biomass comes from the occurrence of polycyclic
aromatic hydrocarbons [6] and the remarkably uniform
delta^13C values for the soot and charcoal of -25.8 +-
0.6permil [13]. The total amount of charcoal and soot (0.11
g/cm^2) is surprsingly large and implies that either a
substantial proportion of biomass burned or that a major
part of the fuel was fossil C. Alternatively, such large
scale fires may result in a higher soot yield. Several
ignition mechanisms have been envisaged for such fires;
immediate blast effects would ignite dry biomass within a
1200 km radius, charge separation during settling of ejecta
is a process that results in lightning in volcanic ash
clouds, or radiated heat from impact ejecta fallout [14].

Diamonds: The discovery of nanometer sized diamonds in a KT
boundary clay from Knudsen's farm, Canada [3], led to the
suggestion that they may be derived from an interstellar
diamond component in the impacting asteroid. Nanometer sized
diamonds (C/delta) found in carbonaceous, ordinary, and
enstatite chondrites have a number of very characteristic
properties: the presence of the exotic noble gas component
Xe (HL), an extremely negative delta^15N value of ca. -343
+- 16 permil with typical N concentrations of 2000-15000
ppm, a cubic structure, and delta^13C values of -32 to -40
permil. Gilmour et al. [4] undertook a systematic study of
acid-resistant residues from three boundary sites in North
America (Brownie Butte and Berwind Canyon) and Europe
(Petriccio) using six independent criteria; diamond size
distribution, C isotopic composition, N isotopic
composition, N abundance, C release profiles obtained in
stepped heating experiments, and N release profiles.

The presence of aproximately 6nm cubic diamonds at the two
North American sites was confirmed by transmission electron
microscopy. The combustion temperature of the majority of
the C in both fireball residues is similar to C/delta
diamonds, but, the C and in particular N isotopic
compositions (delta^13C = -19 to -12 permil, delta^15N~+5
permil) were clearly inconsistent with them being an
interstellar diamond component derived from the impacting
asteroid or comet. It would seem, therefore, that the
diamonds were formed as a result of the impact and there are
several possible mechanisms. The most likely would be either
production by shock alteration of C from the target rock
and/or asteroid or by a mechanism such as chemical vapor
deposition (CVD) producing diamonds in the fireball. The C
isotopic compositions do not match any major crustal
reservoir of C such as carbonate or organic C though could
clearly be obtained by mixing. Indeed, the variations in
delta^13C and indicative of the presence of more than one
component and may suggest the existence of two populations
of diamonds perhaps reflecting several formation processes.

Impact Glass Fluid Inclusions: Glass spherules thought to be
tektites from Haiti have previously been analyzed for their
mineralogy and chemical composition to identify their origin
and mode of formation [15]. They occur in various colors
dependent upon the original target rock and contain bubbles.
To investigate these spherules and the nature of any gas
phase, several dark brown glasses were analyzed by Hough et
al. [5] for their C content and isotope composition, using
stepped combustion analysis and static mass spectrometry.
Optical examination of the glasses showed that they
contained bubbles of gas at pressures slightly above
atmospheric. Individual spherule fragments were found to
yield a total of 0.2 wt% C in two components of different
isotopic composition. One of these components, with an
isotopic composition of -19 permil is apparently common to
all the brown glass spherules. The second more abundant
component had a maximum isotopic composition of -3 permil
but is apparently heterogeneous in its distribution
suggesting that it represents the trapped bubbles.

Organic C: Organic C remains one of the least studied and
poorly understood aspects of the stratigraphy of the KT
boundary event. The boundary clay in marine sequences is
often substantially enriched in organic matter over
background values but there are considerable differences
between sites suggesting strong local environmental controls
[8]. In contrast the greater stratigraphic resolution
possible for North American terrestrial sites reveals an
ejecta and fireball layer depleted in organic C but with a
sharp increase in abundance, with concomitant changes in
isotope composition, in the so called recovery layer
immediately above [16,17]. The isotopic shifts observed seem
to be independent of local environmental conditions with the
same isotopic shift of around -2 permil being apparent at
sites where the boundary is overlayen by coal or shale. Such
an effect might be expected in organic C at non-marine sites
had the isotopic composition of atmosphenc CO2 significantly
differed from the late Creatceous value. This might have
been caused by an input of combustion porduced CO2 from
wildfire, which would be reflected in more negative
delta^13C values for photosynthetically produced C.

Relationships Between Components: Comparison of the data for
the components outlined above may impose constraints on
their origins. For instance, the C isotopic composition of
the gaseous components in the glasses from Haiti are
indicative of a carbonate origin for the C with delta^13C
values of around -2 to -4 permil. If these glasses were
indeed formed in an impact fireball then this isotopic
composition presumably represents that of a substantial
proportion of the C present. However, the nanometer-size
diamonds have delta^13C values of ca. -12 to -16 permil; it
is difficult to see how these two components can have the
same C source if both are synthesized in an impact fireball
unless the fireball itself was heterogeneous. The C source
for the diamonds is problematic, it would seem unlikely from
their isotopic compositions that they have a relationship
with the soot produced in the fires although it is not
inconceivable that nanodiamonds may be produced in such
fires. Similarly if the diamonds were synthesized in the
impact fireball then their isotopic composition is distinct
from that of any fossil C component that may have
contributed to the soot.

References: [1] Wolbach W. S. et al. (1985) Science, 230,
167-170. [2] Wolbach W. S. et al. (1988) Nature, 334, 665-
669. [3] Carlisle D. B. and Bramman D. R. (1991) Nature,
352, 708-709. [4] Gilmour I. et al. (1992) Science, 258,
1624-1626. [5] Hough R. M. et al. (1993) Meteoritics, 28,
364-365. [6] Venkatesan M. I. and Dahl J. (1989) Nature,
338, 57-60. [7] Gilmour I. et al. (1990) GSA Spec. Paper,
247, 383-390. [8] Meyers P. A. and Simoneit B. R. T. (1990)
Org. Geochem., 16, 641-648. [10] Perch-Nielsen K. et al.
(1982) in Geological implication of impacts of large
asteroids and comets on the Earth (L. T. Silver and P. H.
Schultz, eds.), 353-371. [11] Wolbach W. S. and Anders E.
(1989) GCA, 53, 1637-1647. [12] Wolbach W. S. et al. (1990)
GCA, 54, 1133-1146. [13] Wolbach W. S. et al. (1990) GSA
Spec. Paper, 247, 391-400. [14] Melosh H. J. et al. (1990)
Nature, 343, 251-254. [15] Koeberl C. and Sigurdsson H.
(1992) GCA, 56, 2113-2129. [16] Gardner A. F. et al. (1994)
this volume. [17] Hildebrand A. R. and Wolbach W. S. (1989)
LPS XX, 414-415.


McLean D. M.*
   Proposed Law of Nature Linking Impacts, Plume Volcanism, and Milankovitch
   Cycles to Terrestrial Vertebrate Mass Extinctions Via Greenhouse-Embryo
   Death Coupling

A greenhouse-physiological coupling killing mechanism active
among mammals, birds, and reptiles has been identified.
Operating via environmental thermal effects upon maternal
core-skin blood flow critical to survival and development of
embryos, it reduces the flow of blood to the uterine tract.
Today, during hot summers, this phenomena kills embryos on a
vast, global scale. Because of sensitivity of many mammals
to modern heat, a major modern greenhouse could reduce
population numbers on a global scale, and potentially
trigger population collapses in the more vulnerable parts of
the world. In the geological past, the killing mechanism has
likely been triggered into action by greenhouse warming via
impact events, plume volcanism, and Earth orbital variations
(Milankovitch cycles).

Earth's biosphere is maintained and molded by the flow of
energy from the solar energy source to Earth, and on to the
space energy sink (S-E-S) [1]. This S-E-S energy flow
maintains Earth's biosphere and its living components, as
open, intermediate, dissipative, nonequilibrium systems
whose states are dependent upon the rate of energy flowing
through them. Greenhouse gases such as CO2 in the atmosphere
influence the S-E-S energy flow rate. Steady state flow is
necessary for global ecological stability (autopoiesis).

Natural fluctuations of the C cycle such as rapid releases
of CO2 from the mantle, or oceans, disrupt steady state S-E-
S flow. These fluctuations constantly challenge the
biosphere; slowdown of S-E-S energy flow drives it toward
thermodynamical equilibrium and stagnation. Fluctuations
induced by impact events, mantle plume volcanism, and
Milankovitch cycles can grow into structure-breaking waves,
triggering major perturbations of Earth's C cycle, and mass
extinctions.

A major C cycle perturbation involves readjustment of the
outer physiochemical spheres of the E: the atmosphere,
hydrosphere, and lithosphere, and by necessity, the
biosphere. A greenhouse, one manifestation of a major C
cycle perturbation, is the most dangerous natural phenomenon
that life of E can experience. Greenhouse conditions existed
during the Cretaceous Tertiary (KT) mass extinctions of 65
million years ago [2], and the Pleistocene-Holocene (P-H)
mammalian extinctions of 10,000-12,000 years ago. Coupling
climatology to reproductive physiology via effects of
ambient air temperature upon uterine blood flow to
developing embryos accounts for the extinctions via
established physiological principles.

The KT extinctions seem a function of both Late Cretaceous
climatic cooling and KT greenhouse warming that began at or
about the KT boundary. A negative delta^13C excursion
indicates a major C perturbation, and a synchronous negative
delta^18O excursion, climatic warming. The greenhouse lasted
two to three hundred thousand years into Early Tertiary.

Two natural phenomena are candidates for the KT boundary
greenhouse: the Deccan Traps mantle plume volcanism in
India, and the Chicxulub structure on Yucatan, which has
been attributed to extraterrestrial impact event. Ninety
percent of the Deccan Traps' vast volume of tholeiitic lavas
erupted 65 million years ago [3]. The Chicxulub structure
has been dated at about the same age [4]. Massive KT CO2
release via Deccan Traps eruptions [5], or impact [6], or
impact-induced water vapor release [7] would have disrupted
S-E-S energy flow, triggering change of state of the
biosphere manifested in the KT transition extinctions.

Other sources of KT transition CO2-induced greenhouse
conditions were failures of the Williams-Riley marine "pump"
that removes CO2 from the atmosphere-marine mixed layer and
stores it in the deep oceans, and a KT transition marine
transgression starting just below the KT boundary and
extending through early Paleocene (130 m rise) [8] that
flooded highly productive terrestrial plant ecosystems with
low productive marine ecosystems [9]. The KT record contains
no definitive evidences of an "impact winter" global
blackout and refrigeration [10].

A key point in linking climate to embryo survival is that
embryo damage and death can occur at environmental
temperatures that pose little danger to adults. During hot
conditions, adults shunt blood to the skin to transport body
heat to the environment. This action reduces blood flow to
the core, and to the uterine tract. Blood flow to the
uterine tract carries damaging metabolic heat away from
developing embryos [11], and supplies critical O, water,
nutrients, and hormones [12]. Reduced uterine blood flow
causes the uterine tract to overheat, damaging and killing
fertilized eggs during the critical first cleavage. During
later stages of development, reduced uterine blood flow can
produce dwarfing and skeletal abnormalities. For modern
pregnant, lactating European-type cattle, air temperature in
the range of 70-81 degrees F (21-27 degrees C) causes the
core temperature to rise. A rise in uterine temperature of
1.8-2.7 degrees F (1.0-1.5 degrees C) will kill most
embryos. Study of air temperature vs. conception rates of
12,000 Florida cattle via artificial inseminations showed
that on days when the air temperature following insemination
exceeds 86 degrees F (30 degrees C), conception rates fall
from 52% to 32%, and then stay low during the hot summer
months [13].

Vertebrate adaptation to climate involves adjusting size and
S/V ratios, body shape, insulation, and metabolism. Of all,
size is slowest to change in response to sudden climatic
warming. Large Pleistocene mammals adapted to a cold ice age
climate would have had difficulty getting rid of body heat
during rapid warming. The main pulse of the P-H mammalian
extinctions coincided with the abrupt warming about 11,700
years ago that ended the last ice age (indicated in
Greenland ice cores). Those extinctions eliminated primarily
large "big game" mammals, and produced dwarfing and skeletal
abnormalities, reflecting the "signature" of climatic heat
effects upon uterine blood flow to developing embryos [14].
Milankovitch cycles drive the Cenozoic ice age cycles and
also seem to influence mammalian bioevolution and
extinctions.

Among birds, uterine blood flow is reduced by high
environmental temperatures. Studies of modern chickens
indicate that hyperthermia diminishes blood flow to the
ovarian follicles by 40-50%, decreasing shell weight and
quality, and reducing egg production [15]. During the P-H
climatic warming at the end of the last ice age, large birds
experienced more extinctions than any other group except
large mammals [16]. In the face of rapid warming, cold-
adapted large birds would have experienced hyperthermia.
Hyperthermia-induced reduction of uterine blood flow to the
follicles would have reduced fertility and egg production,
triggering collapse of populations.

For moderate to large reptiles (including the dinosaurs),
"gigantothermy," a thermoregulatory strategy based on use of
large body size, low metabolic rate, peripheral tissues as
insulation, and control of blood flow between core and skin
[17], allows successful response to diurnal and seasonal
climatic changes. Via such strategy, the modern leatherback
turtle, the largest marine turtle (1000kg), maintains body
temperature of 86 degrees F (30 degrees C) in Arctic waters
of 50 degrees F (10 degrees C), or less; it has a problem
dumping heat in a warm terrestrial environment [17].
Dinosaurs that were adapting to Late Cretaceous climatic
cooling would have, in the face of KT boundary greenhouse
warming, experienced thermal stress, and likely uterine
blood flow-induced reproductive problems. Hot summers rather
than cold winters likely caused the disappearance of the
dinosaurs [18].

Proposed law of nature: Climate control of uterine blood
supply to the fertilized egg during first cleavage and later
embryo development influence population dynamics,
bioevolution, and extinction.

References: [1] Morowitz H. J. (1979) Energy Flow In
Biology, Ox Bow Press. [2] McLean D. M. (1978) Science, 201,
401-406.[3] Basu A. R. et al. (1993) Science, 261, 902-906.
[4] Sharpton V. L. et al. (1992) Nature, 359, 819-821. [5]
McLean D. M. (1985) Cretaceous Research, 6, 235-259. [6]
O'Keefe J. D. and Ahrens T. J. (1988) LPS 14, 885-886. [7]
Emiliani C. et al. (1981) EPSL, 55, 317-334. [8] Keller G.
et al. (1993) GSA Bull., 105, 979-997. [9] McLean D. M.
(1978) Science, 200, 1060-1062. [10] McLean D. M. (1991) in
Global Biomass Burning: Atmospheric, Climatic, and
Biospheric Implications, MIT Press, 493-503. [11]
Gwazdauskas F. C. et al. (1974) J. Anim. Sci., 39, 87-92.
[12] Senger et al. (1967) J. Exp. Zool., 165, 337-344. [13]
Badinga L. et al. (1985) J. Dairy Sci., 68, 78-85. [14]
McLean D. M. (1981) Am. J. Sci., 281, 1144-1152; (1991) in
Energy and Environment, ASHRAE, 93-100. [15] Wolfenson D. et
al. (1979) Br. Poult. Sci., 20 167-174. [16] Steadman D. W.
and Martin P. S. (1984) in Quaternary Extinctions, Univ.
Arizona Press, 466-477. [17] Spotila J. R. et al (1993)
Modern Geology, 16, 203-227. [18] Cloudsley-Thompson J. L.
and Butt D. K. (1977) Br. J. Herpet., 5, 641-647.


Johnson K.*
   To Be Determined

No abstract available.



                              POSTER PRESENTATIONS

Gardner A. F.   Wright I. P.   Gilmour I.
   Organic Matter Changes Across Nonmarine KT Boundary Sections

Previous work on C and N in KT boundary clays at marine
sites has revealed enrichments for both elements, as well as
corresponding changes in isotopic compositions [1-3]. The
enrichment in N has been attributed to either acid rain [4]
or an anoxic environment [2]. The N enrichment at marine
sites, while being a worldwide phenomenon, is apparently not
universal. For instance, no enrichment was noted at Gubbio,
Italy or Stevns Klint, Denmark [3]. We have examined both C
and N abundance and isotopic compositions from two non-
marine KT sequences in the USA (Brownie Butte, Montana, and
Raton Pass, New Mexico).

At Brownie Butte the fireball layer is separated from an
overlying coal seam by a thin shale band, which is
characterized by a fern spore abundance anomaly and is
thought to represent a resurgence in biological productivity
above the boundary. It has been termed the recovery layer
[5]. A series of 12 samples spanning the boundary, along
with pure samples of the ejecta and fireball layers from
Brownie Butte were treated with HF/HCl to leave a residue
(total C fraction) containing kerogen, elemental C, and
soot. In a similar fashion 16 samples from across the
boundary at Raton Pass, were also prepared. All samples were
then analyzed for C and N. Carbon isotope measurements were
made on a VG SIRA 24 mass spectrometer using sealed tube
combustion and N isotope measurements were made using the
method of Boyd et al. [6,7] on a static vacuum mass
spectrometer.

Results from Brownie Butte (Figs. 1 and 2) show C
concentration to decrease 26 fold in the ejecta layer,
compared to late Cretaceous values, before increasing 18
fold in the fireball layer, followed by a further 4 fold
increase in the layer above. N concentration reflects this
pattern, with a 25 fold decrease in the ejecta layer,
followed by a 10 fold increase. The recovery layer increases
in N by a further 31 fold. The results from Raton Pass
(Figs. 1 and 2) parallel those of Brownie Butte, showing a 9
fold decrease in C concentration at the boundary and 28 fold
increase in the layer above, while N concentration decreased
13 fold at the boundary followed by 44 fold increase
immediately above. Elemental C and soot are concentrated in
the fireball layer at the Raton section [8] though soot does
not comprise the bulk of the C remaining after acid etching.

The C isotopic composition of the total C at both sites
(Fig. 3) shows a marked shift in delta^13C across the
boundary. At Raton Pass, values shift from -24.5 permil
immediately below the boundary to -27.3 permil immediately
above, becoming progressively heavier at -26.3 to -25.6
permil in the overlying coal. The values reported are
similar to those of Hildebrand and Wolbach [8] who also
observed a -2 permil shift in total C isotope compositions
at the boundary. The Brownie Butte samples also show a shift
in their delta^13C values, changing from -26.2 permil below
the boundary to -28.3 permil in the thin shale band, again a
shift of some -2 permil. A similar trend towards heavier
isotopic compositions is observed in the overlying coal as
is seen at Raton Pass.

Organic N isotope composition at Raton Pass (Fig. 4)
averages around +1 permil below and above the boundary,
though there is an apparent shift of around + 1.5 permil in
the lowest Tertiary before returning to Cretaceous values.
Similarly at Brownie Butte (Fig. 4), Cretaceous and Tertiary
delta^15N values range from +2 to +3 permil with a small
(<1%o) shift towards heavier values in the lowermost
Tertiary samples. At both sites the fireball layer is
isotopically lighter at around 0 to 0.5 permil, however the
amount of N in these layers is comparatively low (295 ppm at
Brownie Butte and 7 ppm at Raton Pass).

The enrichments in N abundance observed at marine sites [2-
4] occur in the basal layer and continue throughout the
boundary clay representing a substantial accumulation of
isotopically anomalous N over very short time scales. The
basal layer is thought to be stratigraphically equivalent to
the fireball layer observed in North American sections.
However, the fireball layer itself does not show any
enrichment in N abundance; in fact it is generally depleted
in both organic C and N as might be expected for a layer
that essentially represents atmospheric fallout. An
enrichment in N does occur in the overlying recovery layer
and persists into the coal. However, there is no substantial
change in C/N ratio at either Brownie Butte (C/N=30 to 35)
or Raton (C/N=40 to 50) between Cretaceous and Tertiary
samples implying no additional input of N at or immediately
above the boundary as might be expected had the shock
combination of N2 and O2 in the atmosphere led to the
production of nitric acid rain that had subsequently been
fixed as organic N. C/N ratios are slightly higher in the
fireball layers at both sites (Raton, C/N=62; Brownie Butte,
C/N=61) presumably reflecting the input of elemental C and
soot to this layer.

The increase in both organic C and N above the boundary,
however, reflects a significant environmental change from
conditions preceding the KT boundary. The 0-3 cm layer
coincides with a fern spore abundance anomaly and its
palynology is thought to reflect the first resurgence of
biological productivity immediately following the KT event
[5]. The isotopic changes seen in this layer at both sites
may therefore be a record of environmental change
immediately following the impact. Interestingly, the C
isotopic changes observed in the Raton Pass samples studied
here are similar to those reported by Hildebrand and Wolbach
[8] for a separate set of samples from the Raton Basin where
the boundary clay was not overlain by a coal seam. They are
also similar to those from Brownie Butte where the coal is
separated from the fireball layer by a narrow band of shale.
This would imply that the isotopic compositions reflect
large scale changes in the environmental conditions,
independent of local effects. It has recently been suggested
that the negative shift in C isotopes observed in carbonates
at many marine KT sequences may be the result of
isotopically light CO2 entering the atmosphere as a result
of widespread fires [9]. A similar effect might also be
expected in organic C at non-marine sites, had the isotopic
composition of atmospheric CO2 significantly differed from
the late Cretaceous, value as this would be reflected in
more negative delta^13C values for photosynthetically
produced C. Such a change is apparent at both the sites
studied here, however, such changes in the delta^13C of
organic C are not unusual across stratigraphic boundaries
and more detailed work, specifically on the isotopic
compositions of individual organic compounds is required to
see whether the changes observed are indeed due to such a
drastic change in the Earth's environment.

References: [1] Wolbach W. S. et al. (1990) GSA Spec. Paper,
247, 391-400. [2] Gilmour I. et al. (1990) GSA Spec. Paper,
247, 383-390. [3] Robert F. et al (1990) Meteoritics, 25,
401. [4] Prinn R. G. and Fegley B. (1987) EPSL, 83, 1-15.
[5] Tschundy R. H. et al. (1984) Science, 225, 1030-1032.
[6] Boyd S. R. et al. (1988) J. Phys. E: Sci. Instrum., 21,
876-885. [7] Wright I. P. et al. (1988) J. Phys. E: Sci.
Instrum., 21, 865-875. [8] Hilderbrand A. R. and Wolbach W.
S. (1989) LPS XX, 414-415. [9] Ivany and Salawitch (1993)
Geology, 21, 487-490.


Sutherland F. L.
   Northern Meteoritic Impacts and Southern Volcanic Mantle Plumes: Their Roles
   in Generating Some Differential KT Boundary Extinction Environments

The long, vigorous debate [1,2] for impact [3] or volcanic
[4] causes in KT extinctions [5] has established both
processes as viable events [6,7].

Impact Views: Precise dating links the Chicxulub crater,
Yucatan, with proposed `tektite' deposits in Haiti and
Mexico [8,9] and also correlates them with the Manson crater
USA [10] and other minor craters [11]. These are considered
impact features by most workers, although some prefer
volcanic origins [12]. The nature and correlation of glasses
[13-18] and proposed tsunami deposits [19,20] from
Caribbean-Mexico impact are also under debate.
Interpretations of KT boundary impact deposits in relation
to Chicxulub and Manson events differ [21,22], but favor a
Chicxulub link [23]. KT boundary beds vary in global trace
element signatures [24]. The impact school considers a large
chondritic (?) impactor accounts for the Ir anomaly [25],
high pressure or high temperature minerals (shocked quartz
[26], stishovite [27], diamond [28], Ni spinels [29]) and
extraterrestrial amino acids [30].

Volcanic views: This school contends that voluminous flood
basalt episodes, e.g., Deccan plume [31] and large explosive
eruptions [32] explain Ir anomalies, shocked minerals, and
boundary clay characters [33]. The Ir argument uses Ir
detected in hotspot volcanoes, e.g., present Deccan plume
site [34] and in volcanic horizons in Antarctic ice [35].

Combined views: One scenario triggers flood volcanism by
meteorite impact [36] (and more specifically at KT time
[37]). Another considers impact and volcanism as
coincidental but unrelated events [7].

Palaeontological views: KT mass extinctions caused by impact
and/or volcanism also rely on precise palaeontological
correlations [38,39]. Northern terrestrial [40,41] and
shallow marine [42] palaeobotanical records support some
catastrophic KT extinctions. Northern dinosaur records seem
sharply truncated, with Tertiary remains being reworked [43]
and some ambiguous sections proving Cretaceous [44].
Dinosaur eggs (France, China) show abnormal increases of Se
[45] and pathological protein structure [46] in egg shells
towards the KT boundary. The extinction of the dinosaurs
contrasts with the survival of most large vertebrate groups
[47]. The `classic' KT extinction of planktonic foraminifera
differentially selected the large, ornamented, deeper-
dwelling tropical forms, suggesting extended rather than a
sharp general extinction [48]. Absence of `wildfire' soot
[41], reduced floral extinctions [49], transitional
extinctions in some southern KT sections [50] and presence
of surviving animal groups in Australia [51] suggest some
possible differential extinctions.

Hemispherical views: Available evidence suggests multiple
northern meteorite impacts (Fig. 1), with a Chicxulub impact
giving KT Ir anomalies and extinctions [52] and a Pacific
impact (?) giving Ni spinel fallout [53]. The KT deposits
suggest fractionation of platinum group elements after
impact [25] and a lethal thermal pulse from re-entering
ejecta from a Chicxulub impact was probably regional rather
than global [54]. The southern hemisphere in contrast shows
more starting plume and extant hotspot activity around KT
time (Fig. 1). The main northern plumes (Hawaii, Greenland)
started well in the Cretaceous [7]. Initial Deccan volcanism
probably pre-dates and overlaps the KT boundary by 1-3Ma
[55].

Synthesis: The KT Ir anomaly includes lower level (300 ppt)
wings and a sharp higher level (3000 ppt) peak [56]. The
extended basal wings accommodate the estimated Deccan Ir
budget and the central peak requires sudden Ir input (Fig.
2). This suggests a late Cretaceous biota already exposed to
abnormal volcanism, when subjected to sudden impact effects.
Environmental perturbations from Greenland-North Atlantic
rift margin volcanism continued into the early Tertiary
[57]. Coincidence of major meteorite impact during flood
volcanism can be expected in geological time [58]. The KT
event shows distinct differences to the even larger Permo-
Triassic extinction event [59], which coincided with
voluminous (1.5 x 10^6km^3) Siberian plume lavas [60].

Conclusion: Predominant northern impacts imposed on dominant
southern volcanism gave unevenly disturbed KT environments.
This led to geographically modified extinctions in some
[47,61] but not all [42,62] groups.

Acknowledgements: Royal Society of New South Wales for
adapted figures.

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McClaren D. J. and Goodfellow W. D. (1990) Annu. Rev. EPS,
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F. L. (1993) J. Proc. R. Soc. NSW, 126, 1-25. [8] Swisher C.
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Fig.1, which appears here in the hard copy, shows suggested
meteorite impacts and hotspot volcanism at KT time.

Fig. 2, which appears here in the hard copy, shows an
interpretation of Ir anomaly across KT boundary.


Samoylov V. S.   Benjamini Ch.
   Geochemical Record of the KT Event in Dinosaurian Remains from Mongolia

Geochemical data obtained from Cretaceous-Paleogene faunas
from South Mongolia (Tauson et al., 1990) have shown that
geochemical features of Late Cretaceous faunal remains are
preserved. We present here new investigations of reptilian
remains from the Gobi Desert, a well-known locality for
vertebrate remains of Mesozoic and Cenozoic age.

Dinosaurian remains were collected in South Mongolia from
sediments dated to the Early Neocomian-Aptian-Albian of the
Early Cretaceous and the Cenomanian-Maastrichtian of the
Late Cretaceous. Geochemical features were investigated from
dinosaurian remains over a period of about 70 Ma, nearly to
the time of their extinction. Maastrichtian deposits are
overlain with discomformity and discordance by Late
Paleocene sediments in which dinosaurian remains are absent,
but other reptilian remains have been observed.

Different types of dinosaurian remains (bones, eggshell, and
coprolite), including a large collection of bones from
different parts of dinosaurian skeletons (vertebrae,
extremities, ribs, skull, pelvis) were collected. In
parallel with dinosaurian remains, the Cretaceous sediments
enclosing them, were examinated, as well as other
Cretaceous-Paleogene faunal remains.

Dinosaurian remains were completely fossilized but are
characterized by good external and morphological
preservation. Moreover, the fossilized dinosaurian remains
also preserved their internal textural features and the main
chemical characteristics. Bone remains preserved the
haversian canal texture characteristic for bone tissue of
vertebrates. The haversian canals in osteons were packed by
calcite aggregates, which were surrounded by thin lamellar
apatite crystals.

The bone remains have carbonate-phosphate composition and
were consistently enriched in REE, Y, Sr, Ba, Pb, U, and Th.
Rare-earth elements, yttrium, strontium, barium, and lead
are mainly concentrated in the apatite, while zinc is mostly
accumulated in the carbonate fraction.

Considerable anisotropy was established for the distribution
of fluorine and several trace elements between different
parts of the skeleton (Table 1). The enrichment of bone
remains in Sr, Ba, Pb, REE, and Y is observed in the
following order: ribs -pelvis-skull-extremities-vertebrae.
Morphological types of dinosaurian bone remains are markedly
distinguished by REE-spectrum with the biggest accumulation
of HREE and Y and HREE relative to LREE in vertebrae.

Eggshell has an essentially calcite composition and is
distinguished geochemically from bone remains by sharp
depletion in F, S, and trace elements. REE-spectrum of these
two types of dinosaurian remains is also different. The
geochemical features of eggshell is defined by the chemical
composition of the calcite forming 90% and more of these
remains. Calcite from eggshells was depleted in trace
elements relative to calcite from the bone remains. At the
same time, calcite from eggshell is also significantly
distinguished from epigenetic calcite (Table 1).

Coprolite, as with bone remains, represents carbonate-
phosphate fossil material with approximately the same
proportion of calcite and apatite fractions, but coprolite
was markedly enriched in F, Sr, but was depleted in REE, Y,
and Zn. Besides, LREE, especially La and Ce, contribute more
to the REE spectrum. On the average, the value of LREE/HREE
ratio is 9,8 that is 4,3 times more than for dinosaurian
bone remains.

The epigenetic processes causing alteration of the bone
chemical composition were represented by the carbonatization
and rarely by silicification and baritization. On the whole,
these processes led to the depletion of bones in F, S, Sr,
Pb, REE, Y. The change of the REE spectrum with the
accumulation of LREE relative to HREE is characteristic for
altered bones. Epigenetic processes evidently cannot cause
the accumulation of some trace elements in dinosaurian bone
remains. On the contrary, these processes are the cause of
the depletion in these elements, apart from barium (Table
1). The progressive enrichment of bones in F, Sr, Ba, REE,
Y, Pb, Th, and U and depletion in Zn and LREE relatively to
HREE took place during the Late Cretaceous. Analogous
behavior of these elements is also established during the
Maastrichtian up to K/T boundary. Late Maastrichtian bone
remains are especially enriched by F, Sr, Ba, U, REE, Y, Pb,
Th, and HREE+Y relative LREE. The observed distribution of
elements in dinosaurian bone remains can result from the
different processes.

However, it is difficult to attribute the anisotropy in
distribution of elements between separate skeleton parts to
geological processes. A significant pause in geotectonic and
magmatic processes took place in South Mongolia in the very
Late Cretaceous (from ~80 Ma) and in the very Early Cenozoic
(to ~45 Na). Thus, these processes cannot be involved as a
cause of local dinosaurian extinctions at the Mesozoic-
Cenozoic boundary. Moreover, at the time of Late Jurassic-
Early Cretaceous intensive continental rifting and attendant
magmatism, dinosaurian faunas were blooming.

Epigenetic processes cannot be also considered as the cause
of trace element and volatile accumulation in dinosaurian
bone remains. On the contrary, these processes led to
considerable depletion of bones in trace elements.

In South Mongolia, a direct relation is noted between the
type of sedimentation usually the mudflow deposits and the
conditions, which were favorable for preservation of
dinosaurian remains. The excellent textural and chemical
preservation of dinosaurian remains are attributed to their
fast burial at the formation of mudflow deposits, presumably
under aridic conditions. At the same time, it appears
unlikely that sedimentation and diagenesis affect bone
remain composition. There is no correlation in trace element
composition between bones and the sediment enclosing them,
between bone-bearing and bone-free sediments, and even
between the composition of the bone phosphate fraction and
the apatite from sediments, enclosing these bone remains.
The apatite from sedimentary rocks is depleted in REE and Y
in comparison with bone apatite and REE-spectrum is
obviously different for these two types of phosphate
fractions. It is difficult to explain this distinction in
chemical composition, if both fractions had been subjected
to the same diagenetic processes.

The data on distribution of some trace elements between
dinosaurian skeleton fragments testify to the considerable
anisitropy in the distribution of some volatile and trace
elements between different parts of the skeleton. It is
unlikely that this distribution had been the result of
epigenesis or fossilization. It is impossible to explain by
these processes the difference in elemental accumulation in
different parts of the skeleton, including the very
different concentration of individual rare-Earth elements in
them. The composition of dinosaurian remains was evidently
formed during their lives. Considerable anisotropy of
element distribution in different dinosaurian remains and
different parts of each skeleton confirms this conclusion.

Maastrichtian remains are characterized by maximum contents
of fluorine and some trace elements. These data reflect
changes in dinosaurian life conditions in South Mongolia
during the Late Cretaceous, with special peculiarities in
the Maastrichtian. Data on chemical composition of Late
Cretaceous and Paleocene faunal remains show special
enrichment of all Maastrichtian remains, not only of
dinosaurs but other fauna, some trace elements at the
Mesozoic-Cenozoic boundary and, thus, the special biosphere
conditions of that time.It is interesting that trace
elements and fluorine are especially concentrated in the
dinosaurian vertebrae. Specifically, vertebrae fragments
were enriched in Y and HREE, which are potentially very
toxic. In this context, the data on the partitioning of some
volatiles and trace elements between bones and coprolite is
worth noting. They demonstrate that excretion failed to
remove some key elements from dinosaurian organisms, most
significantly HREE and Y, which are accumulated in
dinosaurian skeletons.

Geochemical studies of dinosaurian remains from South
Mongolia demonstrate that their geochemical features were
determined by original biochemistry of dinosaur bone
tissues. The evolutionary character of change in dinosaurian
bone chemistry indirectly indicates that dinosaurian
extinction in South Mongolia was not the result of an abrupt
catastrophe. This extinction was evidently predetermined by
changes of their living conditions during the Late
Cretaceous.

References: [1] Tauson L. V. et al. (1990) Doklady Akad.
Nauk SSSR, v. 311(1), 200-204 (in Russian).


Burke K.
   Rift-flooding Episodes Around 65 Ma as Causes of Abrupt Sea-level Falls: Did
   the KT Impact Happen During a Time of Frequent Catastrophes?

Catastrophic filling of the kind of subaerial,sub-sea-level
depression commonly formed during the opening and the
closing of ocean basins is the only known mechanism, other
than glacial eustasy, capable of lowering global sea-level
by tens of meters within a few thousand years [1].

At present there is no evidence of the existence of large
continental ice-sheets that could have induced significant
glacial eustatic changes during the Late Cretaceous. But
several continental rift systems were evolving toward ocean
basin formation during the 85 Ma to 50 Ma interval (see
Table 1 and Fig. 1). Episodes of the sudden spilling of
ocean waters into deep subaerial depressions in some of
these rifts could have lowered sea level in discrete, rapid,
and substantial steps and may in that way have contributed
to the exposure of continental shelf areas, to the
destruction of reefs and to the extinction of some life
forms.

Ocean spill events are recognized in closing ocean basins
where evaporites have formed during low-stands as happened
in the Mediterranean at 6 Ma during Messinian time [2] and
in the North Caspian depression during the Late Permian [3].
Spill events are also recognizable from rifts with
evaporites that are linked to ocean opening such as those of
the South Atlantic, formed during Aptian time at about 115
Ma [1] and those of the Gulf of Mexico formed during the
late Jurassic at about 155 Ma [4]. Three of these spills
have all been estimated to have lowered global sea-level by
about 10 m [1] and the Permian spill was probably larger.
The Arctic Ocean was tectonically isolated during the Late
Cretaceous and Early Cenozoic (See Fig. 1 and [5]). Its
waters may also have been isolated from the world ocean for
part of that time. Ocean spills into an Arctic basin, which
then had an area in excess of one million square kilometers
would have reduced global sea level considerably.

A ten-meter lowering of sea level is sufficient to have
catastrophic effects on shallow-water life-forms especially
when, as during the Late Cretaceous, large areas of the
continents are flooded. Catastrophic changes in the area of
continent flooded are also likely to modify the chemistry of
the linked ocean and atmosphere system by rapidly inducing
changes in CO2-concentration. Where a deep continental rift
was filled to the brim by a lake, as the Lake Baikal and
Lake Tanganyika rifts are today, the transition to ocean
would have had no effect on global sea level. If the rift
was partly filled by a lake the transition would have
reduced sea-level by less. In temperate latitudes and near
the equator lake-filled rifts are probable although rift
shoulders are likely to reduce drainage into the rifts. At
high latitudes and in desert latitudes dry rifts are more
likely. Most of the rifts in Table 1 lay in high latitudes
at the critical time and there is a strong possibility that
they were not occupied by deep lakes. Erwin [6]
characterizes the late Permian as involving successive,
rapid, sea-level lowering events. Flooding the North Caspian
depression, a closing ocean basin at the time [1,3] and
flooding possible sub sea-level foreland basins in the
Zechstein and West Texas may have contributed to these
changes.

The long term effects of spill episodes on sea-level are
small because they are dwarfed by changes in spreading
center volume and changes in mass distribution in the
shallow mantle. It is the sudden nature of the spills that
warrents the use of the term catastrophic and it is the same
suddenness that may have led to catastrophic effects on life
forms and the environment.

Conclusions: Because of an unusual likelihood of ocean-
spills associated with an unusually large number of rift to
ocean basin transitions and also because of possible spills
into a then isolated Arctic Ocean basin the 20 my interval
bracketing 65 Ma may have been a time of unusually frequent
episodes of sudden sea-level fall and for that reason an
interval of episodic, rapid, environmental change.

Discriminating the effects of the great Chicxulub impact
from this background of repeated catastrophe represents a
challenge that is likely to require the collaborative
efforts of a variety of specialists including geochemists,
paleobiologists, stratigraphers, and tectonicians. The two
different kinds of catastrophe should have had recognizably
different effects on sea-level, on the environment, and on
biota. Extinction of some life forms would be a feature
common to both kinds of catastrophe.

Possible effects on sea level from rift initiation have been
analyzed by Cathles and Hallam [6]. The model presented here
is somewhat different and indicates some specific examples
as research targets. Testing the model especially by
studying the sequence stratigraphy of the deeply buried
sediments deposited during rift to ocean transitions may
prove feasible.

References: [1] Burke K. and Sengor A. M. C. (1988) Marine
Geology, 83, 309-312. [2] Hsu K. J. et al. (1973) Nature,
242, 240-244. [3] Burke K. (1977) Ann. Rev. EPSL, 5, 371-
398. [4] Burke K. (1975) Geology, 3, 613-616. [5] Burke K.
(1984) Geology, 27, 4, 189-198. [6] Erwin P. (this volume).
[7] Cathles L and Hallam A. (1991) Tectonics.

Table 1, which appears here in the hard copy, shows Late
Cretaceous and Paleocene Rift transitions to oceans and
possible associated global sea level falls. Sea level is
likely to have fallen abruptly at a time within a few
million years of the age of formation of the oldest ocean
floor.

Fig. 1, which appears here in the hard copy, shows isolation
of the arctic ocean basin.


MacLeod K. G.   Huber B. T.   Ward P. D.
   The Biostratigraphy and Peleogeography of Maastrichtian Inoceramids

The global distribution of Maastrichtian inoceramids is now
known in enough detail that the patterns of disappearance
can be used to place first order constraints on
paleoceanographic changes that may have occurred during that
age. The Inoceramidae is an excellent group to focus on in a
study of Maastrichtian events because (1) they were globally
distributed in the early Maastrichtian, (2) they did not
survive the age (i.e., they undergo change during the
interval), and (3) they have left a rich microfossil and
macrofossil record. Some inoceramids grew to be very large;
however, even the largest often passively disaggregated and
are preserved as 100s of millions of characteristic,
columnar, polygonal prisms of calcite ~100 micrometers
across. This taphonomic process has greatly increased the
inoceramid fossil record and provides a means of objectively
estimating changes in their standing population [1]. In
addition, because these prisms commonly occur in Deep Sea
Drilling Project (DSDP) and Ocean Drilling Program (ODP)
cores, it is relatively easy to generate a truly global
database. The existing macrofossil record of inoceramids has
less temporal and spacial resolution but greater taxonomic
resolution than the microfossil record. In concert the
microfossil and macrofossil records of inoceramids
demonstrate that important changes occurred during the
Maastrichtian. These changes are distinct from the
Cretaceous-Tertiary (KT) boundary catastrophe but are part
of the larger KT transition.

One striking aspect of the inoceramid record is how
ubiquitous these bivalves were during the early
Maastrichtian. Inoceramids are known from all continents and
we have documented prism occurrences in early Maastrichtian
strata from deep-sea cores collected in the North Atlantic,
the South Atlantic, the Pacific, the Indian, and the
Southern Ocean. In sections containing inoceramids, prisms
are found in every sample collected across 10s of meters of
section. In land-based sections we have studied, inoceramids
form virtual shell pavements on some bedding planes, and
prism densities >100,000 prisms/50 g samples are typical in
high abundance intervals.

During the mid-Maastrichtian, though, there was a
concentrated, world-wide pulse of extinction among
inoceramids. At the more than 25 sites where we have
recorded this event, inoceramids decline gradually but over
a relatively short stratigraphic interval. The rate at which
the extinction progressed falls between catastrophic [e.g.,
2] and gradual [3] scenarios that have previously been
proposed for the event. One unusual inoceramid taxon,
Tenuipteria, survives after the mid-Maastrichtian event and
disappears at the KT boundary. We think we have recently
found the first known occurrence of Tenuipteria shell
fragments in core material (DSDP hole 605 off Delaware).

Although the major pulse of extinction falls in the mid-
Maastrichtian, this extinction horizon is demonstrably
diachronous on a global scale. In high southern latitudes
(e.g., ODP holes 698A and 700B [4], and 750A) the last
occurrence of inoceramid remains is stratigraphically below
the first appearance of Abathomphalus mayaroensis. At lower
latitude sites in southwestern Europe [1] and in Indian
Ocean ODP holes 754B and 758A, inoceramid remains are found
above that datum. The first appearance of A. mayaroensis is
itself diachronous, but in a direction that accentuates the
diachroneity of the inoceramid extinction. A. mayaroensis
appears earliest in high southern latitudes [5,6] where
inoceramids disappear first. Therefore, there seems to be a
general Antarctic to equator progression in the timing of
the pulse of extinction among inoceramids.

A second trend is that the disappearance of inoceramids
seems to occur earlier in onshore areas than in nearby
offshore areas. This pattern is not yet well constrained,
but it has been observed in four regions. (1) In the Basque
region of northern Spain, the decline in the abundance of
inoceramid macrofossils occurs at a lower level (based on
lithostratigraphic correlation) in sections that represent
more shoreward areas than it does in more offshore areas.
(2) ODP Hole 761B, located off the northwest coast of
Australia, records an early extinction level compared to the
other Indian Ocean sites. (3) In the South Atlantic,
inoceramid prisms disappear earlier in a shoreward site
(DSDP Hole 356) than they do in a more offshore site at the
same latitude (DSDP Hole 21). (4) Finally, inoceramids
disappeared in Campanian strata on Seymour Island [7] while
in offshore ODP sites from high southern latitudes
inoceramid prisms clearly range into Maastrichtian strata.

Although inoceramid remains are ubiquitous in early
Maastrichtian strata, they are not omnipresent. We have
found inoceramids to be less common in Pacific Ocean sites
than in Atlantic or Indian Ocean sites. Perhaps inoceramid
occurrences are reflecting oceanographic differences between
the relatively old, wide Maastrichtian Pacific Ocean and the
relatively young, narrow Maastrichtian Atlantic and Indian
Oceans. Conversely, the scarcity of inoceramids in Pacific
sites may be an artifact of sampling as the recovery of
Cretaceous, calcareous sediments is relatively sparse in the
Pacific Ocean. More problematic is the absence of inoceramid
remains in two ODP sites from Maud Rise (689 and 690).
Compared to nearby, inoceramid-bearing ODP localities (698,
700, and 750), the Maud Rise sites may represent slightly
cooler environments as they were deposited a little further
south; however, foraminiferal assemblages indicate no
significant environmental differences among the sites. At
present we do not have an explanation for the absence of
inoceramids on Maud Rise.

In addition to these global patterns, where the
stratigraphic distribution of whole inoceramids is well
documented, there is an ordered pattern of species
extinction [8,9]. As already noted, only Tenuipteria ranges
through the late Maastrichtian, but there is also structure
in the pattern of disappearance of species of Inoceramus
within the mid-Maastrichtian pulse of extinction. In the
Basque region large platyceramids and endocostids disappear
first; the youngest Inoceramus taxon found is I.
(Trochoceramus) morgani, which is a relatively small form
[9]. Anecdotal evidence suggest that this morphologic
pattern of disappearance is repeated outside the Basque
region, and ongoing research is attempting to confirm these
reports. If the pattern holds up, it suggests that the more
typical, large, flat inoceramids were least tolerant of the
changes occurring during the Maastrichtian.

The pattern of disappearance of inoceramids indicates that
there was a major event during the mid-Maastrichtian, which
affected the benthic environment in all the world's oceans.
The duration and the diachroneity of the inoceramid
extinctions suggests that the forcing mechanism was a
gradual change not a sudden, catastrophic perturbation. If
most Maastrichtian inoceramids were adapted to low-O
environments, a reorganization of ocean circulation leading
to increasing influence of oxygenated, Antarctic bottom
waters fits the constraints imposed by inoceramid
biostratigraphy and paleogeography. In this scenario typical
inoceramids would be expected to survive longest in somewhat
isolated basins in low latitudes. Regardless of the ultimate
fate of this hypothesis, though, changes occurring during
the mid-Maastrichtian provide the opportunity to examine the
interplay of ecological variables on a global scale across a
geologically resolvable interval of time.

References: [1] MacLeod K. G. and Orr W. N. (1993)
Paleobio., 19, 235-250. [2] Kauffman E. G. (1988) Revista
Espanola de Paleontologia, 57-71. [3] Dhondt A. V. (1983)
Zitteliana., 10, 689-701. [4] Huber B. T. (1991) in ODP Sci.
Res., 114, (P. F. Ciesielski et al. eds.), 281-297. [5]
Huber B. T. (1992) Palaeo., Palaeo., Palaeo., 92, 325-360.
[6] Huber B. T. and Watkins D. K. (1992) in AGU Antarctic
Research Series, 56 (J. P. Kennett and D. A. Warnke eds.),
31-60. [7] Zinsmeister W. J. (1993) GSA Ab. Prog., 25, A-
295. [8] Ward P. et al. (1991) Geol. 19, 1181-1184. [9]
MacLeod K. G. (in press) J. Paleo.


Heymann D.   Wolbach W. S.   Chibante L. P. F.   Smalley R. E.
   Search for Extractable Fullerenes in Clays from the KT Boundary of the
   Woodside Creek and Chancet Rock sites, New Zealand

Fullerenes were first discovered in a geologic environment
by Buseck et al. [1]. The occurrence of significantly
increased concentrations of C, mostly soot, and of Ir at the
KT boundaries of at least eleven sites worldwide has been
interpreted as evidence for the raging of major wildfires in
the aftermath of the "Terminal Cretaceous Impact" [2-4]. The
recent discovery that soot generated by the free burning of
toluene and candle wax contains some 50-100 ppm extractable
fullerenes [5] suggested a chance that fullerenes are
associated with the soot at KT boundaries. A search for
extractable C(sub)60 and C(sub)70 fullerenes as markers for
fires of wood, oil, or gas. We are grateful to Dr. R. R.
Brooks of Massey University, Palmerston North, New Zealand
for having donated these samples for our study.

Table 1 describes the samples. (WC)KTBa and (WC)KTBb were
splits from a homogenized boundary clay from the Woodside
Creek site. (WC)KTBa was first extracted prior to, then
extracted again after demineralization. (WC)KTBb and (FR)KTBa
were only extracted after demineralization. Also
available from Woodside Creek, but not shown in the table,
were two samples from a shale located at some 32 cm above
the KT boundary. These were named (WC)KTa and (WC)KTb after
demineralization.

All glasware was new and was heated in an oven in air at 500
degrees C. The samples were digested with toluene at room
temperature 30 minutes. Solutions and solids were separated
by centrifugation. The solution was evaporated to dry at
reduced pressure and 45 degrees C. Then the flasks of the
(WC) samples were rinsed with 0.5 ml and the flask of the
(FR) sample with 0.1 ml toluene. Procedural blanks were
always done.

Twenty-five microliter of solution were injected with a
dedicated pipette into the High Pressure Liquid
Chromatograph. UV-Visible spectra were taken in the range
310-430 nm every 4 s, but the processing of the
chromatograms was done at 330 nm. Prior to a series of
analyses, "instrumental blanks" were analyzed by the
repeated injections of fresh toluene. The retention times of
the C(sub)60 and C(sub)70 are 4.44 and 6.91 min.
respectively. At the end of a series of blanks and samples,
the instrument was calibrated by the injection of only 4.8
ng C(sub)60 and 5.3 ng C(sub)70. The limit of detection was
about 0.2 ng C(sub)60 injected, that is to say 1.7 x 10^11
molecules.

Figure 1 shows the enlarged chromatogram for the injection
of 25 microliters from (WC)KTBb#. The chromatogram has a
clear-cut peak at the expected retention time of C(sub)60,
riding on the "tail" of a strong "oil" peak. Unfortunately,
even the combined solutions of all (WC) extractions
contained too little C(sub)60 for the acquisition of an
acceptable UV-Vis spectrum. No signal significantly above
noise at the retention time of C(sub)70 was found in any of
the samples.

Figure 2 summarizes the analyses of extracts from (WC)KTBa,
(WC)KTBa#, (WC)KTBb#, and (FR)KTBa#. In every subfigure, the
numbers on the X-coordinate represent the sequence of
injections of samples and blanks. The Y-coordinate
represents the integrated peak areas at 4.44 min. It is seen
that only injections from extracts of boundary samples yield
peak areas significantly above the blank values. We adopted
75 +- 50 AUS for corrections. We surmise that the boundary
samples contain C(sub)60.

Table 2 presents the numerical results. In addition to the
error from blank corrections, there is a substantial
stochastic error of +-8% from the injected volumes as
deduced from repeated injections of standard solution.

The results show that the amounts of C(sub)60 per unit
surface are in the nanogram range, and that the
"concentration" in the residue, mostly soot, is in the 0.1-
0.2 ppm range. There may be several reasons why these
amounts are so much smaller than those found in soot from
the free burning of toluene and candle wax [5]. The burning
of wood may not produce as much fullerene as the burning of
toluene and wax. The fullerenes produced by the KT
wildfires, regardless of what was burning then, were exposed
to the "perils of waters, winds, and the rocks", and
fullerenes are especially sensitive to ozone [6]. Since
wildfires occur frequently, albeit usually on much smaller
scales, it seems possible that lignites and coals contain
small amounts of fullerenes.

The failure of finding C(sub)70 is not disturbing when one
considers that the predicted C(sub)70 peak area for an
assumed 70/60 ratio of 0.2 would have been barely above
background.

References: [1] Buseck P. R. et al. (1992) Science, 257,
215-217. [2] Alvarez L. et al. (1980) Science, 208, 1095-
1108. [3] Wolbach W. S. et al. (1985) Science, 230, 167-170.
[4] Anders E. et al. (1991) In Global Biomass Burning (J. S.
Levine ed.), 485-492, MIT Press (this is only a selection of
many more papers on the subject of wildfires). [5] Chibante
L. P. F. et al., Science, submitted. [6] Chibante L. P. F.
and Heymann D. (1993) GCA, 57, 1879-1881.





                           Friday, February 11, 1994
                 SESSION 6: OTHER EXTINCTIONS AND OTHER CAUSES
                                 2:30 - 6:00 PM

Chair(s):  A. Montanari
           W. S. Wolbach


Rigby J. K. Jr.*   Landis G. P.   Hengst R. A.
   Can an Extraterrestrial Impact Event Explain the Pattern of Biotic Crisis
   Near the KT Boundary?

There are three basic questions regarding the
Cretaceous/Tertiary (KT) boundary: (1) Was there an
extraterrestrial impact event at or near a stratigraphic
interval that preserves one of the Earth's largest recorded
biotic crises? (2) Did that extraterrestrial impact event
cause the biotic crisis? (3) Are there other processes, not
related to the extraterrestrial impact, that are more robust
in their explanation of sequence, scope and selectivity of
KT extinctions?

While the first question remains an open issue in some
circles, progress is being made in answering questions (2)
and (3). Any extinction event will have to be substantiated
on the basis of the preserved geologic record, whatever its
weaknesses, because it is the only available data and is the
only means by which our ideas regarding extinction can be
tested. Even reworking of older materials into younger rocks
has been used to explain some of the record inequities.
Discoveries made during the 1993 field season in Montana's
continental section may not totally resolve the reworked
issue but they definitely place the bulk of the
circumstantial evidence on the side of limited, if any,
reworking of fossils. Rock record constraints now demand
that several biologic trends associated with KT boundary
deposits must be answered in order to sustain a viable
extinction hypothesis. These constraints fall into two
general categories: (1) Faunal and floral selectivity of
extinction and (2) Conditions that anticipate the boundary
event and warn of crisis.

Extinction selectivity is difficult to explain by a single
event; examples of such selectivity include: (1) Dinosaurs
are said to become extinct at the KT boundary, yet those
species most closely related to living reptiles and
amphibians that would have the most difficulty surviving the
cold conditions specified by the impact model, survive and
do well; (2) Oceanic siliceous and carbonate photosynthetic
plankton, inhabiting the same cubic meter of water, suffer
widely different extinction rates; (3) Mammals, in the
process of radiating, continue more-or-less uninterrupted
and increase diversity across the boundary; and (4)
Ammonites appear to suffer extinction before the KT boundary
in some geologic sections, yet apparently continue across
the boundary in others; showing variation in extinction
pressures with respect to latitude.

Several groups of organisms appear to anticipate the biotic
crisis. Rudist clams blossom in the Cretaceous, yet suffer
extinction before the boundary as do inoceramid clams. In
North America, two-thirds of dinosaur lineages become
extinct between the time of maximum diversity 10 m.y. before
the KT and the KT boundary; only 12-13 species of the
approximately 40 species (depending upon taxonomic
preference) remain prior to the KT event. We are approaching
consensus regarding the magnitude of KT boundary
extinctions, but it is virtually impossible to explain long
term declines and selective extinction with sudden
cataclysmic scenarios. Perhaps there are more robust
explanations that do a better job of explaining what is no
longer a "knife edge" but a less well defined and protracted
extinction.

The Pele Hypothesis [1] provides a unique model that relates
a number of well established geological events to mantle
processes. Among these are O blooms predicted from increased
photosynthetic activities made possible by increased levels
of CO2 from mantle activity. Cerling [2] shows that
Cretaceous atmospheric CO2 was considerably more abundant
than at present; concentrations reaching 10 to 12 times
modern abundance during much of the Cretaceous and
importantly near the KT boundary. Berner [3] shows that such
CO2 may be sufficient to create the Cretaceous greenhouse
effect. Changes in CO2 concentrations rapidly and profoundly
affect organic material accumulation rates, which were also
affected during episodes of eustatic adjustment. These
factors may have led to C sequestering in the form of coal
and oil deposits, which in turn produced an increase in
atmospheric O.

Consider the effects of increased O levels on the biosphere.
Landis and others (1993) have shown atmospheric O2
concentrations in excess of 30% during the Cretaceous. O2
concentrations ranged from a high of 35% to a low of 28%;
considerably higher than at present. Oxygen is integral to
the process of energy production in both plant and animal
species [4]. It is logical to seek verification of these
unique atmospheric conditions in organisms that evolved to
survive in these conditions. Terrestrial vertebrates of the
Cretaceous are reasonably well known. The sauropod dinosaur,
Apatosaurus, is one such animal and analysis of its bone
constrained anatomy provides a number of obvious
physiological limitations [5]. These include an elongated
neck with considerable dead air space, extremely small
nostrils for the size of the animal, a large bulk with
attendant demands for O, and no obvious mechanism for
efficient, and deep, ventilation mechanics. Hengst et al.
[6] conservatively estimated at 30 liters. Ventilation was
probably achieved by rib movements alone with maximal
capabilities approximately four times that of tidal volumes.
Thus, 30 breaths were needed to refresh the lung. This
implies that sauropods had a high potential for CO2
retention and consequent acidosis. They probably had limited
exercise capabilities that were further limited by
crocodilian-type lung areas and low O carrying capacities of
the blood [7]. It was concluded that although minimal effort
would have been possible in 21% O2 concentrations there was
insufficient reserve for stressful conditions. High O
concentrations create increased O pressures sufficient to
allow a respectable range of activity intensities.

Our research has indicated that the low tolerance to O
stress may well explain the progressive decline in species
variety as the KT boundary is approached. It is also
important to consider the effect that limited O transport
might have had on other, vigorous activities. For instance,
the O available in migration over mountainous areas would
cause considerably larger problems than a similar migration
near sea level.

References: [1] Landis G. P. et al. (1993) GSA, 25, 362. [2]
Cerling T. E. (1991) AJS, 291, 377-400. [3] Berner R. A.
(1991) AJS, 291, 339-376. [4] McAlester A. L. (1970) J.
Paleontology, 44, 405-409. [5] Spotilla J. R. et al. (1991)
Mod. Geol., 16, 203-227. [6] Hengst R. A. et al. (1993) GSA
25, 297. [7] Jones J. H. et al. (1989) FASEB J., 3, A234.


Lerbekmo J. F.*   Sweet A. R.
   A Short Normal Magnetic Interval at the KT Boundary: A Measure of Continuity
   of Record Across the Boundary and Synchroneity of Boundary Events

Intensive detailed study of the KT boundary interval during
the last decade or so has led to stratigraphic arguments
over extinctions and catastrophic events that hinge on
exceptionally fine-tuned chronostratigraphy. The science
progressed rapidly from determining that the KT boundary
occurred in the upper half of chron 29r to recognizing that
a global Ir anomaly associated with shocked quartz probably
represents the time of impact of a large extraterrestrial
body. Such an impact could be expected to be responsible for
some or all of the multiple widespread extinctions that
collectively had come to be recognized as the end of the
Mesozoic Era. The Ir anomaly has provided a stratigraphic
pinpoint against which to measure the synchroneity of the
extinctions. Paleontologists have since argued that some of
these extinctions occurred before the Ir event, some
considerably later, and that some extinctions that appear to
be synchronous with the Ir horizon are only apparently so
because of a hiatus at the boundary.[1,2]. Therefore an
independent method of measuring sedimentary continuity
across the border is required in order to assess the
presence or absence of a recognizable hiatus.
Magnetostratigraphy might provide such information if there
were a short term magnetic event associated with the
boundary interval.

Detailed magnetic sampling across the KT boundary in eight
continental sections from the Red Deer Valley in central
Alberta southeastward through Saskatchewan to the Hell Creek
area of northeastern Montana indicates that the KT boundary
occurs within a short normal polarity interval within
reversed magnetozone 29r. The KT boundary in these sections
is defined by polynomorph extinctions and an Ir anomaly. The
boundary clay is also present in four of the sections and
occurs locally nearby in the other four. Choice of section
was based largely upon character of the outcrop and type of
lithology, sandstones being avoided as much as possible.
Samples were collected at stratigraphic intervals of 0.1 to
0.3 m near the boundary where lithology permitted. Samples
were obtained by driving an oriented one-inch diameter core
barrel into the rock and extruding the core into a vial of
the same diameter [3]. Magnetic measurements were made on a
Molspin magnetometer after AF and thermal demagnetization.

The base of the normal polarity interval occurs from 0.7 to
1.4 m below the KT boundary (mean = 1.1 m, n = 7) and
extends to 0.8 to 1.3 m above (mean = 1.1, n = 5) sections
with sandstones in the boundary interval have not been
included). Four sections are illustrated in Fig. 1. The
length of time represented by this normal polarity interval
can be estimated from its proportion of magnetozone 29r.
Magnetochron 29r lasted about 0.6 m.y. [4]. In the Red Deer
Valley and Frenchman River section magnetozone 29r is about
30 m thick [5,6]. Thus the mean rate of sedimentation over
this interval in these sections is ~50 m/m.y. In these two
sections the average thickness of the normal interval below
the KT boundary is 1.1 m; the average interval above the
boundary is 1.2 m. If the rate of sedimentation in the
vicinity of the boundary is similar to the mean for 29r, the
normal interval began some 21,000 years before the boundary
event that produced the Ir anomaly, and lasted for about
24,000 years afterward.

Keller and others have argued that most deep marine KT
boundary sections show one or more hiatuses at or near the
boundary [2]. Early Paleocene foraminiferal zonation is
detailed, enabling short hiatuses to be detected. The first
Paleocene zone (Po) is estimated to have lasted only about
50,000 years [1]. However, the latest Maastrichtian zone
extends downward through 29r, and therefore, a short hiatus
below the boundary might be difficult to detect
paleontologically.

References: [1] Keller G. (1989) GSA Bull., 101, 1408-1419
[2] MacLeod N. and Keller G. (1991) GSA Bull., 103, 1439-
1457. [3] Lerbekmo J. F. (1990) Sed. Geol., 66, 295-299 [4]
Harland W. B. et al. (1982,1989) Cambridge Earth Sci. Ser.,
Geol. Time Scale. [5] Lerbekmo J. F. and Coulter K. C.
(1985) CJES, 22, 567-583. [6] Lerbekmo J. F. (1985) BCPG,
35, 454-459. Fig. 1. Magnetostratigraphy of four KT boundary
sections. Right-hand columns show magnetic polarity: black =
normal; white = reversed. (a) R. V. = Red Deer Valley,
Alberta; F. R. = Frenchman River, Saskatchewan; (b) M. C. =
Morgan (Rock) Creek, Saskatchewan; H. C. = Hell Creek,
Montana.


Sheehan P. M.*   Coorough P. J.
   Were all Extinction Events Caused by Impacts?

Extraterrestrial impacts are firmly implicated in several of
the five major Phanerozoic extinction events. A critical
issue now is whether extraterrestrial events have been the
only mechanism that produced physical changes of sufficient
magnitude to cause major extinction events [1,2].

While we believe the evidence is overwhelming that the KT
extinction event was caused by an impact [3,4], we also find
that an event of similar or larger size [5] near the end of
the Ordovician is best explained by terrestrial causes [6-
9].

The Ordovician extinction event (End-O extinction event)
occurred near the end of the Ordovician, but the interval of
extinction was completed prior to the newly established
Ordovician-Silurian boundary [6]. In spite of extensive
field studies, a convincing signature of an associated
impact has not been found [10-13]. However, a prominent
glaciation does coincide with the End-O extinction event.

The Late Ordovician Glaciation and Extinction: During the
Late Ordovician sea level was high, and broad epicontinental
seas covered most continental plates. The Baltic,
Laurentian, and Siberian plates were isolated and inhabited
by zoogeographically discrete faunas. Extensive glacial
features in North Africa document a Late Ordovician
glaciation that lasted for 2 or 3 million years [14,6,8].
Sea level declined as water was transferred to the ice caps,
and the epicontinental seas were drained. River drainage
systems were incised into the carbonate platforms. Albedo
changes caused by progressive exposure of the sea floor may
have intensified the glaciation [15]. Drop-stones from
icebergs were present in high-latitude oceans [16]. The
glaciation ended abruptly, as evidenced by rapid eustatic
rise in global sea level. Oxygen and C isotopic studies are
consistent with ocean chemistry changes expected during the
glaciation [17-19].

The End-O extinction event tracked the glacial epoch.
Epicontinental sea faunas were decimated by the regression
that marked the beginning of the glaciation [20,8]. Around
the margins of many continents a new fauna called the
Hirnantia-fauna appeared and replaced older stocks. Many
elements of the Hirnantia-fauna originated in high
latitudes. The appearance of the Hirnantia-fauna commonly
corresponds with a change from warm-water, carbonate rocks
to cool-water, siliceous rocks.

As the glaciation intensified, planktonic groups, such as
graptolites, progressively became restricted to the tropics
[20]. The glaciation increased oceanic circulation, which in
turn oxygenated previously anoxic areas of the oceans,
reducing the habitat of some planktic organisms including
graptolites [21], but providing new habitat for other
benthic organisms [8]. A pulse of extinction during an
interglacial event may have eliminated some benthic animals
[6]. At the end of the glacial epoch climatic warming, sea
level rise, and the return of anoxic waters dealt the final
blow to the ecosystem [6].

Ecologic patterns changed significantly during the
extinction interval. Long standing communities of animals
were decimated. New communities evolved during the glacial
interval, but these communities were eliminated when the
glaciation ended. A third set of communities was established
in the Silurian, and recovery of the ecosystem required
several million years [22].

Zoogeographic patterns were also disrupted. In the Late
Ordovician highly endemic faunas formed distinct
zoogeographic provinces in the epicontinental seas on each
of the isolated continental plates. These provinces were
eliminated by the extinction and survivors were dominated by
animals with wide geographic distribution. The post-
extinction interval was characterized by the some of the
most cosmopolitan faunas in the geologic record [7]. Tens of
millions of years of evolution in isolated seaways were
required before prior levels of provincialism were attained.

Study of a Non-Impact Related Extinction Event: Study of the
End-O extinction event is more difficult than studying the
KT event because an impact clay is missing, so there is no
world-wide event horizon to which phenomena can be tied.
Correlation in the Ordovician relies primarily on fossil
data and the extinction reduced the number of species that
were available for correlation. Graptolites and conodonts,
the two groups most useful for correlation, were especially
hard hit. Post-extinction species tended to have
exceptionally long geologic ranges making correlation
following the extinction difficult. Some events, such as the
appearance of the Hirnantia-Fauna in two different areas,
may be mistakenly taken to be synchronous. The
Hirnantia-fauna was a cold water fauna that apparently moved
toward the tropics as the glaciation intensified. Thus, the
first appearance of the Hirnantia-fauna probably happened at
different times in different places. Eustatic sea level rise
and fall may prove to be the most accurate method of
correlation during this interval.

Comparison of Impact and Non-Impact Extinction Events: Even
though the End-O extinction event had a terrestrial cause,
it was still catastrophic. The physical perturbation of the
environment was beyond the ability of ecosystem to adapt
through normal evolutionary processes.

The primary difference between the KT and End-O extinction
events was the duration of the events. The KT event was
instantaneous on a geologic time scale, and post-impact
perturbation of the physical environment lasted less than a
million years. The End-O event had an extinction interval of
2 to 3 million years with several pulses as the ice caps
advanced and retreated.

In spite of these differences, the effect on the Cretaceous
and Ordovician biotas was quite similar. An impact and a
glaciation are capable of producing similar levels of
extinction at family and generic levels [5,22]. In both
cases long intervals of ecologic stability were disrupted,
and new ecologic communities and provinces were established
[23].

There is currently a paradigm shift underway in paleontology
from one that views evolution as pervasively gradual to a
paradigm of gradual evolution punctuated by occasional
catastrophic events that are accompanied by restructuring of
the entire ecologic landscape. Determining whether or not
these catastrophic events include both terrestrial and
extraterrestrial events is imperative.

If some extinction events are not caused by impacts, there
is a wild card in the current search for periodicity of
extinction events. If one of the 5 major Phanerozoic
extinction events had a terrestrial cause, many smaller
events also may have had terrestrial causes. The history of
life might include a group of extinction events caused by
periodic impacts and another group of extinction events with
non-periodic, terrestrial causes.

References: [1] McLaren D. J. and Goodfellow W. D. (1990)
Ann. Rev. Earth and Planet. Sci., 18, 123-171. [2] Raup D.
M. (1990) GSA Spec. Paper, 247, 27-32. [3] Sheehan P. M. et
al. (1991) Science, 254, 835-839. [4] Sheehan P. M. and
Fastovsky D. E. (1992) Geology, 20, 556-560. [5] Sepkoski J.
J. (1986) in Patterns and Process in the History of Life,
(D. M. Raup and D. Jablonski, eds.), Springer-Verlag,
Berlin, 277-296. [6] Brenchley P. (1989) in Mass Extinctions
Processes and Evidence (S. K. Donovan, ed.), Columbia Press,
104-132. [7] Sheehan P. M. and Coorough P. J. (1990) Geol.
Soc. London, Mem., 12, 181-187. [8] Sheehan P. M. (1988) New
Mex. Bur. Mines. Mem., 44, 405-415. [9] Owen A. W. et al.
(1991) Geol. Surv. Canada Paper, 90-9, 179-190. [10] Orth C.
J. et al. (1986) Geology, 14, 433-436. [11] Wilde P. et al.
(1986) Science, 233, 339-341. [12] Orth C. J. et al. (1990)
GSA Special Paper, 247, 45-59. [13] Wang K. et al. (1992)
Geology, 20, 39-42. [14] Bennacef A. (1971) Amer. Assoc.
Petrol. Geol. Bull., 55, 2225-2245. [15] Gildner R. F. and
Cisne J. L. (1987) Paleoceanography, 2, 177-183. [16]
Robardet M. and Dore F. (1988) Palaeogeog. Climat. Ecol.,
66, 19-31. [17] Marshall J. D. and Middleton P. D. (1990) J.
Geol. Soc. London, 147, 1-4. [18] Middleton P. D. et al.
(1991) Geol. Surv. Canada Paper, 90-9, 313-323. [19] Wilde
P. and Berry W. B. N. (1986) in Global Bio-Events (O.
Walliser, ed.), Springer-Verlag, Berlin, 75-91. [20]
Skevington D. (1978) Alcheringa, 2, 21-26. [21] Melchin M.
J. and Mitchell C. E. (1991) Geol. Surv. Canada, Paper 90-9,
143-154. [22] Sepkoski J. J. (1990) J. Geol. Soc. London,
146, 7-19. [23] Sheehan P. M. (1991) in The Unity of
Evolutionary Biology (E. C. Dudley, ed.), Discorides Press,
Portland OR, 103-118.

Acknowledgements: Work was supported by N.S.F. Grant EAR-
9004589.


Erwin D. H.*
   The End-Permian Mass Extinction: A Complex, Multicausal Extinction

The end-Permian mass extinction was the most extensive in
the history of life and remains one of the most complex.
Understanding its causes is particularly important because
it anchors the putative 26 myr pattern of periodic
extinction. However, there is no good evidence for an impact
and this extinction appears to be more complex than others,
involving at least three phases [1,2]. The first began with
the onset of a marine regression during the Late Permian and
resulting elimination of most marine basins, reduction in
habitat area, and increased climatic instability; the first
pulse of tetrapod extinctions occurred in South Africa at
this time. The second phase involved increased regression in
many areas (although apparently not in South China) and
heightened climatic instability and environmental
degradation. Release of gas hydrates, oxidation of marine
carbon and the eruption of the Siberian flood basalts
occurred during this phase. The final phase of the
extinction episode began with the earliest Triassic marine
regression and destruction of nearshore continental
habitats. Some evidence suggests oceanic anoxia may have
developed during the final phase of the extinction although
it appears to have been insufficient to be the sole cause of
the extinction.

Some 50% of marine families and perhaps over 90% of marine
species were eliminated during the last few million years of
the Permian [1,3]. Preservational problems and geochemical
evidence suggest the extinction may have persisted into the
earliest Triassic. Extinction patterns were highly
selective: conodonts, non-fusulinid foraminifera, most
bivalves, nautiloid cephalopods, and bellerophontid
gastropods were largely unaffected, while blastoids,
camerate crinoids, and tabulate corals had been declining
well before the end of the period and disappeared entirely
by the latest Permian. The last trilobites, stenolaemate
bryozoans, rugose corals, and crinoids persisted at low
diversity into the final, Changxingian stage. The extinction
primarily affected the fusulinid foraminifera, several
groups of articulate brachiopods, and ammonoid cephalopods;
cidaroid echinoids, other gastropods, sponges, and some
brachiopod groups declined in the latest Permian but
rebounded fairly quickly during the Early Triassic.

Late Permian faunas are highly provincial however, and the
biogeographic component of the extinction is only now being
assessed. The best data for rapid extinction come from South
China [4-6] an isolated tectonic block during the late
Permian. It remains unclear to what extent the data from
South China are representative of global diversity patterns.
Biogeographic, taphonomic, and other problems continue to
plague analysis of extinction data, obscuring important
information on the rate and pattern of extinction. Recently
developed conodont biostratigraphic correlations hold the
promise of answering such questions [7]. It does seem fairly
clear that sessile, epifaunal filter-feeders, and nearshore
taxa suffered higher extinction rates than other taxa.

On land, 21 of 27 (75%) tetrapod families disappeared [8].
In the Karoo Basin of South Africa two extinction peaks have
been described from the Late Permian [9] although sampling
problems raise important questions. Of the 27 orders of
insects recorded from the Permian, eight disappeared and an
additional seven suffered considerable reductions; four of
the latter recovered during the Triassic. This was the most
extensive ordinal extinction in insect history [10]. Plants
show little evidence for mass extinction, although both
pollen and fungal spores show significant changes at the
Permo-Triassic boundary [1,11]. These changes may be
associated with the destruction of nearshore communities
during the rapid Early Triassic transgression.

Determining the cause of the extinction is complicated by
the multitude of Late Permian geological changes. Although
the supercontinent of Pangea had formed tens of millions of
years earlier, a widespread regression during the terminal
Permian exposed most of Pangea [12] leading to widespread
climatic instability [13]. The rapid eruption of the
Siberian flood basalts [14], significant shifts in C, O, S,
and Sr isotopes [1,12,15,16], and increasing evidence for
some marine anoxia [17] complicate the picture. The
delta^13C evidence has received considerable attention but
appears to be equally consistent with erosion and oxidation
of marine carbonates [12,15], formation of anoxic bottom
waters [17-20], and release of methane gas hydrates during
regression [1].

Recently proposed causes for the end-Permian extinction
include volcanism-induced cooling, extra-terrestrial impact,
and global anoxia (reviewed in [1]). Although no Ir anomaly
has been identified at the P/Tr boundary [21],
microspherules, apparently of volcanic origin, are
widespread [e.g., 4]. However, despite the apparent force of
the eruption of the Siberian traps, the eruption appears to
have begun after the extinction. Additionally, the eruption
appears to have been insufficient to trigger an extinction
this large. Anoxia hypotheses have a related problem:
calculations suggest that the observed delta^13C shift may
have been insufficient to reduce atmospheric O levels
sufficiently to induce extinction [1,2].

The most plausible current explanation of the end-Permian
mass extinction appears to involve multiple elements in a
complex extinction, as outlined above. Testing this
scenario, and the others proposed, requires high-resolution,
global, analyses of survival and extinction with particular
attention to various biases (taphonomic, taxonomic, etc.),
which may bias the data. Recent biostratigraphic advances
[7] now make this possible and such studies are currently
underway in the US and China.

The end-Permian mass extinction differs sharply from the
end-Cretaceous in length, the pattern of extinction, and in
the complexity of associated geological events.
Additionally, there is little evidence for extraterrestrial
impact at the close of the Permian and C, O, S, and Sr
isotopic patterns are significantly different [22]. These
sharp differences pose important problems for attempts to
link these two extinctions to a common cause.

References: [1] Erwin D. H. (1993) The Great Paleozoic
Crisis, Columbia Press, 327. [2] Erwin D. H. (1993) in
press, Nature. [3] Raup D. M. (1979) Science, 206, 217-218.
[4] Yang Z. Y. et al. (1993) Permo-Triassic Events of South
China, Geological Publishing House, Beijing, 153. [5] Yang
Z. Y. et al. (1987) Ministry of Geology and Mineral
Resources, Geological Memoir, Ser., 2, 6,1-379. [6] Yang Z.
Y. and Li Z. S. (1992) in Sweet (W. C. et al. eds.)
Cambridge, 9-20. [7] Sweet W. C. (1992) in Sweet (W. C. et
al. eds.), Permo-Triassic Events in the Eastern Tethys,
Cambridge Press, 120-133. [8] Maxwell W. D. (1992)
Palaeontology, 35, 571-584. [9] King G. M. (1991) Hist.
Biol., 5, 239-255. [10] Labandiera C. C. and Sepkoski J. J.
(1993) Science, 261, 310-315. [11] Visscher H. and Brugman
W. A. (1988) Mem. Soc. Geol. Ital., 34, 121-128. [12] Holser
W. T. and Magaritz M. (1987) Mod. Geol., 11, 155-180. [13]
Parrish J. M. et al. (1986) The Ecology and Biology of
Mammal-Like Reptiles (Hotton N. ed), Smithsonian Inst.
Press, 109-131. [14] Campbell I. H. et al. (1992) Science,
258, 1760-1763. [15] Holser W. T. et al. (1991) Abh. Geolog.
Bund., 45, 213-232, [16] Baud A. et al. (1989) Geologische
Rundschau., 78, 649-677. [17] Wignall P. B. and Hallam A.
(1992) Palaeogeog. Palaeoclimat. Palaeoecol., 93, 21-46.
[18] Wignall P. B. and Hallam A. (1993) Palaeogeog.
Palaeoclimat. Palaeoecol., 102, 215-237. [19] Hoffman A. et
al. (1990) Mod. Geol., 14, 211-221. [20] Malkowski K. et al.
(1989) Nature, 337, 64-68. [21] Clark D. L. et al. (1986)
Science, 233, 984-986. [22] Holser W. T. and Magaritz M.
(1992) GCA, 56, 3297-3309.


Claeys P.*   Kyte F. T.   Casier J.-G.
   Frasnian-Famennian Boundary: Mass Extinctions, Anoxic Oceans, Microtektite
   Layers but Not Much Iridium?

The Cretaceous-Tertiary boundary (KTB) debate has triggered
intense search for other possible impact-induced mass
extinction events in Earth history. So far, of the five
major mass extinctions of organisms in the last 600 my [1],
only the most recent one, the KTB 65 Ma ago, presents
abundant evidence for a major impact associated with the
faunal disappearance.

The Frasnian-Famennian boundary (FFB) mass extinction took
place approximately 365 Ma ago in the Late Devonian. The
International Subcommission on Devonian Stratigraphy
recently defined the FFB at the base of the Famennian Lower
triangularis conodont Zone (Fig. 1). This boundary is marked
by a drastic reduction in marine biomass and ecosystem
diversity. The highly productive coral-stromatoporoid reefs
of the Frasnian disappeared, many brachiopods and
trilobites, all tentaculitids were wiped out; conodonts,
goniatites, ammonoids, cricoconarids, and ostracods were
severely affected [2]. McGhee [3] estimates that 20% of all
families and approximately 50% of all genera disappeared.
The time frame of the extinction varies according to
different authors: stepwise process within the last conodont
zones of the Frasnian [4], extinction of various species
ranging over 6 conodont zones [3], an instantaneous
worldwide extinction event occurring precisely at the FFB
[2,5,6].

Glassy spherules displaying morphological and chemical
similarities to microtektites have been found near the FFB
in two Late Devonian sections: Hony and Senzeille located in
the Dinant Basin in Belgium [7,8]. These spherules range in
size from 100 to 500 micrometers, have smooth or etched
surfaces, and rounded or teardrop shapes. Their chemical
composition is variable with mainly Ca- and K-rich end
members. The spherules have a water content of 0.001 wt%
comparable to that of tektites from the Cenozoic
strewnfields [8]. Lechatelierite and partially melted quartz
grains may be present in the glass matrix [8]. In Hony the
spherules are found at the very base of the Famennian Lower
triangularis conodont zone, only 2 to 3 cm above where
Sandberg et al. [9] placed the FFB. In Senzeille the exact
location of the spherule layer is unclear because of poor
conodont biostratigraphy. Similar glassy spherules have been
described in the Early Famennian crepida conodont Zone in
South China [10]. According to the conodont zonation
timescale [11] the Chinese spherules are approximately 1.5
to 2 my younger than the ones recovered in Belgium (Fig. 1).
They are stratigraphically associated with a 300 pg/g Ir
anomaly in the Caning basin (Australia) [12].

The Hony section was investigated to determine if a positive
Ir anomaly coincided with the spherule-bearing layer. A
succession of 42 samples distributed across the FFB were
analyzed by radiochemical neutron activation. Near the
spherule layer the samples were collected approximately
every 2 to 3 cm. The results are presented in Fig. 2. No
significant positive Ir anomaly was detected in the 2 m of
section investigated. The one-point Ir increase up to 83
pg/g over a background of ~40 pg/g right at the spherule
layer is inconclusive. Two similar spot increases occur in
basal Famennian shales above the spherule layer. The Hony Ir
result demonstrates that there is no Ir anomaly comparable
in magnitude to that observed at the KTB. As in the
Cenozoic, microtektite events are generally not associated
with major Ir anomalies [13].

Trace-element analyses in the Hony section reveal that the
chalcophile elements Zn, As, and Sb are correlated and
increase near the FFB (Fig. 3). An increase in these
elements may be linked to suboxic conditions. During the
Late Paleozoic the Paleotethys ocean was probably O
stratified [14]. Ostracod fauna assemblages associated with
the FFB are indicative of dysaerobic conditions [15].
Joachimski and Buggish [16] describe positive delta^13C
anomalies just below the FFB in sections in France and
Germany. These anomalies coincide with the deposition in the
uppermost Frasnian of black limestones and shales known as
the lower and upper Kellwasser horizons [16]. These
Kellewasser horizons are not identifiable in the Hony
section based on lithology alone. However, the increase in
Zn, As, and Sb suggests that perhaps in Hony there was also
an input of disaerobic water into shallow marine
environments at the FFB.

The FFB is marked by a succession of major events including
overflow of O-depleted water on the shelf in Western Europe,
impact evidence in Belgium and China, and worldwide
extinction of shallow marine species (Fig. 1). These events
are spread over 3 to 4 condont zones or roughly 2 my. [11].
At this point the relationships between these events and
thus the cause(s) of the mass extinction are unclear. Part
of this problem may be due to the old age (365 Ma) of the
event, the biostratigraphic resolution available, the
difficulties in worldwide correlations, and too few well-
preserved FFB sections.

It has been proposed that the influx of low-O water on the
shelf could be responsible for the FFB mass extinction [16].
In the Dinant Basin during the entire Frasnian the coral-
stromatoporoid reefs are repeatedly buried by shale
deposited under O-depleted conditions [15] and each time the
reefs reappear when conditions return to more oxygenated.
Only at the FFB do they finally disappear to be replaced in
the Famennian by cyanobacterial reefs [2].

The impact hypothesis also poses problems. The stratigraphic
relationships between the Belgian and Chinese glassy
spherules is puzzling. Do these two occurrences correspond
to one global impact event or to two smaller events
separated by 1.5 my? The occurrence of microtektitelike
glass in two Belgian sections lends support to a possible
impact closely associated with the FFB. The lack of a
positive Ir anomaly in Hony indicates that the FFB impact
differs, perhaps in magnitude or type of bolide, from the
KTB event. It appears more similar to the four tektite-
producing impacts that took place in the Cenozoic.

References: [1] Raup D. M. and Sepkoski J. J. Jr. (1982)
Science, 215, 1501-1503. [2] McLaren D. J. (1970) J.
Paleont., 44, 801-815. [3] McGhee G. R. (1989) in Mass
Extinctions: Processes and Evidences (S. K. Donavan, ed.),
133-151, Belhaven Press, London. [4] Becker T. R. et al.
(1991) Hist. Biol., 5, 183-196. [5] McLaren D. J. (1985)
Geology, 13, 170-172. [6] McLaren D. J. and Goodfellow W. D.
(1990) Annu. Rev. EPSL, 18, 123-71. [7] Claeys P. et al.
(1992) Science, 257, 1102-1104. [8] Claeys P. and Casier J.-
G. (1993) EPSL, submitted. [9] Sandberg C. A. et al. (1988)
Cour. Forsch-Inst. Senckenberg, 102, 267-307. [10] Wang K.
(1992) Science, 256, 1546-1549. [11] Ziegler W. and C. A.
Sandberg (1990) Cour. Forsch-Inst. Senckenberg, 121, 1-115.
[12] Nicoll R. S. and P. E. Playford (1988) Geol. Soc.
Austalia Abstract, 291, 296. [13] Glass B. P. (1990)
Tectonophysics, 171, 393-404. [14] Kammer T. et al. (1986)
Lethaia, 19, 109-121. [15] Casier J.-G. (1987) Rev.
Paleobiol., 6, 193-204. [16] Joachimski M. M. and Buggisch
W. (1993) Geology, 21, 675-678.

Fig. 1, which appears here in the hard copy, shows Late
Devonian stratigraphy, conodont zones [11], and
stratigraphic position of major events. The mass extinction
is interpreted to be (a) gradual over several conodont zones
across the FFB [3,4] or (b) instantaneous worldwide at the
top of the linguiformis zone [6].

Fig. 2, which appears here in the hard copy, shows the Ir
profile in the Hony section (Belgium). No significant
anomaly is present at the spherule layer.

Fig. 3, which appears here in the hard copy, shows
chalcophile element profiles across the FFB in Hony
(Belgium). The increase in Zn, As, and Sb may indicate the
influx of suboxic water on the shelf at the FFB.


Warme J. E.*
   Catastrophic Alamo Breccia, Upper Devonian, Southeastern Nevada

An anomalous sedimentary deposit, informally named the Alamo
Breccia after the local settlement of Alamo in southeastern
Nevada, occurs as a single bed in the Upper Devonian. It is
interpreted as a gigantic debris flow and turbidite
generated by a shallow-water landslide. It is anomalous
because it formed upon the surface of a long-lived carbonate
platform and not just at the platform edge.

To date, the Alamo Breccia has been identified in seven
mountain ranges, covering an area of about 50 x 150 km (7500
square kilometers). Before tectonic shortening by thrust
faulting, the area may originally have been greater.
Thickness of the Alamo Breccia ranges from approximately 30
to 130 meters, averaging 80 meters; a conservative volume
estimate is 600 cubic kilometers.

All evidence indicates that the Breccia was deposited as a
single event, probably within a day. It is dated by
conodonts as early Frasnian, or the early part of the early
Late Devonian, approximately 375 million years ago (Sandberg
and Warme, 1993). The composition of the Alamo Breccia is
almost entirely platform limestone and dolomite, except in
some locations where it is secondarily partially or
completely dolomitized. Fragments represent the spectrum of
carbonate lithologies present on the Devonian platform; no
clasts of deepwater derivation have been recognized.Lower
and upper contacts are sharp. The lower contact is erosive
but usually follows approximately the same underlying
stratum for hundreds of meters. The upper contact is usually
sharp, but appears gradational with overlying mudstones at
some localities. Internally the Alamo Breccia is a graded
bed. It has been zoned into four zones, two (A, B) of which
are always present and two others (C, D) that are common but
not universal (Warme et al., 1993). From the top downward,
Zone A is an exquisitely graded bed, two to several meters
thick. Clasts range from 2-3 decimeters or more in diameter
at the base, becoming gradually smaller upward to the
capping sand or mud. Downward Zone A merges with Zone B of
larger clasts, ranging up to 10 meters or more in largest
dimension, with relatively finer matrix. Zone B may extend
to the base of the Breccia, but in most localities is
interrupted downward by one or more huge clasts (Zone C)
commonly hundreds of meters long and ten or more meters
thick. Zone C clasts are generally parallel or subparallel
to regional bedding surfaces. Zone D, a detachment horizon
showing bedrock dilation and fluidization, may or may not be
present between Zone C clasts and underlying intact bedrock.

Breccia fabric is characterized by overall rough grading,
superb grading in Zone A, and general (sub)parallel
orientation of tabular clasts (including those in Zone C).
Imbrication of clasts is rare, and shows no consistent
paleocurrent direction.

The Alamo Breccia is anomalous, suggesting unusual
derivation, for several reasons: (1) It occurs within the
framework of a carbonate platform. Underlying and overlying
beds represent shallowing-upward platform cycles of the
Guilmette Formation, commonly capped by exposure surfaces
(desiccation cracks, karst). Only at its thickest (130 m)
and westernmost (western Timpiute Range) exposure is it
bracketed by beds representing deeper-water, off-platform
paleoenvironments, which are the normal milieu of such mass
flows. (2) Its lower boundary, an eroded surface of
detachment, represents an apparently horizontal surface over
a distance of at least 30 kilometers landward from the
seaward platform edge and over an area of several thousand
square kilometers. (3) The Alamo Breccia is the only such
intra-platform mass-flow recognized from Cambrian to
Mississippian time.
A working hypothesis for the origin of the Alamo Breccia is
that it was triggered by an earthquake, perhaps spawned by a
nearby bolide. Circumstantial evidence for such an impact
is: (1) The singular occurrence of such a deposit. (2) The
presence of numerous unique quartz grains, found in
conodont-sample residues only from within the Breccia and
not from adjoining beds. These anomalous grains have blotchy
extinction under cross-polarized light and are peripherally
studded with metallic oxide crystals. They may represent
altered tektites or other impact debris. (3) A weak Ir
anomaly that exists near top of the Breccia in one locality.
(4) Conodonts representing deeper-water biofacies that occur
only in the Breccia, not in confining beds (Charles
Sandberg, 1993, personal communication).
The anomalous conodont occurrence is consistent with
physical evidence that the detachment and debris flow
creating the Alamo Breccia was concurrent with, or
immediately followed by, a tsunami or similar wave that
uprushed landward far beyond the detachment headwall. Other
evidence for such a wave includes the observation that the
Alamo Breccia fills the entire vertical space left by the
slide; the slide scar is brim full of debris swept into it,
from the adjoining platform, as a single graded bed. In
addition, three localities, tens of kilometers beyond the
distribution of the thick Alamo Breccia, show a single
thinner (1-5 meter) graded bed that was apparently deposited
by the uprush, stranded above sealevel, and then karsted
before subsequent flooding by the next sealevel cycle.

Whether or not the Alamo Breccia was triggered by
extraterrestrial impact, it represents a local but
significant event in the Devonian history of Nevada and
perhaps more distant regions.



                              POSTER PRESENTATIONS

Fischer A. G.   Kitz N.
   A 74.5 Million Year Stress Schedule and its Reflection in the Stratigraphic
   Periods

Sepkoski's and Raup and Sepkoski's work on biotic crises
(various papers) and improvement of radiometric
geochronology provide the basis for a new evaluation of
periodic biospheric stress. Plotting the six strongest
biotic crises in terms of age shows a remarkable regularity
(Table 1): With exception of the greatest of them (the
unique 1st-order Permo-Triassic crisis) they define
successive steps in a 74.5 million year schedule. These
coincide (or very nearly coincide) with Period boundaries-
end Cambrian, end Ordovician, end Devonian, end Triassic,
and end Cretaceous. Two stops on this schedule are not
marked by severe (2nd-order) crises, but match the 3rd-order
crises at end Carboniferous and end Jurassic The Siluro-
Devonian boundary falls off the 74.5 Ma schedule, and has no
associated crisis: it reflects regional bias on the part of
the founding fathers of stratigraphy. The Permo-Triassic
boundary and crisis fall off the schedule, and we attribute
them to unidentified complications. No crisis marks the
Tertiary-Quaternary boundary; rather, the 74.5 million year
stop lies 8 million years in the future.

Thus Phanerozoic time was segmented by biotic stresses
recurring at 74.5 Ma intervals. 3rd-order crises seem to
have bisected these segments in Paleozoic time and to have
trisected them in the Mesozoic and Cenozoic. The cause of
these stresses remains uncertain, but we incline to
interactions with galactic forces: the period of the solar
system's oscillation transverse to the mid-galactic plane
has been estimated at between 67 (Oort, 1960, 1965) and 80
Ma (Tumpler & Weaver, 1968).


van den Bergh S.
   Supernovae and Mass Extinctions

Shklovsky and others have suggested that some of the major
extinctions in the geological record might have been
triggered by explosions of nearby supernovae. The frequency
of such extinction events will depend on the Galactic
supernova frequency and on the distance up to which a
supernova explosion will produce lethal effects upon
terrestrial life. In the present note it will be assumed
(Seward [4]) that a killer supernova has to occur so close
to Earth that it will be embedded in a young, active,
supernova remnant. Such young remnants typically have radii
~3 pc (1 x 10^19 cm).

Larger (more pessimistic?) killer radii have been adopted by
Ruderman [3], Romig [2], and by Ellis and Schramm [1]. From
observations of historical supernovae van den Bergh [6]
finds that core-collapse (types Ib and II) supernovae occur
within 4 kpc of the Sun at a rate of 0.2 +- 0.1 per century.
Adopting a layer thickness of 0.3 kpc for the Galactic disk
this corresponds to a rate of ~1.3 x 10^-4 supernovae pc^-3
Gyr^-1. Including supernovae of type Ia will increase the
total supernova rate to ~1.5 x 10^-4 supernovae pc^-3 Gyr^-
1. For a lethal radius of R pc the rate of killer events
will therefore be 1.7 (R/3)^3 x 10^-2 supernovae per Gyr.
However, a frequency of a few extinctions per Gyr is
required to account for the extinctions observed during the
phanerozoic. With R(extinction) ~3pc the Galactic supernova
frequency is therefore too low by two orders of magnitude to
account for the major extinctions in the geological record.

References. [1] Ellis J. and Schramm D. N. (1993) preprint.
[2] Romig J. H. (1975) Ph.D. Dissertation, University of
Colorado. [3] Ruderman M. A. (1974) Science, 184, 1079-1081.
[4] Seward F. D. (1978) J. British Interplanetary Soc., 31,
83-92. [5] Shklovsky I. S. (1969) Supernovae, Wiley, p. 15.
[6] van den Bergh S. (1993) Comments on Astrophysics, in
press.


Muller R. A.
   Extraterrestrial Accretion and Glacial Cycles

We propose that the ~100 kyr cycle seen in terrestrial
glaciation is due to changes in meteor flux that come from
changes in the Earth's orbit. This model can explain a 70
kyr "anomalous" period in climate data and the apparent
discrepancy between present extraterrestrial fluxes and
those in oceanic sediments. It can be tested by measuring Ir
densities in sediments and ice during glacials and
interglacials.

Milankovitch [1] attributed the changes in the Earth's ice
coverage to perturbations in the motion of the Earth and the
resulting changes in insolation in the northern hemisphere.
The strongest effects were expected to come from changes in
the Earth's obliquity (the tilt of the Earth's axis with
respect to its ecliptic), and from a precessional term that
accounts for the delay between solstice and perihelion. The
success of this mechanism has been argued by Imbrie et al.
[2]. The most compelling evidence is the presence of Earth
orbital frequencies in Fourier analysis of proxy climate
measurements such as CO2 and delta^18O. In a recent review
and reanalysis of available data, MacDonald [3] showed that
inclination, obliquity, and precession frequencies account
for eight of nine statistically significant delta^18O
frequencies in an eastern Pacific core. The "only anomalous
line" in the data with no theoretical counterpart was a
delta^18O peak at 0.014 +- 0.001/kyr.

However the insolation mechanism proposed by Milankovitch
has difficulty accounting for the magnitudes and phases of
the climate cycles. The spectrum is dominated by a 100 kyr
period, usually attributed to the oscillation of
eccentricity e. However yearly average insolation varies as
(1/2)e^2, giving changes in insolation of only 10^-3.
MacDonald [3] calculated the solar insolation at 60 degrees
N at the primary frequencies for (obliquity, precession,
eccentricity) to be in the ratio (1, 5, 0.1); in sharp
disagreement with the delta^18O data, which show the climate
variations to be in the ratios (1, 0.4, 4.4).

Milankovitch explained the strong eccentricity dependence by
arguing that the insolation effect of obliquity and
precession are biggest when the eccentricity is greatest and
that the effect is nonlinear. Other suggestions to solve the
puzzle postulate resonances and time constants in the
geologic system to enhance ice coverage at periods near 100
kyr and to cause delays without enhancing the 20 and 40 kyr
periods. However recent delta^18O measurements with precise
dates show that the warming associated with several glacial
terminations begins well before the start of above-average
insolation, and this has been interpreted as disproving
Milankovitch's mechanism. [4,5].

The correspondence of observed climate frequencies with
those of the Earth's orbit argue strongly for a causal
connection, but the failure to predict the correct
amplitudes and phases suggest that the insolation mechanism
of Milankovitch may be incorrect, at least for the 100 kyr
period. We suggest that this cycle might be due to a
different mechanism: extraterrestrial accretion. This
mechanism yields the correct frequencies, can account for
the "anomalous line" of MacDonald, gives a new
interpretation to recent accretion measurements, and makes
predictions that can be tested in the near future. Accretion
does not account for the periods near 20 and 40 kyr, and we
assume that the Milankovitch solar insolation mechanism
correctly accounts for these.

Mechanism of Extraterrestrial Accretion: As the orbit of the
Earth changes, it passes through different parts of the
sun's Zodiacal ring, and should encounter different meteor
streams. Changes in meteor flux can affect the density of
dust and aerosols in the upper atmosphere. The fact that
high altitude dust can affect global climate is generally
accepted. The eruption of Krakatoa has been estimated to
cause global cooling >1C, and climate changes have been
attributed to El Chichon and other eruptions that injected
megatons of material into the stratosphere. Measurements of
Ir in oceanic sediments [6] show that long-term global
average flux from extraterrestrial materials is 60-120
kiloton/yr [6]; comparable over 10-100 years to that from a
large volcanic eruption. The meteors may not come in at a
steady rate but in bursts as we cross the meteor streams
(with peaks much higher than the 60-120 kton/yr average) and
in cycles as the Earth's orbit oscillates. The shortest
variation detectable (the Nyquist period) in the best
sediment records [6] ~600 kyr, so a 100 kyr oscillation
would have been missed. Clube [7] analyzed the possibility
of large unobserved meteor streams and argued that the
Zodiacal ring of the Sun is largely meteoroid in composition
rather than dust. He concluded that large unobserved streams
exist, and suggested that encounters with them could cause
climate excursions and extinction events; however he did not
propose a relationship with the known climate cycles.

A single meteor stream can contain the mass of one comet
~10^5 megatons. The Earth can capture about 10^-4 of the
stream with each passage ~10 megatons ~1 large volcano
equivalent/year, as long as the orbits intersect. The
cumulative effect would be very much larger than Krakatoa.

We do not propose that the climate effects are due to a
single meteor stream, rather that the solar ring has many,
in nearly circular orbits with semimajor axis ~1 AU, so that
their own orbital perturbation frequencies (but not
necessarily their phases) closely match those of the Earth.
We do not explain how such rings were formed, but for our
model to work, they must have been present for the last
million years.

Another orbital parameter expected to have little effect on
climate, because of its negligible effect on insolation, has
been the inclination of the Earth's orbital plane. Changes
in inclination also cause the Earth to probe different
regions of interplanetary debris, so we predict correlations
between inclination and climate. The long-term variation of
the inclination was calculated by Berger [8] who showed that
the behavior is quasi periodic with a period of 70 kyr, and
a fundamental frequency of 0.0144/kyr. Note that this
frequency is, within errors, equal to the one unexplained
peak in the delta^18O data [3]; thus we explain the one
"anomalous" frequency.

We can gain more insight into this effect, and possibly
understand the relative intensities of the spectral lines,
if we chose coordinates based on the invariable plane of the
solar system, perpendicular to the total angular momentum
vector of the planets, approximately the plane of Jupiter's
orbit. In these coordinates the oscillations of inclination
are nearly sinusoidal with peak-to-peak amplitude reduced by
two, and period changed to 100 kyr rather than 70 kyr. (The
change simplifies the behavior because the primary
perturber, Jupiter, has a nearly stationary orbit in this
system.) In this frame the precession of the inclination
vector, Omega, now shows a virtually steady advance of one
cycle every 70 kyr. In the invariable frame, the inclination
i oscillates with a 100 kyr period, nearly identical to that
of the eccentricity e. Changes in i and e both cause the
Earth to sweep out new material with a frequency 0.01/kyr
identical to that of the strongest line seen in the climate
data. The `anomalous' line [3] at 70 kyr period is Omega,
the frequency of precession of the inclination vector (the
advance of the line of nodes) measured in the invariable
plane.

Accretion Measurements: Our proposed mechanism predicts a
substantial reduction in the flux of meteoritic material in
the present era compared to the long term average. In fact
just such a reduction is known. In a recent review, Rochia
et al. [9] found 6 of 7 published determinations of recent
influx measured ~10 kiloton/yr, yet the long-term flux [6]
was 60 to 120 kilotons/yr. They speculate that the recent
low flux is a fluctuation, due to few large meteoroid
impacts recently. Yet the most straightforward measurement,
that of Ir in Antarctic snow, gave 10 kiloton/yr despite the
presence of the Tunguska event of 1908 ~0.1 to 1 megaton.

We offer a different interpretation, that the discrepancy is
not from the absence of large impacts, but is due to the
fact that interglacials (including the present one) are
periods of low extraterrestrial meteoroid flux.

References [1] Milankovitch M. (1941) Royal Serbian Acad.
Spec. Pub., 132. [2] Imbrie J. et al. (1984) in Milankovitch
and Climate (A. Berger et al. eds.), Reidel Publishing, 269-
305. [3] MacDonald G. (1990) in Global Climate and
Ecosystems Change (G. MacDonald and L. Sertoria, eds.),
Plenum NY, 1-95. [4] Winograd I. et al. (1992) Science, 258,
255-260. [5] Broecker W. and Denton G. (1989) GCA, 53, 2465-
2501. [6] Kyte F. and Wasson J. (1986) Science, 232, 1225-
1229. [7] Clube S. (1987) Phil. Trans. R. Soc. Lond. A, 323,
421-436. [8] Berger A. (1976) Astron. Astrophys., 51, 127-
135. [9] Rocchia R. et al. (1990) GSA Spec. Paper, 247, 189-
193.


Turekian K.
   New Insights from the Osmium Record

No abstract available.


Sheldon W. R.
   Ozone Control of Biological Activity During Earth's History, Including the
   KT Catastrophe

There have been brief periods since the beginning of the
Cambrian some 600 million years ago when mass extinctions
destroyed a significant fraction of living species. The most
widely studied of these events is the catastrophe at the KT
boundary that ended the long dominance of the dinosaurs. In
addition to mass extinctions there is another profound
discontinuity in the history of Earth's biota, the explosion
of life at the end of the Precambrian era, an episode that
is not explained well at all. For some 3 billion years
before the Cambrian, life had been present on Earth, but
maintained a low level of activity, an aspect of the biota
that is puzzling, especially during the last 2/3 of that
period. During the last 2 billion years before the Cambrian,
conditions at the Earth's surface were suitable for a
burgeoning of the biota, according to most criteria: the
oceans neither boiled nor were frozen solid during this
time, and the atmosphere contained sufficient O for the
development of animals. The purpose of this paper is to
suggest that mass extinctions and the lackluster behavior of
the Precambrian biota share a common cause: an inadequate
amount of ozone in the atmosphere.

Calculations of atmospheric ozone O3 have been made with the
concentration of O2 in the atmosphere as a variable
parameter [1,2], but with the solar spectrum at the top of
the atmosphere assumed to be that which is observed at
present. On the basis of these calculations it is of
interest to note that the present ozone atmosphere is only
some 30 percent above the critical value of 7 x 10^18 O3
cm^-2, a level that provides a "full screen" for the biota;
however an O2 concentration only 10 percent of the present
atmospheric level (PAL) creates an ozone atmosphere only 20
percent below the critical value. Consequently it has been
assumed that when the O2 concentration reached the
neighborhood of 10^-1 PAL, the biota found an environment on
the Earth's surface relatively free of degradation from
solar ultraviolet (uv) radiation. Here that concept is
questioned and a scenario is described in which a robust O2
atmosphere could be present, but an O3 atmosphere anywhere
near the critical value would not be formed.

Present concern for the Earth's ozone layer is focused on
chemical processes in the atmosphere that destroy O3; the
ozone production rate is constant and not considered to be a
potential problem. In the geologic past such was certainly
not the case and the main point of this paper is that during
the Earth's history the O3 production rate has been the
controlling factor for the biota. Specifically, it is
suggested that cometary activity during the Precambrian and
comet showers during mass extinction events provided a
supply of volatile material to the inner solar system that
selectively absorbed uv photons capable of photolyzing the
O2 molecule (and thus precluded O3 production); at the same
time the solar flux at near uv wavelenths arrived at Earth
unattenuated, resulting in an extremely harsh environment
for the biota at the Earth's surface and at ocean depths
down to several tens of meters.

The comets under consideration consisted of volatile
material that accumulated in the region of the nebular disk
beyound Jupiter's orbit. Then gravitational perturbations by
the four giant planets redistributed a portion of these
cometary bodies to form the Oort comet cloud at the outer
boundary of the solar system [3]. At the same time a much
larger number of these bodies were expelled from the solar
system, or were disrupted and vaporized by passing in the
vicinity of the Sun [4,5].

In order to estimate the amount of cometary volatile
material that is required to have a critical effect on O3
production in the atmosphere it is assumed that the rate of
O photolysis (O2 + hv --> O + O) required for O3 production
is proportional to the flux of photons in the extreme uv
that drive the reaction; further it is assumed that a
reduction of the O3 production rate of 1/e of its present
value would result in an environment sufficiently harsh at
the Earth's surface to account for the Precambrian biota or
mass extinctions. In the present atmosphere the flux of 200
nm photons is reduced to 1 over e of its value at the top of
the atmosphere by ~10^23 O2 molecules per cm^2. However, the
cometary volatile material would consist of water (H2O) and
ammonia (NH3, molecules with uv absorption cross-sections
greater than that of O2 by factors of 10 and 10^6,
respectively [2]. Thus an average concentration of ~7 x 10^8
cm^-3 of H2O, or ~7 x 10^3 cm^-3 of NH3 would be required
along the path connecting the Earth and the Sun. If the
cometary material is constrained to a disk 0.1 AU thick the
total amount of H2O would be 2 x 10^25g, or 10^20g of NH3.
Comparing the collision cross-sections of the Earth and the
disk (a factor of 5 x 10^7), it is concluded that during the
same period of time cometary transport would deposit 4 x
10^17g H2O on Earth, or 2 x 10^12g NH3. Assuming a residence
time of one year for vaporized material in the disk
(corresponding to an average velocity of 50 km s^-1 for the
outward transport of the vapor), during the 3 x 10^9 years
of cometary activity in the Precambrian 1.2 x 10^27g H2O, or
6 x 10^21g NH3 would have been deposited on Earth.

It is appropriate to consider two aspects of this proposed
scenario for the effect of comets on the history of Earth's
biota: whether it is possible and what, if any, further
conclusions would result. The most direct quantitative
comparison involves the total amounts of H2O or NH3
predicted to have been deposited on Earth by cometary
activity [6,7]. The amount of H2O alone that would be
required exceeds the amount on Earth by more than two orders
of magnitude, but the amount of NH3 required is about equal
to the mass of Earth's atmosphere. Thus the proposed
scenario seems to meet the zeroth order test, epsecially
where NH3 is concerned. However it should be noted that the
required amount of volatile material could be an
overestimate since the absence of an O3 atmosphere for
perhaps only 1% of the time during the Precambrian could
suffice to suppress the development of the biota on the
Earth's surface. Another consideration is the fact that
solar luminosity was less during the Precambrian era. Also,
the amount of cometary volatile material that has been
deposited on Earth may be more than is found at present,
since some of the volatile material may have been removed by
impact erosion. In addition, the uv absorption cross-
sections of cometary H2O and NH3 may be larger than the
values measured in the laboratory, since in comets they
could be in the form of cluster molecules [8]. Finally, the
continuous introduction of H2O and NH3 at the top of the
atmosphere would not only supress O3 production by uv
absorption, but the photolyzed products would catalytically
destroy O3 by the well-known reactions at work in the
stratosphere at present [9].

Since suppression of the O3 atmosphere by comets in the
inner solar system appears to be possible, it is of interest
to note that the same mechanism would resolve two other
current problems involving the history of Earth and Mars.
The likelihood of NH3 in the Precambrian atmosphere was
suggested [10] in order to provide the necessary greenhouse
effect when solar luminosity was less than its present
value, and thus reconcile calculated ocean temperatures with
the observation that the oceans had not frozen. However,
calculation of NH3 photolysis [11] indicated a lifetime for
NH3 in the atmosphere that was significantly less than that
which was required. The presence of NH3 and H2O in the inner
solar system according to the scenario presented here could
reduce the rate of NH3 photolysis in the Earth's atmosphere
to a level, which would permit the small amount of NH3 that
had been suggested [10]. In a similar way the efficient
greenhouse nature of NH3, in conjunction with its shielding
from solar uv radiation by cometary outgassing in the inner
solar system could account for the warmer temperatures on
Mars that are needed to explain the fluvial features that
have been observed there.

To summarize the main suggestion proposed here:
discontinuities in the history of Earth's biota can be
explained by the single unifying suggestion that low levels
of O3 production are controlled by cometary activity.
Precambrian biological activity is explained along with mass
extinctions. Where mass extinctions since the Cambrian are
concerned, comet showers from the remnants of the nebular
disk lasting for thousands to millions of years provide a
model consistent with the paleontological record, which
shows biological degradation lasting for similar periods and
becoming increasingly destructive with time until the event
suddenly ends. Since the model proposed here appears to
answer some outstanding questions it should be investigated
further.

This work was supported by NSF Grant ATM-9206417.

References: [1] Ratner M. I. and Walker J. C. G. (1972) J.
Atmos. Sci., 29, 803. [2] Levine J. S. (1982) J. Molec.
Evol., 18, 161. [3] Oort J. H. (1950) Bull. Inst. Neth., 11,
91. [4] Hills J. G. (1981) Ap. J., 86, 1730. [5] Fernandez
J. A. (1980) Icarus, 42, 406. [6] Oro J. et al., (1990) Ann.
Rev. EPS, 18, 317. [7] Delsemme A. H. (1992) Origins of Life
and Evolution of the Biosphere, 21, 279. [8] Frank L. A. and
Sigworth J. B. (1993) Rev. Geophys., 31, 1. [9] Turco R. P.
(1985) The Photochemistry of Atmosphere, (J. S. Levine,
ed.), Academic Press, 77. [10] Sagan C. and Mullen G. (1972)
Science, 177, 52. [11] Kuhn W. R. and Atreya S. K. (1979)
Icarus, 37, 207.


Courtillot V. E.   Jaeger J. J.   Feraud G.
   The Influence of Large Igneous Provinces on Mass Extinctions: Where Do We
   Stand?

As proponents in the late 1980s of the volcanic scenario for
the KTB mass extinctions, and not having worked on the
subject for the last few years, we find this conference an
opportunity to critically evaluate the ever-expanding
litterature and to attempt to summarize where we stand now.

A number of publications have reported results from sections
that were later found to be incomplete, underlining the need
for careful interdisciplinary basic field geology. For
instance, early interpretation of the Mimbral section as an
instantaneous deposit, or of the Chixculub structure as a
KTB impact crater, have been challenged since on simple
geological grounds [1,2]. Sedimentary features such as
glasses or spherules previously attributed to an impact have
been shown to give an ambiguous message, to say the least
[3,4]. The most complete sections (e.g. El Kef) apparently
record a prolonged period of ecological disruption beginning
some 200,000 years prior to the KTB (as defined by the Ir
anomaly) and lasting hundreds of thousands of years
afterward [5].

Dating of continental flood basalts has reinforced the
correlation between them and mass extinctions [e.g., 6]. The
coincidence of the age of the Permo-Triassic boundary and
the Siberian traps, which had been predicted by several
authors including ourselves [7], has subsequently been
verified [8]. A claim by Venkatesan et al. [9] that the
Deccan traps predate the KTB by at least 1 million years has
been shown ill-founded on two counts [10]: The argon
plateaus have suffered recoil effects leading to age
overestimates and the magnetostratigraphic interpretation
contains an internal contradiction. Given the few estimates
of absolute age of the KTB [11], the coincidence of Deccan
volcanism and KTB remains tenable.

Claims that impacts could generate flood basalts, however
interesting, do not seem founded [12]. Observations such as
time constraints, lack of iridium for boundaries other than
KT [13], and probable involvement of the lower mantle [14],
seem to argue against such a view. For instance, should the
Deccan traps have been triggered by impact, this would place
the timing of impact within chron 30N, several hundred
thousand years before the Ir anomaly.

We believe that extensive work on large igneous provinces
[14,15] has emphasized their major geodynamical role and
potential environmental impact: More harmful flood basalts
may result from a combination of large size, rapid
emplacement, detrimental chemical composition, and subaerial
emplacement, all variable parameters. They remain very
strong candidates as causes of most mass extinction events.

On the other hand, the Ir anomaly (which remains a rather
unique feature in the geological record [13]) and the
shocked quartz [e.g. 16] are strong evidence for an impact,
with no appropriate explanation in terms of volcanism as
yet. Many other lines of evidence cited in the last decade
as unambiguous proofs of impact have subsequently been shown
ambiguous (e.g., spherules, spinels,...[17]). There is no
doubt that large impacts have occurred on Earth fairly
recently, but their biological consequences remain to be
assessed. Large (40 to 80 km diameter) craters such as
Montagnais or Kara and Ust Kara are dated from some 10 Ma
after and before the KTB, respectively, and have had no
significant effect on extinction rates.

It may well be that an unusual impact did coincide with the
KTB and accelerated the extinction of some 30% of the
species that dissappeared in the crisis. But the extinction
event, likely due to volcanism, was well on by that time and
the role of impacts seems to have been largely overstated in
this last decade. We expect further insights from detailed
analysis both of the Narsapur (off the Deccan traps and the
Bombay coast) and of the projected Chixculub drill cores.

References:  [1] Keller G. (1993) Marine Micropaleontology,
21, 1-45; Keller G. et al. (1993) GSA Bull., 105, 979-997;
Keller G. et al. (1993) Geology, 21, 776-780. [2] Meyerhoff
A. A. et al. (1993) submitted to Geology. [3] Jihanno C. et
al. (1992) Earth Planet. Sci. Lett., 109, 229-241. [4]
Officer C. B. and Lyons J. B. (1993) EPSL., 118, 349-351.
[5] Keller G., and Barrera E. (1990) in GSA Spec. Paper 247,
563-575. [6] Stothers R.B. (1993) GRL, 20, 1399-1402. [7]
Courtillot V. (1990) Sci. Am., 263, 85-92. [8] Renne P. R.
and Basu A. R. (1991) Science, 253, 176-179. [9] Venkatesan
T. R. et al. (1993) EPSL, 119, 181-189. [10] Firaud G. and
Courtillot V. (1993) submitted to EPSL. [11] Baadsgaard H.
et al. (1988) Can. J. Earth Sci., 25, 1088-1097; Gillot
P.Y. et al. (1991) C. R. Acad. Sci. Paris, 313, 193-199;
Odin G. S. (1989) Bull. Soc. Giol. France, 8, 145-152; Izett
G. A. et al. (1991) Science, 252, 1539-1542; Swisher C. C.
et al. (1992) Science, 257, 954-958. [12] White R. (1989)
Eos, 70, 1490-1491. [13] Orth C. J. et al. (1990) in GSA
Spec. Paper 247, 45-59. [14] Coffin M. F. and Eldholm O.
(1993) Rev. Geophys., in press. [15] Richards M. A. et al.
(1989) Science, 246, 103-107. [16] Goltrant O. et al. (1992)
Terra Nova, 4, 405-412; Goltrant et al. (1991) EPSL, 106,
103-115. [17] Swinburne N. C. et al. (1993) abstract; Robin
E. et al. (1992) EPSL, 108, 181-190.


Bhandari N.   Shukla P. N.   Ghevaria Z. G.   Sundaram S. M.
   KT Boundary in Deccan Intertrappeans: Chemical Anomalies and Their
   Implications

We have identified the KT boundary layer in Deccan
intertrappean sediments based on anomalously high
concentrations of Os and Ir. Concentration of Os (~1.5
ng/g), about 20 times more than in the adjacent sediments,
have been found in a ~1cm thick chocolate band within white
shale in the third intertrappean sediments (I-III) at Anjar.

The Anjar trap sequence consists of seven alkaline basalt
flows with five intertrappean sediments, which appear to be
brackish water shallow continental deposits and one red bole
horizon. The site is located south of Anjar in Kutch, which
forms the western periphery of Deccan plateau. Two of these
traps (FII and FIII) have been dated by Pande et al. [1] in
our laboratory by ^39Ar/^40Ar method to be 65.5 +- 0.7 and
65.4 +- 0.7 Ma respectively, coincident with the KT boundary
event (65 Ma [2]). Dinosaur bones and upper Cretaceous
fossils have been found in intertrappeans I-II and I-III.
The I-III contains the uppermost horizon containing in situ
dinosaur fossils [3]. The intertrappean sediments I-III
consist of a shale-limestone-chert-clay sequence that
contains this Ir rich thin and compacted chocolate layer
within the white shale.

Os, Ir, and about 25 other elements were measured by RNAA
and INAA following the procedure described by Bhandari et
al. [4]. High Os and Ir in sample 27B, representing the
chocolate layer, is accompanied by high Fe, Co, Sb, Zn, Se,
As, and relatively low Th, Hf, Sc compared to the adjacent
shales and the underlying basalt F-III. Ag is found to be 36
ng/g in the sample 27B and has the highest concentration in
sample 24A, located just above 27B. The enrichment factors
for various elements, compared to their concentrations in
basalt F-III, are Sb(795), Se(14.5), Fe(4.1), Co (2.6),
Zn(1.9), Ni(1.8), and Zr(1.2). Sc, Hf, Th, Ta, and Na are
all relatively depleted (by factors of 5 or more) and so are
rare Earth elements (by factors of ~1.5 to 6). Depth
profiles of Os, Fe, and Sb are shown in Fig. 1. Co profile
closely follows the Fe profile whereas the Zn and Se
profiles are similar to Sb profile. Cr profile, on the other
hand, is not similar to that of Fe or cobalt. Based on these
chemical anomalies, some of which (Ir, Os) are diagnostic of
the KT boundary, we identify this layer as the KTB horizon.

Two possible scenarios, one envisaging bolide impact (comet
or asteroid) and the other involving Deccan volcanism have
been proposed based on the discovery of anomalously high Ir
(and other platinum group elements) concentration in KT
boundary in marine sediments [5]. Our search for similar
chemical anomalies within the Deccan is based on the
realization that identification and chemical
characterization of the KTB layer within Deccan may help us
to resolve this debate between volcanic and extraterrestrial
origin of high PGE concentration.

The concentration of Ir in Anjar I-III (sample 27B) is not
only the highest among Anjar intertrappeans but also 10 to
20 times higher compared to many Deccan intertrappean
sediments [6] from the eastern periphery of the Deccan
plateau i.e., from Nagpur and Jabalpur [7].

The question arises whether this chocolate layer represents
the fallout resulting from the bolide impact or a volcanic
deposit resulting from the Deccan eruptions. Furthermore,
whether this layer represents the primary concentration of
elements or it has undergone some secondary processes of
alteration after its deposition has to be established before
its implications to the source of KTB anomalies can be
understood.

The high concentration of Zn and Sb in marine sediments at
the KTB was considered by Gilmour and Anders [8] to be of
oceanic origin. The continental sections, such as Anjar,
have the advantage that there is little oceanic interference
and the material deposits in a short period of time.
Therefore, the presence of high Zn and Sb, together with Se
etc. in I-III indicates that their enrichment at the KTB may
be due to their high concentration in the source material of
the dust rather than any oceanic processes.

Several of these elements for which an enhancement is seen
are chalcophile and volatile. The Fe abundance is also very
high (~39%). The element pattern is not consistent with any
meteorite composition and therefore also with the asteroidal
impact. Comets, on the other hand, are believed to be rich
in volatile and possibly chalcophile elements [9]. Many of
the elements (Sb, Se, Zn, and As) for which enhancements are
seen are known to be enriched in volcanic emissions [10] as
well, but our earlier study of these chalcophile elements in
other Deccan intertrappean sediments [6] as well as of
samples other than 27 in the Anjar section does not show
anomalously high concentration of these elements. A complete
mineralogical, chemical, and isotopic characterization of
this sample, which is now in progress, should enable us to
distinguish between various source materials for the KT
boundary layer.

References: [1] Pande K. et al. (1988) Mem. Geol. Soc. of
India, 10, 145. [2] Izett G. A. et al. (1991) Science, 252,
1539. [3] Ghevaria Z. G. (1988) Current Science, 57, 248.
[4] Bhandari N. et al. (1993) Chem. Geol. (in press). [5]
Alvarez L. W. et al. (1980) Science, 208, 1095. [6] Bhandari
N. et al. (1993) Chem. Geol., 103, 129. [7] Bhandari N. et
al. (1993) PALEO (in press). [8] Gilmour I. and Anders E.
(1989) GCA, 53, 503. [9] Clube S. V. M. and Napier W. M.
(1986) In Smoluchowski (R. Bahcall et al., eds.), The galaxy
and the Solar System, 260-285. [10] Zoller W. H. et al.
(1983) Science, 222, 1118-1123.

Fig.1, which appears here in the hard copy, shows Os, Fe,
and Sb profiles in Anjar intertrappeans. Some data points
are displaced along the depth for sake of clarity. The peak
concentrations occur in the sample 27B. The upper small peak
occurs in sample 24. The lowest sample is basalt F-III.


Hansen H. J.
   Elementary Carbon Associated with Volcanic Eruptions: Relevance for the KT
   Boundary Problem

Volcanic glass particles from 24 different Icelandic
eruptions (basaltic and intermediate composition) were
analyzed for their C content. The values range from 400 to
1100 ppm.

Volcanic glass dissolved in HF shows presence of hollow
spheres of graphite with diameters in the range of 1-15
microns.

The hollow spheres are interpreted as coatings of the walls
of the gas vesicles in the glass.

Glass grains of basaltic composition from the Paleocene Fur
Formation, Denmark, showed bulk delta^13C value of -16 while
the solid C alone showed almost -28.

The eruption of Hekla, Iceland, January 19th, 1991 took
place during a snow period resulting in a sandwiched
complete ashlayer in the snow. The fraction of the ash layer
smaller than 125 microns had a C content of 400 ppm. After
cleaning by ultrasonic vibrations the fine fraction showed
only 100 ppm of C.

From these observations it can be concluded that volcanoes
produce elementary C as part of the eruptional products.
Some of the C is contained in gas vesicles in the glass,
while other C is part of the ash. Hollow C spheres are thus
indicators of volcanism and they stand a good chance of
surviving later diagenetic alteration of the glass. The
smallest spheres 1-2 microns in diameter survives sediment
compaction.

Samples from the grey chalk underlying the KT boundary clay
at Stevns Klint and the boundary clay itself contain C
spheres indicating that a volcanic component is present. The
isotopic value of the solid C is in good agreement with the
values from the grey chalk.

Whitish spheres from the "altered tectite layer" at Beloc,
Haiti, after dissolution showed presence of C spheres
pointing to a volcanic rather than a tectite origin.

Various tectites dissolved in HF did not show any C residue.


Liu Y.-G.   Schmitt R. A.
   KT Phenomena in the Context of Tectonic Seafloor Rearrangements

Various processes have been proposed for mass extinctions in
the Earth'shistory. Rampino and Caldeira [1] showed that
major geologic events over the past 250 Ma were periodic and
interrelated and seem to involve jumps in sea-floor
spreading associated with continental rifting, volcanism,
enhancedorogeny, global sea-level changes, and fluctuations
in climate. Lyle et al. [2] and Owen and Rea [3] found that
sea-floor hydrothermal activities fluctuate by as much as an
order of magnitude and that episodes of greatest
hydrothermal flux correspond to times of major tectonic sea-
floor arrangements (TSR). The ridge jumps and changes in
ridge orientation may significantly increase hydrothermal
activities by fracturing oceanic crust and providing
seawater access to deep-seated crustal heat sources.
Enhanced leaching of Ca^2+ ion into the hydrothermal
solutions reacts with oceanic bicarbonate to yield CO2 [3].
Rea et al [4] noted that 20% of the preindustrial
atmospheric CO2 may be attributed to the present mid-oceanic
ridge hydrothermal activities. Itwas estimated from
geochemical evidences that one or two orders of magnitude of
hydrothermal activity occurred at the Paleocene/Eocene
boundary. The CO2-induced climate change may be the cause of
the extinction and evolution at the Paleocene/Eocene
boundary.
We [5] found P(sub)CO2/P(sub)CO2^o peaks at ~17, ~53, and
~63 Ma, where P(sub)CO2 values relative to the present
P(sub)CO2^o were calculated from Ce^As (Ce anomalies) of
Pacific seawater recorded by Ce^A*s recorded in deep sea
carbonate sediments. The correspondence between our
calculated P(sub)CO2 enhancements and the TSR-HA
(hydrothermal activities) associated with major TSRs [1]
strongly support the hypothesis that the pH of the deep
(>600 m) Pacific Ocean was lowered by the CO2 generated by
enhanced TSR-HA at these times.

Hydrothermal solutions are strongly enriched in Mn and Fe by
10^6X, Co by ~3000X, Zn by ~8000X [6], and in REEs by 10-
10^4X (avg. ~10^3X) [7]. Therefore, in addition to Ce^A
changes, which indicate pH changes, the enrichments of the
above elements in sediments signify the enhanced TSR-HA.
Because Mn at ~0.017 micrograms/L and Co at ~0.0015
micrograms/L are quite low in present seawater [8], their
signals should be observable at the postulated TSR-HA events
especially in carbonate sediments with relatively low
amounts of detrital clay. As the hydrothermal plumes mix
with seawater, Fe^2+ is rapidly oxidized to ferric-
oxyhydroxides forming particulates while the slow oxidation
of Mn serves to keep most of the Mn in solution for
dispersal throughout the oceans [9]. Likewise, Co is
dispersed and kept in solution.

However, the REEs are largely coprecipitated and scavenged
with the Fe-oxyhydroxides near the vent [10]. We and Michel
et al. [11] have obtained (Eu/Sm) ratios of 0.24-0.27 for
~300 carbonates from Pacific D.S.D.P. Holes 316, 317A, 317B,
577, 577A, 577B, and the Laytonville Limestones, and these
ratios overlap the present Eu/Sm = 0.26 +- 0.01 ratio in the
Pacific [12]. These observations firmly support the
contention [10] that the bulk (say, >95%) ofthe REEs in the
hydrothermal plumes are scavenged by the ferric-
oxyhydroxides that precipitate out of the hydrothermal-
seawater mixture close to the hydrothermal vents.

Since the REEs, Mn and Co precipitate out of seawater onto
the detritus and ferric-oxyhydroxide coatings and also
because the La^*/Th, Mn^*/Th, and Co^*/Th (Th is an
excellent detritus proxy and is well determined by INAA)
indicate a surface/volume effect (i.e., these ratios
generally increase with lower Th abundances) we have
eliminated the surface/volume effect of the detritus by
correlating Mn*/La* and Co*/La* ratios. Mn^*, Co^*, and La^*
values are seawater precipitated abundances and are
corrected for Mn, Co, and La in the NASC-like detritus.

From Table 1, we clearly observe enhanced Mn^*/La^* and
Co^*/La^* ratios in the 15.7-19 Ma interval, with a max at
~17 Ma. This corresponds to the enhanced Mn abundances of
~10-40X reported [2] in 17-18.5 Ma metalliferous sediments
from DSDP Leg 92, East Pacific Rise, and attributed to TSR-
HA.

The Shatsky Plateau (SP) data indicate a number of
interesting observations. There is a hiatus in SP cores
between 6-50 Ma and no older samples than 66.24 Ma were
cored. The background levels of Ce^A* in the time intervals
of 65.46-66.24, 65.30-65.31, 65.25-65.27, 64.95-65.11 (the
K/T-Ir event), 58-58.9, 20-31, and 3.2-15.5 Ma are the same
within error and equal to the Ce^A* in present deep Pacific
seawater of 0.06 +- 0.01 [12]. Since Ce^A* is controlled by
the P(sub)CO2^o[5], P(sub)CO2 during those intervals was
the same as the present P(sub)CO2^o. The Mn^*/La^* and
Co^*/La^* follow the general pattern of Ce^A* with the
average background values of Mn*/La* = 8.3 +- 1.1 and
Co^*/La^* = 0.084 +- 0.007. Relative to the above background
levels, Ce^A*, Mn^*/La^* and Co^*/La^* in the 62.0-64.25 and
50.1-55.0 Ma intervals are quite high and indicate major
TSR-HA. We also note minor enhancements of Ce^A* at
intervals of 64.59-64.80 and 65.15-65.24 Ma and very minor
enhancements of Ce^A* at 65.28-65.30 and 65.34-65.46 Ma but
relatively pronounced enhancements of Mn^*/La^* and
Co^*/La^* during these intervals. They are designated as
minor or very minor TSR-HA.

The inferred major TSR at ~53 Ma corresponds to the broad
delta^18O minimum between 24N and 22N chron and to a rising
delta^13C signal for the same chron intervals [4]. These
parameters coupled with our observations are consistent with
a greenhouse effect induced by higher P(sub)CO2 due to TSR.
The major TSR-HA event at ~63 Ma started 0.75 Ma after the
K/T-Ir event at 65.0 Ma [13] and lasted for 2.3 Ma from
64.25-62.0 Ma. An average high Ce^A* corresponds to a P(sub)
CO2 ~1.9X the present P(sub)CO2. At that time, seafloor
spreading in an E-W direction about the Gakkel Ridge
resulted in the opening of the Eurasian Basin. The extensive
islandic plume was activated at that time. The high
Mn^*/La^* and Co^*/La^* ratios suggest that hydrothermal
activities were very extensive in the world's oceans.

As proposed by [2,3], TSR-HA would lower the oceans' pH.
Some trace element poisoning would have occurred if a
significant fraction of elements like As, Sb, Pb, Bi, etc.
remained in the hydrothermal plume and dispersed throughout
the oceans. Therefore, some of the extinctions before the
K/T-Ir boundary [14] may be attributed to either pH or
poisoning effects or to a combination of both effects. The
absence of TSR-HA and a normal pH like that present in the
deep (>600m) oceans at the K/T-Ir boundary negates poisoning
and pH effects. Of course, the pH of the mixed oceanic layer
was probably lowered from the acidic rain caused by the
atmospheric effects of bolide transit [15], thereby
resulting in extensive planktonic extinctions at K/T time,
65.0 Ma.

References: [1] Rampino M. R. and Caldeira K. (1993) EPSL,
114, 215-227. [2] Lyle M. et al. (1987) GRL, 14, 595-598.
[3] Owen R. M. and Rea D. K. (1985) Science, 227, 166-169.
[4] Rea D. K. et al. (1990) Palaeogeo. Palaeoclim.
Palaeoecol., 79, 117-128. [5] Liu Y.-G. and Schmitt R. A.
(1993) LPS XXIV, 883-884. [6] Von Damm K. L. et al. (1985)
GCA, 49, 2197-2220. [7] Michard A. et al. (1983) Nature,
303, 795-797. [8] Martin J. H. and Gordon R. M. (1988) Deep
Sea Res., 35, 177-196; Nature, 314, 524-525. [9] Klinkhammer
G. and Hudson A. (1986) EPSL, 79, 241-249. [10] German C. R.
et al. (1990) Nature, 345, 516-518. [11] Michel H. et al.
(1985) Init. Repts. DSDP, 86, 533-538. [12] Piepgras D. J.
and Jacobsen S. B. (1992) GCA, 56, 1851-1862. [13]
McWilliams M. O. et al. (1992) EOS, 363; Sharpton V. L. et
al. (1992) Nature, 359, 819-821; Swisher C. C. et al. (1992)
Science, 257, 954-958. [14] Keller G. (1989) GSA Bull., 101,
1408-1419. [15] Prinn R. G. and Fegley B. Jr. (1987) EPSL,
83, 1-15. [16] Anders E. and Grevesse N. (1989) GCA, 53,
197-214.


Dalziel I. W.
   Impacts, Extinctions, Volcanism, Glaciations, and Tectonics: Matches and
   Mismatches

The debate concerning possible relations between impacts,
extinction events, and volcanism has recently taken a new
turn. Diamictites and associated sedimentary deposits long
regarded by geologists as glaciogenic, have been
reinterpreted as impact-related. Going further, the Permo-
Carboniferous diamictites that are widespread in the
southern continents and India are now put forward as
evidence that fragmentation of the Gondwana supercontinent
in the Mesozoic was a direct result of meteorite impact. In
an abstract at a meeting of the American Geophysical Union,
and in an article in the popular press, one member of the
earth science community has made a specific claim to
identify the site of the supercontinent-destroying bolide on
the Falkland/Malvinas Plateau. It is claimed by this
scientist that the Cape fold belt in Africa represents a
"breaking wave" of deformation resulting from this impact,
and that fractures in the clasts of the Dwyka diamictite in
southern Africa represent impact-induced cataclasis of the
target rock. These hypotheses fly in the face of the well-
established tectonic history of the Gondwana supercontinent
in several respects.

1. The suggested site of impact is the Lafonian
(Falkland/Malvinas) microplate that was displaced relative
to both South America and Africa after the time of the
proposed impact and before opening of the South Atlantic
Ocean basin. Hence the suggested geometric relationship of
this site to surrounding features such as the Cape Fold Belt
is incorrect (see Fig. 1).

2. Gondwana reconstructions based on sea-floor spreading and
paleomagnetic data show that the Gondwanide fold belt is
almost rectilinear from Argentina, through southern Africa,
to the Pensacola Mountains of Antarctica. It is not circular
in plan view (see Fig. 1).

3. The Gondwana diamictites and associated deposits are
diachronous and track the paleomagnetically determined
position of the south pole across the supercontinent in the
Devonian, Carboniferous, and Permian.

4. Gondwanide deformation is also diachronous, even within
the Cape fold belt it extended from ca. 278-230 Ma. It took
place behind an active subducting margin of the Pacific
Ocean.

5. The fracturing of the clasts in the diamictites can be
directly related to strain during a mid-to Late Permian or
Triassic phase of deformation in the Gondwanide fold belt.
Glossopteris-bearing Permian strata overlie the diamictites
everywhere, and were deformed with them during the
Gondwanide folding.

Leaving aside the issue of obvious distinctions (textural,
metamorphic etc.) between modern tillites and deposits such
as the Onaping Formation of the Sudbury basin that are
widely accepted as being impact-related, it is unfortunate
that the largest extinction event in earth history should
have been related in such a cavalier fashion to geologic
features spanning several tens of millions of years (indeed
even hundreds of millions of years to judge by maps shown at
AGU and published, with attribution, in the popular press).
Geologists clearly need to consider impact of
extraterrestrial bodies as a major agent of tectonic as well
as environmental change. But as students of impact and its
effect on our planet reach back in time from the KT
boundary, it is all the more important for them to look
carefully at its complex tectonic history in relation to the
geologically instantaneous events that they invoke.

Fig. 1, which appears here in the hard copy, shows (Left)
Gondwana during the Late Triassic (230 Ma) after the
Gondwanide orogeny; based on the reconstruction of Grunow et
al. (1991), intraplate deformation in Africa after Daly et
al. (1991), and reconstruction of the Gondwanide orogen
after Dalziel and Grunow (1991). Arc magmatism from
Pankhurst (1990). The positioning of the incipient
Karoo/Bouvet plume (circle) follows that of Whate and
McKenzie (1989). AP = Antarctic Peninsula block; EWM =
Ellsworth-Whitmore Mountains block; FI = Falkland Islands
terrane (shape schematic); K = Karoo Basin; LM = Lebombo
monocline; MBL = Marie Byrd Land; NSL = Neuschwabenland; SNZ
= south New Zealand (South Island, Campbell Plateau, and
Chatham Rise); TI - Thurston Island block. Dashed circles
are confidence circles about paleomagnetic poles (asterisks
and crosses) of Grunow et al. (1992) and are identified by
letters as above. The reconstructions in Figs. 5 through 10
are all polar stereographic projections centered on the
paleomagnitic south pole (cross) determined for East
Antarctica (eat).

(Right) Gondwana in the Middle Jurassic (175 Ma) after the
rotation of the Falkland Island and Ellsworth-Whitmore
Mountains blocks; based on the reconstruction of Grunow et
al. (1991). The location of the magmatic provinces follows
Cox (1978), Dalziel et al. (1987), White and McKenzie
(1989), and Storey and Alabaster (1991). Additional
abbreviations: BSB = Byrd Subglacial Basin (including
Bentley Subglacial Trough); DM = Dufek Massif; EW = Explora
wedge; TM = Theron Mountains.


Webb G. E.
   The Frasnian-Famennian Extinction Event: Dominance of Extrinsic Over
   Intrinsic Factors in the Recovery of Reef Communities

Extinction events are recognized in many different ways and
may be seen as short-term biotic diversity minima, numerous
coincidental extinctions, or decreases in originations.
Amounts of community-ecological disruption and resulting lag
time before ecosystem recovery have also been important in
shaping our perceptions of an event. The Late Devonian
Frasnian-Famennian (FF) extinction event provides an example
for which the cause, or causes, and duration of the event
are controversial [1]. Less controversial is the importance
of an abrupt change in global reef communities at the
boundary. Microbes, corals, and stromatoporoids constructed
Frasnian shelf edge reefs. It is widely accepted that reef-
building corals and stromatoporoids perished during the FF
event and that comparable reef communities, and hence,
reefs, were not abundant again until the Permian. The
intervening, impoverished record of Carboniferous reefs is
considered lag time during which reef communities gradually
recovered as new constructors and ecological relationships
evolved after the FF event [2,3]. This intrinsic,
evolutionary community recovery was termed erathemic
succession [4] and its length, following the FF event, has
been used to imply that the event was particularly severe.
However, the extended length of upper Paleozoic reef
recovery has been attributed partly to subsequent ecologic
perturbations caused by external agents [5]. Recent research
[6,7] supports this hypothesis and, further, suggests that
intrinsic erathemic succession played only a small role in
upper Paleozoic temporal reef distribution. Post-extinction
lag times are inappropriate proxy indicators of extinction
severity because they can be subject to subsequent,
unrelated, extrinsic controls.

Shelf edge reefs were globally abundant in the Frasnian but
the majority were terminated well before the end of the
stage. The remainder were terminated at the FF boundary
except for a few reefs with stromatoporoids and corals that
survived for a short time into the Famennian [8]. Other
reefs survived longer without stromatoporoids and corals
[9], and those on the Russian Platform apparently were not
affected by the FF event [10]. Hence, the effect of the FF
event on global reef communities has been overstated [11].
Additional stromatoporoid-bearing reefs were initiated in
the Famennian of Canada [12]. All reefs, except for those on
the Russian Platform, ceased growth by the Devonian-
Carboniferous (DC) boundary. The DC boundary may be a more
important marker for global reef communities than the FF
boundary. Lower Tournaisian reefs are known only from Russia
[10]. By the middle Tournaisian, deep-water bioherms
(Waulsortian mounds) were abundant on ramps, which were the
dominant carbonate setting [13,14], and shallow water reefs
were known only in eastern Australia [7] and Russia [10]. By
the Visean, shallow-water reefs were abundant globally on
numerous shallow carbonate shelves. Reefs declined
dramatically by the early Namurian, however, owing to
clastic influxes onto many shelves and tectonic uplift and
regression of epeiric seas. Early Namurian reefs were rare
and few reefs, with the notable exception of the Akiyoshi
atoll complex of Japan, survived past the Mississippian-
Pennsylvanian boundary [7].

Reef communities were clearly set back by events in the Late
Devonian, but slow recovery in the Carboniferous was not
caused by slow, intrinsic, erathemic succession. Important
reef-building microbes, such as those that built Frasnian
and Famennian reefs in the Canning and Bonaparte Basins of
Western Australia were available to build reefs as late as
the late Visean. New reef-building corals were developing by
the middle Tournaisian in eastern Australia. The implication
that reef communities did not exist because of the lack or
scarcity of preserved reefs is unfounded. Lower
Carboniferous bioherm communities, although largely endemic,
were not limited by the lack of constructors. Rather, their
distribution was tightly constrained by proximate factors,
such as local tectonostratigraphic settings and sea level-
sedimentation histories [7]. The global pattern of temporal
reef distribution represents a composite of these
individual, local histories. However, global ultimate causes
are probably responsible for many of the proximate factors,
thus lending a global pattern to the composite. The
following global factors affected Late Devonian-Lower
Carboniferous reef distribution. (1) Abundant, anoxic black
shales have been linked with the FF event [15] and imply
relative instability of oceanic stratification that may have
led to periodic poisoning of shallow-water habitats from the
Frasnian to Tournaisian. (2) The development of deeper-
rooting trees in the Late Devonian may have increased the
rate and depth of soil formation and hence the influx of
terrigenous sediment and nutrients to shallow waters [16].
Clastic sediments smother reefs and abundant nutrients favor
non-skeletonized fleshy algae over reef-building organisms.
(3) The long-term regressional trend towards greater
continentality [17] had several adverse affects on reefs.
Shallow carbonate shelves were lost through exposure and the
influx of terrigenous sediments increased owing to the
lowered base level and enlarged area for subaerial erosion.
The associated relative change in shallow bathymetric
profiles and increased production of mobile, especially
oolitic, shoaling sands [18], also adversely affected reefs.
(4) Climatic changes associated with the global shift from
the Devonian "greenhouse" climate to the mid-Carboniferous
"icehouse" climate [17] also may have been important because
most reef builders appear to have preferred tropical
climates. Rapid glacio-eustatic sea level fluctuations also
may have disturbed some reef communities. (5) The accretion
of Pangaea, with resulting tectonism and loss of continental
margins, limited reef development in some areas and may have
changed global oceanic circulation patterns [19]. (6) Major
changes in ocean chemistry, such as the shift from calcite-
to aragonite-favoring seas at the Mississippian-
Pennsylvanian boundary [20], also may have caused
perturbations in reef communities.

In conclusion, there is no doubt that a major change took
place in global reef communities during the Late Devonian.
However, it may be preferable to attribute the change to
multiple events rather than to a single FF event.
Furthermore, the apparent long recovery time for global reef
communities resulted not from the severity of the disruption
of the community at the FF boundary, but from a multitude of
subsequent, local disruptions. These local disruptions were
driven by an extended period of profound, global changes in
tectonics, sea level, climate, ocean chemistry, and even the
terrestrial biosphere. The intrinsic, evolutionary recovery
rate of post-Frasnian reef- and bioherm-building organisms
appears to have played only a limited role in the length of
recovery. Therefore, extrinsic, environmental factors played
a more important role in the recovery and distribution of
post-Frasnian reefs than did intrinsic, erathemic
succession. The apparent "lag time" before recovery of upper
Paleozoic reefs is a poor indicator of the severity of an FF
extinction event.

References: [1] McGhee G. R. Jr. (1989) Mass Extinctions-
Processes and Evidence (S. K. Donovan, ed.), Columbia, New
York, 133-151. [2] Sheehan P. M. (1985) Geology, 13, 46-49.
[3] West R. R. (1988) Palaios, 3, 152-169. [4] Copper P.
(1989) Mem. Ass. Australas. Palaeonts., 8, 371-385. [5]
Copper P. (1988) Palaios, 3, 136-151. [6] Horbury A. D.
(1992) Sed. Geol., 79, 117-137. [7] Webb G. E. (1993) Carb.
to Jur. Pangea, Progr. & Abs., 329. [8] Burchette T. P.
(1981) SEPM Spec. Publ., 30, 85-142. [9] Mory A. J. and
Beere G. M. (1989) Bull. Geol. Surv. W. Austr., 134, 1-184.
[10] Ulmishek G. F. (1988) Can. Soc. Petrol. Geol. Mem.,
14/1, 527-549. [11] Stearn C. W. (1987) Geology, 15, 677-
679. [12] Stearn C. W. et al. (1987) Palaios, 2, 560-570.
[13] Ahr W. M. (1989) SEPM Spec. Publ., 44, 203-212. [14]
Wright V. P. and Faulkner T. J. (1990) Geol. J., 25, 139-
144. [15] Goodfellow W. D. et al. (1988) Can. Ass. Petrol.
Geol. Mem., 14/3, 9-21. [16] Algeo T. J. and Maynard J. B.
(1993) GSA Ann. Meeting, A-83. [17] Fischer A. G. (1984)
Biotic Crises in Ecological and Evolutionary Time (M. M.
Nitecki, ed.), Academic, New York, 103-131. [18] Wilkinson
B. H. et al. (1985) J. Sed. Pet., 55, 171-183. [19] Stanley
S. M. (1988) Palaios, 3, 228-232. [20] Sandberg P. A. (1983)
Nature, 305, 19-22.


Wolbach W. S.   Roegge D. R.   Gilmour I.   Gardner A.
   The Permian-Triassic of the Gartnerkofel-1 Core (Carnic Alps, Austria):
   Organic Carbon Isotope Variation

The extensive study of the Permian-Triassic (P/Tr) boundary
sequence of the Gartnerkofel-1 Core, Carnic Alps, Austria,
has included detailed descriptions of its stratigraphy [1],
mineralogy [2,3], geochemistry of common and trace elements
[4-6], and stable C and O isotopes in carbonates [4, 7, 8].
The results of these studies are consistent with gradual
sea-level regression, leading to increased erosion of
organic material and metals [4, 8]. Oxidation of organic C
in the ocean would lead to oceanic anoxia. A reducing ocean
would allow increased accumulation of organic C and
precipitation of metallic sulfides [8]. While organic C
abundances have been measured [4], until now no study of
organic C stable isotopes has been conducted across the P/Tr
sequence.

Procedures: Bulk C residues were isolated from a suite of
P/Tr sequence samples from the Gartnerkofel-1 Core, Carnic
Alps, Austria, using the HCl and HF-HCl procedures of
Wolbach and Anders [9]. An attempt was made to separate
organic from any elemental C using extended acid dichromate
oxidation [9]. The residues were combusted to CO2 for
mass-spectrometric analysis, yielding isotopic data, and
weights for organic C. Carbon isotope measurements were made
on a VG SIRA 24 mass spectrometer using sealed tube
combustion.

Organic C Abundance: The brown color of all carbonaceous
residues, the total and rapid disappearance of residue
aliquots upon dichromate oxidation, and their C content
(<<1OO%) were consistent with these residues being composed
completely of organic C, with no elemental C component. Thus
all carbonaceous residues from the suite of P/Tr samples
studied are assumed to be 100% organic.

Organic C concentration increases sharply 15x to 4600 ppm in
the Triassic (Tr), 8.5 m above the top of the 6.5 m thick
Tesero Horizon (TH), thought to contain the P/Tr boundary
(Fig. 1a). The organic concentration remains elevated for
1.5 m, then drops back to baseline values. At first glance,
the increase in organic C concentration seems to parallel a
similarly sharp increase in S concentration (S data from
[5]) (Fig. 1b). Expanded depth scales over the regions of
increase (insets in Figs. 1a, 1b), however, show that the C
increase actually occurs 5 m above the first of two S peaks.

The sequence of rocks from ~245 to ~170 m represents roughly
3 Ma, or ~40,000 yr/m [8]. Thus the first increase in S
occurred ~200,000 yr before the C increase, assuming
constant sedimentation rates. This implies that although
more S-rich sediments were being deposited, any C that was
also being eroded as a result of the regression was being
oxidized in the water column and not being accumulated in
sediments. The destruction of C would gradually deplete the
ocean of O2, leading to anoxia, elevated precipitation of
metallic (and other) sulfides, and the observed increase in
preserved oceanic C. That the C enrichment is likely to be
oceanic in origin is supported by the observation that the
carbonaceous residues were easily oxidized by dichromate:
residual organic matter from land is generally much more
difficult to destroy [9]. The second increase in S overlaps
the C peak, and thus might represent increased S in organic
matter from sulfate reducing bacteria.

Organic C Isotopes: The isotopic composition of organic C
(delta^13C(sub)Org) decreases by 6-8 permil in the Triassic,
beginning 12 m above the TH and persisting for nearly 45 m
before returning to baseline values (Fig. 1c). This isotopic
shift falls well within a negative ~4 permil shift over a
similar stratigraphic interval observed for delta^13C of
carbonate C (delta^13C(sub)Carb; carbonate data from [7])
(Fig. 1d). This interval represents ~2 Ma.

The start of the negative shifts for delta^13C(sub)Org and
delta^13C(sub)Carb coincide with the first increase in S,
corresponding to increased erosion. Delta^13C of organic C
is on average ~25 permil lighter than that for carbonate C.
Thus incorporation of light CO2 from eroded, oxidized C into
forming carbonates and photosynthetic marine species would
cause both to become isotopically lighter. Increasing anoxia
of the ocean would eventually result in increased
preservation of marine organic C and S. Increased abundances
would be expected after the initial isotopic shifts, which
is observed.

The increased preservation of marine organic C would
preferentially tie up ^12C, increasing delta^13C of
carbonates. In fact, the shift of delta^13C(sub)Carb back to
heavier values begins immediately after the organic C
abundance peak. As erosion slows, the supply of eroded C
would decrease and anoxia would diminish. With no light CO2
source, delta^13C(sub)Carb and delta^13C(sub)Org would
return to normal.

Recent measurements on the age and duration of the Siberian
Traps flood basalts have placed them synchronous with the
Permian-Triassic Boundary [10,11] and that they were erupted
over an extremely short time interval (900,000 + 800,000 yrs
[10]). Such large scale volcanism would have lead to
substantial quantities of dust and SO2 in the atmosphere
with subsequent climate changes that may have caused the
global regression and subsequent changes in erosion rates.

To summarize, organic C abundances and delta^13C(sub)Org
across the P/Tr boundary seem consistent with the occurrence
of sea-level regression leading to erosion, oceanic anoxia,
and increased precipitation, events that have been suggested
as the cause of other geochemical changes taking place at
that time [8].

We want to thank Roman Schmitt for donating the samples and
J. Gibson for making the C isotope measurements.

References: [1] Boeckelmann K. (1991) Abh. Geol. B.-A., 45,
17-36. [2] Fenninger A. (1991) Abh. Geol. B.-A., 45, 53-60.
[3] Boeckelmann K. and Magaritz M. (1991) Abh. Geol. B.-A.,
45, 61-68. [4] Holser W. T. et al. (1989) Nature, 337, 39-
44. [5] Klein P. (1991) Abh. Geol. B.-A., 45, 109-121. [6]
Attrep M. Jr. et al. (1991) Abh. Geol. B.-A., 45, 123137.
[7] Magaritz M. and Holser W. T. (1991) Abh. Geol. B.-A.,
46, 149-163. [8] Holser W. T. et al. (1991) Abh. Geol. B.-
A., 45, 213-232. [9] Wolbach W. S. and Anders E. (1989) GCA,
53, 1637-1647. [10] Renne P. R. and Basu A. R. (1991)
Science, 253, 176-179. [11] Campbell I. H. et al. (1992)
Science, 258, 1760-1763.


Brinkhuis H.   Visscher H.
   New Evidence for Terrestrial Ecosystem Collapse at the Cretaceous/Tertiary
   and Permian/Triassic Boundaries

A new Cretaceous/Tertiary (KT) boundary site was recently
discovered in ancient man-made caves at Geulhem, in the
vicinity of the Maastrichtian type-section in The
Netherlands. Micropaleontological and palynological evidence
indicates that this section includes an extraordinarily
well-developed earliest Danian succession. At the locality,
the earliest Danian 'boundary clay' interval is represented
by several individual clay layers separated by more
calcareous units and totals some 125 cm.

The section can be confidently assigned to the latest
Maastrichtian and earliest Danian. The KT transition is
complete in terms of its micropaleontological and
palynological record. In the latest Maastrichtian portion of
the section, the uppermost part of the Thalassiphora
pelagica Subzone of the Palynodinium grallator
(dinoflagellate cyst) Zone can be recognized. The earliest
Danian interval is represented by the NP1 (pars;
nannoplankton; A. J. T. Romein, pers. comm.), P0 (planktonic
foraminifer; J. Smit, W. J. Zachariasse, pers. comm.) and
Carpatella cornuta (dinoflagellate cyst) Zones. Preliminary
evidence from other disciplines (studies in progress),
confirms the completeness of this newly found KT boundary
site.

In addition to rich dinoflagellate cyst assemblages, that
cross the KT boundary without any evidence of extinction,
the boundary clay layers yield unparalleled abundances of
bryophyte spores. This phenomenon is interpreted to reflect
a conquest and domination of the terrestrial domain by
bryophytes following a major terrestrial ecosystem collapse.
Bryophytes are well-known to be resistant to adverse
environmental conditions, and are the first to colonize
spaces where higher land plants have disappeared. Since the
pattern is consistent over 125 cm of sediment following the
KT boundary, it is suggested that this phase lasted at least
5000 years prior to recovery of the terrestrial ecosystem
and re-establishment of stable vegetations.

Re-interpretation of palynological successions reported
earlier from sequences straddling the Permian/Triassic
(P/Tr) boundary in southern Europe now suggests great
similarity with the KT succession discussed above. It was
already demonstrated that at the P/Tr junction a reflection
of a world-wide collapse of terrestrial ecosystems with
concomitant loss of standing biomass is indicated by
unprecedented abundances of fungal remains. Directly
succeeding this 'fungal event', one may frequently note high
abundances of palynomorphs generally interpreted as marine
phytoplankton of unknown affinity (so-called acritarchs).
However, restudy of these palynomorphs now suggests that
many of these elements may, in fact, be assigned to
bryophyte spores.

Thus, both the Paleozoic/Mesozoic and Mesozoic/Cenozoic
transitions appear to be marked by an overall similar array
of events, both reflecting severe disruption of the
terrestrial ecosystem, followed by gradual recovery. It is
proposed that these biotic crises in terrestrial ecosystems
could have been triggered by the effects of acidifying
pollution of the atmosphere. Volcanic emissions or ejecta
effects of meteorite impact(s) may be taken into
consideration as the underlying mechanisms.


Little C.
   The Pliensbachian/Toarcian (Lower Jurassic) Extinction Event

In the fossil record of marine genera during the last 250
Ma. Raup and Sepkoski [1] have identified eight events where
extinction rates were significantly higher than background
rates. These are often called `mass' extinctions. The
authors postulate that they were spaced with a 26 Ma.
periodicity. The Permo-Triassic and Cretaceous-Tertiary (KT)
extinctions were the largest of the eight, and are the best
documented. One of the smaller-scale events is at the
Pliensbachian-Toarcian boundary (Lower Jurassic, 186 Ma.), a
peak implied by the 26 Ma. periodicity hypothesis. The
periodicity itself suggests a single causative mechanism,
such as bolide impacts or terrestrial processes such as
volcanism or sea-level change. Hallam [2] has suggested the
later as a cause of the Pliensbachian-Toarcian event and
claims it does not have global distribution.

I have studied various Pliensbachian-Toarcian sections in
North-West Europe to gain an understanding of the timing and
magnitude of the event. An excellent ammonite
biostratigraphic scheme, first developed by Quenstedt and
others over 130 years ago, allows detailed correlation
between sections, with a resolution of perhaps 250 ka. The
section on the Yorkshire coast in Northern England is one of
the thickest and most complete in Europe, consisting of 110
meters of siliciclastic and oolitic iron-rich marine
sediments ranging in age from the Upper Pliensbachian
margaritatus Zone to the mid Toarcian bifrons Zone (Fig. 1).
Macrofaunal species diversity and abundance were recorded at
one meter intervals, or less, from this section. A careful
sampling procedure was employed to assess the role of
collecting effort in diversity/abundance counts.

During the late Pliensbachian, there was a diverse range of
benthic species, including infaunal and epifaunal bivalves,
brachiopods, and crinoids. The equally diverse nekton
included belemnites and ammonites. These faunas cross the
Pliensbachian-Toarcian boundary without showing any
significant extinctions. There was a 22% reduction in
diversity 50 cm above the boundary beneath a decimeter-scale
layer of laminated bituminous shale, a lithology indicative
of sediment anoxia/dysaerobia. The typical Pliensbachian
faunas continued into the lowest zone (tenuicostatum) of the
Toarcian until the last sub-zone (semicelatum). Here there
was a 77% (17 from 22 species) reduction in diversity. This
event is intimately associated with the onset of bituminous
shale deposition, which formed a thick unit, the Jet Rock
Member (falciferum Zone), in the basin. The benthos was the
most severely affected (87% diversity reduction), the two
surviving species being epifaunal bivalves. Among the
nekton, the belemnites (two species) became extinct, while
the rapid turnover of ammonite species, characteristic of
the group, continued unchanged. The other two surviving
species were pseudoplanktonic crinoids, which attached to
floating logs [3]. Thus there was a gradient of extinction
from infaunal benthos to epifaunal benthos to nektic
belemnites, ammonites, and pseudoplanktonic crinoids. This
pattern is consistent with that expected from a long period
of sediment anoxia/dysaerobia.

The Jet Rock Member contains an impoverished benthic fauna
of two or three species of epifaunal bivalves, often
extremely abundant, scattered fish debris and, especially in
the top half (falciferum Sub-zone), an increasing diversity
of immigrant nektic ammonites and belemnite species. Benthic
diversity does not increase again until the resumption of
grey shale deposition, with normal sediment O
concentrations, in the mid Toarcian Alum Shale Member
(bifrons Zone). These species are different from those of
the lowest Toarcian. However, at generic level, a certain
amount of pseudo-extinction occurs, as shown by shared
genera in sediments below and above the Jet Rock Member.

The extinction event recorded in the Yorkshire
Pliensbachian-Toarcian section lasts slightly longer than
one ammonite biozone, a time interval of perhaps 1 Ma.

The sections in Baden-Wurttemburg, S-W Germany show very
similar faunal and facies changes to the Yorkshire section,
although they are thinner [4]. Faunas pass relatively
unchanged across the Pliensbachian-Toarcian boundary into
the lowest Zone (tenuicostatum) of the Toarcian. As in
Yorkshire, there is a decimeter-scale bituminous shale layer
(Tafelfleins) in the first Sub-zone (paltum) of the
tenuicostatum Zone. The main extinction event occurs at the
start of the Jet Rock equivalent, the Posidonienscheifer
(falciferum Zone). The bituminous shales of this unit again
contain very low-diversity, high-abundance benthic faunas,
diverse nekton, and pseudoplanktonic crinoids. Benthic
diversity does not increase until the top of the bifrons
Zone.

This pattern of extinction, linked to falciferum Zone-age
bituminous shales, is common in Pliensbachian-Toarcian
sections in North-West Europe. Indeed, contemporaneous
facies are developed in sections in Italy, Austria, Hungary,
and Greece [5], and are expected to show similar faunal
trends. Outside Europe, lower Toarcian age bituminous shales
are found in the Canadian Arctic, Western Canada,
Madagascar, offshore Western Australia and Japan. However,
sections in Chile show neither bituminous shale development
nor an associated extinction event [6]. This implies that
the event does not have a global distribution, nor is it
independent of facies change from normal marine to O-
deficient conditions.

The Toarcian is generally agreed to be a time of maximum
marine transgression in N-W Europe. One model suggests that
these bituminous shales on epicontinental margins are
characteristic of sediments deposited during the early
stages of a rapid transgression [7].

In summary: (1) The Pliensbachian-Toarcian event actually
occurs one ammonite biozone higher in the lower Toarcian,
perhaps 1 Ma. later, than at the boundary between the two
stages. (2) The event is not global as it is not found in
Chilean sections. (3) The event is intimately associated
with bituminous shale deposition and is therefore facies-
dependent.

As yet, no signatures of bolide impact (Ir anomalies,
tectites, shocked quartz, etc.) have been found. The early
Toarcian event thus seems to be a poor candidate for an
impact-related extinction. It can more easily be explained
by widespread continental-margin sediment anoxia lasting on
the order of 1 Ma. perhaps related to the rapid early stages
of eustatic transgression.

References: [1] Raup D. M. and Sepkoski J. J. (1986)
Science, 231, 833-836. [2] Hallam A. (1986) Nature, 319,
765-768. [3] Simms M. J. (1986) Palaeontology, 29, 475-493.
[4] Riegraf W. et al. (1980) Der Posidonienschiefer.
Biostratigraphie, Fauna und Fazies des sudwestdeutschen
Untertoarciums (Lias e). Enke, Stuttgart, 195. [5] Aberhan
M. (1992) Beringeria, Wrzbuger geowissenschaftlische
Mitteilungen, 5, 1-174. [6] Jenkyns H. C. (1988) American
Journal of Science, 288, 101-151. [7] Wignall P. B. (1991)
Geology, 19, 167-170.


Preisinger A.   Aslanian S.
   Event-, Bio-, and Magnetostratigraphy of the KT Boundary Sections in the
   East Balkan Area, Bulgaria

Cretaceous/Tertiary boundary sections in the East Balkan
area, south of the thrust and nappes of the Balkan Mountain
have been identified by biostratigraphic, mineralogical,
geochemical, and magnetostratigraphic methods. From these
investigations a real time KT boundary was detected only on
the coast at the Black Sea near the village Bjala, 35 km
south of Varna [1]. The hemipelagic sediments of profile
Bjala 2b and Bjala 2c show an Ir enrichment in the boundary
clay, a minimum of CaCO3 and delta ^13C, shocked quartzes, a
mass extinction of Cretacerous nannoplankton species as well
as a bloom of survivors at the KT boundary and the first
appearance of new nannoplankton species after the KT event.
Both profiles lie within the reversal paleomagnetic Chron
29R. The comparison of biostratigraphic and
magnetostratigraphic results provides a time scale for the
evolution of these new nannoplankton species [2].

References: [1] Preisinger A. et al.(1993)
Palaeogeograph.,Palaeoclimatol.,Palaeoecol., 104. [2]
Preisinger A. et al. (1993) Geologica Bulgaria, submitted.


Preisinger A.
   Cyclostratigraphy of the KT Boundary Section on the Coast of the Black Sea
   Near Bjala (Bulgaria)

Cretaceous/Tertiary (KT) boundaries on the coast of the
Black Sea near Bjala, 35km south of Varna have been
identified by biostratigraphic, mineralogical, geochemical
and magnetostratigraphic methods [1].

In the hemipelagic profile Bjala 2b oriented rock cores were
drilled for magnetostratigraphic studies. The measurements
show the normal and reversed zones from CHRON 29R to CHRON
26R. Using biostratigraphic investigations, the Tertiary
(Dan) could be subdivided into nannofossil zones NP1 to NP4.
Since the profile Bjala 2b shows CaCO3 cycles between wt%40
to wt%80 these precessional cycles in Late Cretaceous-Early
Tertiary marine sediments could be used for
cyclostratigraphy. The comparison of biostratigraphic,
magnetostratigraphic and cyclostratigraphic data with the KT
event allows the determination of a time scale in the range
from 65 to 62,5 M.y. Climatic changes could be determined by
means of delta ^18O determinations of the nannofossil from
the survivor Thoracosphaera operculata.

References: [1] Preisinger A. et al. (1993) Palaeogeogr.,
Palaeoclimatol., Palaeoecol., 104.





                          Saturday, February 12, 1994
                  SESSION 7: IMPACT EVENTS AND IMPACT MODELS
                                9:00 - 12:00 PM

Chair(s):  V. Sharpton
           C. Koeberl


Ginsburg R. N.*   Asaro F.   Attrep Jr. M.   Canudo J. I.   Crocket J. H.
Krahenbuhl U.   Masters B.   Millard Jr. H. T.   Olsson R. K.   Orth C. J.
Orue-Etxbarria X.   Quintana L. R.   Rocchia R.
   Results of Blind Tests to Resolve Controversies: Iridium at Gubbio;
   Extinctions at El Kef

Reimold W. U.*
   Epithermal Witwatersrand Gold Mineralization--Caused by the Vredefort
   Mega-Impact Event?

Masaitis [1] recently reviewed many geological and
morphological aspects that might render impact structures
interesting and sometimes important economic geological
targets. However, the role of hydrothermal processes that
might be triggered by impact events and that, in connection
with the effects of structural geological modification in
and around an impact crater, might lead to mineralization of
economic magnitudes has not yet been explored. Considering
the enormous energies released on impact of even small
projectiles (e.g., [2,3]), this aspect should clearly be
emphasized.

Dressler and Reimold [4] drew attention to the particular
setting of the Vredefort Dome near the geographical center
of the preserved Witwatersrand Basin (South Africa) with its
exceptional gold and uranium deposits. An impact origin for
the Vredefort structure was first suggested by Daly [5], and
since the recognition of, particularly, shatter cones [6]
and coesite and stishovite [7] this view has been accepted
by the majority of geologists. However, this hypothesis has
not remained unchallenged, and the discovery of structural
complexities and of temporal relationships between the
various deformation phenomena and geological events in the
area of the Vredefort Dome led to critique of the impact
model for this structure [8,9]. Modeling the size of the 2
Ga [10] Vredefort Structure on the basis of radial
distribution of shock effects, such as shatter cones or
planar microdeformations in quartz [11], resulted in
diameter estimates ranging from 180 to 300 km, with
preference given to the upper limit. This places the
Vredefort Structure into the same size range as the buried
Chicxulub crater structure, for which a 300 km diameter has
just been determined [12]. Clearly the lack of any other
impact structure of such size (the next largest would be
the, like Vredefort, deeply eroded Sudbury Structure that
probably originally measured < 200 km in diameter [13])
implies that continued detailed study of the exposed
remnants of the Vredefort Structure is of utmost importance
for the understanding of large impact cratering events and
the resulting formations.

Various groups have studied the role of hydrothermal
alteration in impact structures (e.g., 14-17]). It was
suggested that within the confines of an impact structure
extensive circulation of impact-heated fluids could take
place [18]. Such effects are not only known from large
impact structures (e.g., [15]), but were noted in very small
structures as well (e.g., [16]). Like the origin of the
Vredefort Structure by either endogenic or exogenic
processes, the formation of the Witwatersrand ores has been
controversial. An avidly conducted argument between those
workers favoring a detrital origin of the Witwatersrand gold
and a second school of 'hydrothermalists' has been carried
out [19]. To date many geologists studying the Witwatersrand
Basin support the so-called `Modified Placer Hypothesis' for
the formation of these ores, by partial remobilization of
primary detrital deposits. However, source and nature of the
fluids that caused large-scale remobilization of gold and
other base metals throughout the ca. 40000 km^2 of the
Witwatersrand Basin have not been identified or resolved
yet. Some workers believe that these fluids were liberated
during the extrusion of the 2.7 Ga Ventersdorp Lavas, while
others have related a basin-wide 250-400 degrees C
metamorphic phase of post-Ventersdorp age with base metal
remobilization [20] and suggested an association with either
the Bushveld magmatic phase at about 2.065 Ga or with the
Vredefort event (e.g., [21]).

It has been known for several years that extensive
pseudotachylite occurences are not restricted to the area of
the Vredefort Dome, but also occur at multiple stratigraphic
levels within the surrounding Witwatersrand Basin [22,23].
These occurences are mainly linked to bedding-plane parallel
faults or to normal faults. Some of the former occurences
are closely related to important reefs. Like in Vredefort,
several generations of pseudotachylites and other fault
rocks were identified in the Basin. Planar microdeformation
features in quartz are observed in Witwatersrand
pseudotachylites, too, but are less abundant than in
Vredefort lithologies. Recently several bedding-fault-
related occurences of pseudotachylite and their respective
host rocks were studied in petrographic and chemical detail
[24-26] and dated with the ^40Ar-^39Ar stepheating technique
[25,27]. We could demonstrate that pseudotachylites
associated with, but younger than various reef horizons can
be highly enriched in Au and other base metals (e.g., Zn,
Pb, Cu). Distinct alteration zones mark the contacts between
pseudotachylite-bearing fault zones and the country rocks
(e.g., at the Ventersdorp Contact Reef, VCR, [25,26]).
Pseudotachylites, too, contain hydrothermally deposited
mineral phases, such as sericite, chlorite, and various
sulfides and oxides, in form of vesicle fillings or matrix
minerals. They contain Au either as primary detrital grains
that obviously are derived from the precursor rocks, or as
tiny, secondary crystallites in the matrix. Detailed fluid
inclusion analysis of all lithologies in the VCR fault zone
[26] relates the alteration and deposition of secondary Au
to a 250-350 degrees C metamorphic event. The fluids were
clearly not of magmatic, but of sedimentary-metamorphic
origin. ^40Ar-^39Ar chronology on 7 pseudotachylite
specimens from the VCR fault zone [27] from geographically
well separated locations in the West Rand and Central Rand
gold fields consistently yielded 2.0 Ga ages. These results
separate the pseudotachylite-forming event from the Bushveld
magmatic phase at 2.065 Ga ago. The fact that
pseudotachylites with hydrothermal mineral phases also
yielded well-defined 2 Ga ages is regarded as evidence for
autometasomatism at that time. Because the Vredefort event
is believed to have occurred at just about 2 Ga ago [10], it
is currently debated whether this widespread hydrothermal
alteration, associated with basin-wide, large-scale
remobilization of Au and other base metals is a direct
consequence of the Vredefort impact event at that time. It
is proposed that a series of bedding-parallel faults,
dipping at shallow angles towards the Vredefort Dome (e.g.,
[23]), acted as channels for fluids that originated nearer
to the center of the structure. No evidence for significant
lateral movements, nor for distal accumulations of Au and
base metals, have been observed to date. It is therefore
thought that, while these processes affected a major part of
the Witwatersrand basin, actual chemical transport did not
exceed distances wider than individual fault/alteration
zones (several meters at maximum). Further fluid studies on
Vredefort lithologies and other Witwatersrand reefs and
fault rocks, as well as metamorphic and chronological
studies are in progress and will be applied to test this
model. Should it be proven right, it would have significant
consequences both for the understanding of hydrothermal
systems associated with impact structures (are they really
confined to the interior of crater structures, or do these
effects further extend into crater floor and crater
environs?) and for the geological and geochemical
exploration in and around large impact structures.

In this context it should be recalled that Duane and Reimold
[28,29] proposed an impact origin for the ca. 800 km-in-
diameter Simpson Desert depression in Central Australia.
This region is surrounded by a belt comprising a host of ore
deposits of mainly hydrothermal origin and of vast economic
importance.

References: [1] Masaitis V. L. (1992) Meteoritics, 27, 21-
27. [2] Melosh H. J. (1989) Impact Cratering: A Geologic
Process, Oxford Press, 245. [3] Dence M. R. et al. (1977) in
Impact and Explosion Cratering, Pergamon Press, 247-275. [4]
Dressler B. O. and Reimold W. U. (1988) in Int. Conf. on
Global Catastrophes in Earth History, LPI, 673, 42-43. [5]
Daly R. A. (1947) J. Geol., 55, 125-145. [6] Hargraves R. B.
(1961) Trans. Geol. Soc. S. Afr., 64, 147-154. [7] Martini
J. E. J. (1978) Nature, 272, 715-717. [8] Nicolaysen L. O.
(1972) GSA, 132, 605-620. [9] Reimold W. U. (1992) Sudbury,
LPI, 790, 59-60. [10] Walraven F. et al. (1990)
Tectonophys., 171, 23-48. [11] Therriault A. M. et al.
(1993) LPS XXIV, 1419-1420. [12] Sharpton V. L. et al.
(1993) Science, 261. [13] Grieve R. A. F. et al. (1991) JGR,
96, 22753-22764. [14] Stoeffler D. et al. (1977) Geol.
Bavar., 75, 163-189. [15] Newsom H. E. (1980) Icarus, 44,
207-216. [16] Newsom H. E. et al. (1987) JGR, 91, E239-E251.
[17] Komor S. C. et al. (1988) Geology, 16, 711-715. [18]
Koeberl C. et al. (1989) GCA, 53, 2113-2118. [19] Pretorius
D. A. (1991) Econ. Geol. Mon., 8, 139-163. [20] Phillips G.
N. and Myers R. E. (1988) Econ. Geol. Mon., 6, 598-608. [21]
Frimmel H. E. et al. (1993) Econ. Geol., 88, 249-265. [22]
Killick A. M. and Reimold W. U. (1990) S. Afr. J. Geol., 93,
350-365. [23] Fletcher P. and Reimold W. U. (1989) S. Afr.
J. Geol., 92, 223-234. [24] Reimold W. U. and Koeberl C.
(1991) Terra abstracts, 3, 386. [25] Reimold W. U. et al.
(1993) The VCR Revisited, GSSA, Western Transvaal Br., 55-
58. [26] Boer R. H. et al. (1993) The VCR Revisited, GSSA,
Western Transvaal Br., 92-94. [27] Trieloff M. et al. (1992)
Sudbury, LPI, 790, 75-77. [28] Duane M. J. and Reimold W. U.
(1989) LPS, XX, 252-253. [29] Duane M. J. and Reimold W. U.
(1990) LPS XXI, 301-302.


Warren P. H.*   Claeys P.   Cedillo E.
   Where Are the Chicxulub Coarse-grained, Igneously Layered Impact Melt Rocks
   Analogous to Those at Sudbury?

The Chicxulub basin is an impact structure with circular
gravity anomalies that suggest as many as four rings: the
most prominent one having a diameter (D) of roughly 170 km,
and the biggest (marginally significant) having D ~300 km
(if real, this implies the 170-km ring is close to the rim
of the transient crater) [1]. Hildebrand et al. [2] advocate
a structural D of ~170 km and a transient crater D of ~90
km. Even if the structure's D is "only" 170 km, it must have
had catastrophic effects, and its age of 65.01 +- 0.08 Ma
[3] coincides with the KT boundary.

Impacts this large should produce enormous volumes of impact
melt. Figure 1 shows impact melt volume as a function of
transient crater size, estimated by two methods: (1) a
power-law relationship [4] and (2) application of equation
7.10.2 of Melosh [5] along with an unnumbered equation (for
estimating volume displaced as a function of transient
crater D) on his p. 119. Method (2) consistently gives
roughly 40% higher melt volumes than method (1), and the
absolute value of any such estimation is probably uncertain
by at least a factor of 2. However, comparison with observed
craters [4] confirms the slope of the (melt volume)/D
relationship. An additional area of uncertainty is the
impact velocity; a range of 12-70 km/s (encompassing nearly
all conceivable major impacts) would shift the estimated
melt volumes by a factor of 0.9-1.5. The velocity used in
Fig. 1 is slightly greater than the estimated average
terrestrial impact velocity of 17 km/s [6]. The nominal
"crater diameter" implied by a given transient crater D
[6,7] is also shown, albeit the "crater diameter" concept is
hardly applicable to a multiring basin. Fig. 1 also shows
curves for melt volume translated into depth of deepest
melting, by modeling the initial melt distribution as
confined to volumes of either spherical or hemispherical
shape; and for idealized thickness of an impact melt
"sheet," modeled as a total aggregation of the melt into a
volume of shallow-cylinder shape, with the cylinder's
diameter equal to the transient crater D.

Of course, impact melt will hardly aggregate into a near-
surface sheet with 100% efficiency, but a melt
compositionally similar to average Chicxulub melt [8] will
have a much lower density than any plausible country rock,
and thus should tend to efficiently rise to near the
surface. The only comparable Earth structure, in terms of
size and melt composition, is Sudbury. The Sudbury Igneous
Complex was once widely assumed to represent mainly
endogenous melt, formed only indirectly in response to the
impact. Many recent studies [e.g., 9] suggest that the
Sudbury structure is larger than previously supposed (its
transient crater D is now estimated at roughly 100 km), and
also that virtually the entire igneous complex represents
impact melt. By this view, the original thickness of the
aggregated Sudbury melt sheet was roughly 2.5 to 4 km [10];
i.e., lower, but only by a factor of 2-3, compared to the
idealized melt sheet thickness (Fig. 1).

The Sudbury Igneous Complex [10] is a mass of medium-grained
rocks with strong vertical compositional layering. Thus, it
is often likened to layered igneous intrusions of the
Skaergaard-Stillwater variety, even though fine-scale
igneous layering is absent at Sudbury. The Sudbury rocks
range from a mafic norite with pyroxenes that have Mg/(Mg +
Fe) as high as 78 mol%, to a thick capping granophyre
(roughly half of the total complex), consisting of ~90%
quartz + feldspar with only 10% mafic minerals, including
pyroxenes with Mg/(Mg + Fe) as low as 25 mol%.

All of the Chicxulub impact melt rocks described thus far
appear more fine-grained than any significant portion of the
Sudbury Igneous Complex. From the C1 core, near the center
of the structure, the only impact melt-like rocks are
described as "glass" or "microcrystalline" [11], or else
"fine- to medium-grained coherent crystalline" [8]. Core S1,
roughly 26 km from center of structure, yielded "thick units
of andesitic glass" [Hildebrand et al., 1991]. From core Y6,
roughly 50 km from center of structure, impact melt-like
rocks have been described as either microcrystalline (N14,
N17), or (N19) fine-grained to glassy melt rock clasts set
in a "medium to coarse-grained" melt rock matrix [11,12].

We have studied three Chicxulub thin sections: one from
C1/N9, immediately (<=4 m) above the N10 sample of Sharpton
et al. [8]; and two from Y6/N19, the same position (to
within 2.5 m) as a sample described as medium-coarse-grained
by Sharpton et al. [8]. The impact melt portions of all our
samples are fine-grained. Grains in C1/N9 are mainly ~0.3 mm
in max. dimension (the largest, possibly a relict from a
target lithology, is 1.6 mm), and the section includes at
least 6 clasts of very fine-grained (~20 mm) material (Fig.
2a). The melt-rock portions of the two Y6/N19 sections are
dominated (80 vol%) by grains <10 mm across (Fig. 2b). The
extraordinarily wide range of feldspar compositions (Fig. 3)
suggests a clast-rich impact melt origin [13]. Even if
elsewhere in Y6/N19 the texture is medium-coarse-grained
[8], this can hardly represent an igneous cumulate rock, a
la customary interpretations of the Sudbury Complex. For
comparison, Fig. 2c shows the texture of a Sudbury rock only
150 m from the edge of the complex, in which grain size
(max. dimension) averages ~0.75 mm (10]). Interior Sudbury
rocks are twice this coarse, and uniformly lack perceptible
clasts. Moreover, the uppermost Sudbury-IC rocks are far
more granitic than any Chicxulub melt rock yet described.

Comprising ~half of thin section Y6/N19-11 is a large
fragment of a granophyric or myrmekitic rock of
extraordinary mineralogy: the only major minerals are quartz
+ feldspar (Fig. 3), and ~8% anhedral, locally vermicular,
anhydrite. Origin of the anhydrite by a late-stage reaction
process appears unlikely, on grounds of texture and mass
balance. Apart from the predominance of anhydrite in place
of more "normal" minor phases, this fragment is similar to
the granophyre layer than comprises the upper half of the
Sudbury Complex. The fragment is definitely not analogous,
however, because it contains quartz with multiple sets of
planar elements indicative of shock metamorphism. Thus, this
material was part of the basement at the time of the impact.

Atop the igneous complex at Sudbury is a thick (up to 1.6
km) breccia layer, the Onaping Formation, interpreted [14,9]
as a welded mass of back-fallen ejecta mixed with
subordinate, heterogeneously distributed impact melt.
Microbreccias with textures roughly similar to the Chicxulub
impact melts are fairly common in the Onaping formation, but
only as thin "veinlets" or subrounded fragments no more than
"a few m" across [14]. The Onaping also contains many fine-
grained, often fluidal textured "melt bodies" with typical
dimensions of the order 100 m. Possibly the available
samples of Chicxulub impact melt are all from an Onaping
analog. If so, deeper drilling may eventually reveal a much
coarser and more differentiated igneous complex. The great
heterogeneity in siderophile (Ir) concentrations found by
Sharpton et al. [8] would be less surprising for a suite of
Onaping-like samples than for samples of the main mass of
impact melt. Weighing against this Onaping analogy, however,
are the large vertical dimensions of apparently uniformly
fine-grained, little-differentiated impact melt rocks in
cores C1 and especially Y6 (where the "microcrystalline
andesite" is ~380 m thick) [11].

If indeed the Chicxulub impact melt is nowhere as coarse-
grained and compositionally layered as the Sudbury Complex,
the simplest explanation would be that the total yield of
impact melt was much greater at Sudbury. Yet the Sudbury
structure appears, based on evidence that is in many
respects more direct, to be similar in size, if not smaller,
than Chicxulub. Perhaps the Chicxulub structure is actually
smaller than Sudbury. Or perhaps recent interpretations of
the Sudbury Complex as nearly pure impact melt have gone too
far--a complicating factor is that the Sudbury impact hit a
region already warmed by the Penokean Orogeny [14].

Future deep drilling near the center of the Chicxulub
structure can be expected to clearly indicate which of these
two models accurately describes the distribution and end
state of the Chicxulub impact melt, thus helping to clarify
the role of impact melting in early planetary
differentiation [9], as well as the key question of the
magnitude of the Chicxulub impact.

Acknowledgment: We thank John Christie for petrographic
advice.

References: [1] Sharpton V. L. et al. (1993) Science, 261,
1564-1567. [2] Hildebrand A. R. et al. (1993) SOMEXPAL Conf.
(Abstr.) Mexico City. [3] Swisher C. C. et al. (1993)
Science, 257, 954-958. [4] Grieve R. A. F. and Cintala M. J.
(1992) Meteoritics, 27, 526-538. [5] Melosh H. J. (1989)
Impact Cratering. [6] Chyba C. F. (1991) Icarus, 92, 217-
233. [7] Croft S. K. (1985) LPSC, 15, C828-C842. [8]
Sharpton V. L. et al. (1992) Nature, 359, 819-821. [9]
Grieve R. A. F. et al. (1991) JGR, 96, 22753-22764. [10]
Naldrett A. J. and Hewins R. H. (1984) in The Geology and
Ore Deposits of the Sudbury Structure, 235-251. [11]
Hildebrand A. R. et al. (1991) Geology, 19, 867-871. [12]
Kring D. A. and Boynton W. V. (1992) Nature, 358, 141-144.
[13] Cedillo-P. E. (1992) GSA Abstr., A333. [14] Peredery W.
V. and Morrison G. G. (1984) in The Geology and Ore Deposits
of the Sudbury Structure, 491-511.

Fig. 1, which appears here in the hard copy, shows model
predictions for scale of impact melting.
Fig. 2, which appears here in the hard copy, shows thin-
section views of Chicxulub impact melts and a peripheral
Sudbury-IC rock [after 10], all at similar scale. Left third
of (a) shows part of the anhydrite-rich granophyre.
Fig. 3, which appears here in the hard copy, shows An-Ab-Or
proportions in feldspars and feldspathic glasses.


Anderson R. R.*   Witzke B. J.   Hartung J. B.
   Impact Materials Recovered by Research Core Drilling in the Manson Impact
   Structure, Iowa

In 1992 the Iowa Department of Natural Resources Geological
Survey Bureau and US Geological Survey Branch of
Astrogeology completed the last of 12 research cores drilled
in the Late Cretaceous Manson Impact Structure. In excess of
1200 m of drill core was recovered from the three major
crater terranes, the terrace terrane, crater moat, and
central peak. Almost all of the cored rocks were impact
materials (materials emplaced as a direct result of the
impact). The six primary impact materials encountered
include

(1) Crystalline Basement Blocks: Large blocks of crystalline
basement materials cut by pseudotachylite veins; (2)
Crystalline Clast Breccia with Sandy Matrix: Clasts of
brecciated crystalline rocks in a matrix of disaggregated
crystalline rock and mineral and rare melt-rock fragments;
(3) Crystalline Clast Breccia with Melt-Rock Matrix: Clasts
of brecciated crystalline rocks, melt-rock clasts, and
mineral grains (dominantly quartz) in a melt-rock matrix;
(4) Keweenawan Clast Breccia: Clasts of brecciated dark gray
to black Keweenawan (Middle Proterozoic) shales, siltstones,
and sandstones mixed with light gray-green, devitrified
melt-rocks in a dark gray argillaceous matrix; (5)
Phanerozoic Clast Breccia: A matrix-dominated breccia
composed primarily of clasts of Cretaceous marine shale with
lesser numbers of Paleozoic carbonate clasts, rare clasts of
Proterozoic Keweenawan clastic rocks, and very rare clasts
of crystalline rock and melt-rock in a medium gray
calcareous sandy shale matrix; and (6) Impact Ejecta: An
overturned flap of Proterozoic Keweenawan Red Clastics and
Paleozoic strata.

The Crystalline Basement Blocks (CBB) that form the core of
the central peak and were encountered in four cores. They
are dominated by Proterozoic gneisses and granites but also
include minor occurrences of other igneous and metamorphic
lithologies. All blocks display abundant parallel
deformation features (pdf's) and other impact deformation
features. Pseudotachylite dikes of melt-rock and crystalline
rock breccia intrude these blocks. The Crystalline Clast
Breccia with Sandy Matrix (SM) lies above the CBB on the
central peak and was encountered in five cores. Although
this unit is dominated by crystalline rock clasts, rare
clasts of melt-rock and siltstone reflect minor mixing with
overlying units. Pdf's and other impact deformation features
are pervasive. The SM is interpreted as impact-brecciated
rocks from below the transient crater floor. The SM grades
upward into the Crystalline Clast Breccia with Melt Rock
Matrix (MRM), recovered in five cores. Melt-rock clasts and
regions of apparent melt flow banding are common. The
abundant matrix in this unit is mostly isotropic, suggestive
of melt material; however, apparent zones of solid-state
diffusion [1] suggest that very fine-grained unmelted
material may also be present. The central portion of the MRM
was apparently the hottest, displaying the maximum
melt/clast ratio, sanidine and recrystallized quartz, and
zones of solid-state diffusion. The MRM also displays rare
siltstone clasts and crystalline rock clasts with melt-rock
mantles (probable airborne accretion) suggesting minor
mixing of overlying materials. All nonrecrystallized
material display pdf's and other impact deformation
features. The MRM is interpreted as having originated in a
hotter area neared the transient floor than the SM; however,
a fall-back origin for this unit has also been suggested
[2]. A mixture of SM and MRM was encountered in two cores,
possibly by slumping or sliding during formation of the
central peak. The uppermost of the central peak impact units
is the Keweenawan Clast Breccia (KCB), encountered in four
cores. The KCB is a mixture of unmelted and partially melted
dark gray to black shale-dominated Proterozoic clastic rocks
mixed with melt-rock. Only very rare grains with pdf's have
been observed in Keweenawan sandstone clasts. This
relatively thin unit is interpreted as having been at the
crater floor when the transient crater growth stopped. The
Keweenawan Clastic rocks were apparently displaced downward
during transient crater formation, then lifted up on the
crest of the central peak.

On the terrace terrane an overturned flap of late-stage
Crater Ejecta (CE) was encountered in one core. This unit
included an inverted sequence of Keweenawan Red Clastics
overlying Cambrian, Ordovician, and Devonian marine
carbonate and clastic rocks. While all stratigraphic units
in this interval appear to be present, they are represented
by only about 20% of their normal thicknesses. This core
reached the maximum depth attainable by the drill rig that
was used. The possibility of encountering the structurally
preserved Upper Cretaceous ground surface only about 100 m
lower (~500 m below land surface) presents a tempting target
for future drilling.

Among the most interesting materials encountered in the
Manson core drilling was the Phanerozoic Clast Breccia
(PCB). The dominance of rock clasts from the shallowest pre-
impact stratigraphic units (clast abundance is inversely
proportional to pre-impact depth) and the scarcity of shock-
melted or -deformed minerals is similar to Ries Crater Bunte
breccia [3,4]. Bunte breccia is, by definition, primary
crater ejecta formed by the combined effects of ground surge
and secondary cratering, and as such is restricted to
regions outside of the transient crater. However, PCB in the
Manson Crater was encountered in eight cores, within all
regions of the crater (Fig. 1), implying that if this
material had a Bunte breccialike origin, it must have been
transported back into the crater by some unknown dynamic
process. One possible medium for such transport is water,
displaced by the bolide impacting into a shallow marine
environment, rushing back into the evacuated crater region.
The presence of marine conditions in the Manson area does
not preclude a latest Cretaceous impact, since erosion has
left an equivocal record of conditions at that time.
Recovery of the structurally preserved impact surface by
core drilling in the crater terrace would resolve the
questions of impact environment and age.

References: [1] Reagan M. K. et al. (1993) EOS, 74. [2] Bell
M. S. et al. (1993) GSA Abst., 25. [3] Horz F. (1981) GSA
Spec. Paper, 190, 39-56. [4] Newsom H. E. et al. (1990) GSA
Spec. Paper, 247, 195-206.


Hsu K. J.*   Shields G.   Hollander D.
   Gaia, Cambrian Explosion, and KT Catastrophe

In a paper on Is Gaia Endothermic?, Hsu suggested that the
Earth's climate cooled down considerablv during the
Proterozoic, when atmospheric CO2 was increasingly tied up
in stromatolitic carbonates. The consequence was the late
Proterozoic glaciation. Two bigs steps forward in the
biologic evolution took place at that time. First the
metazoans appeared and flourished. The Ediacaran faunas had
no hard parts, and decrease the efficiency of biological
communities in storing C as inorganic C in sediments. Then
the first skeletal organisms appeared, and their skeletons
were mainly phosphatic. Both of those seemed to have
successfully competed against cyanobacteria for living
space. A prediction was made that a reduction in the biomass
of the cyanobacteria biocoenoses subsequent to the evolution
of the Ediacaran and small shelly faunas should have
increased the atmospheric partial pressure of CO2. At the
same time, more terrestrial Ca should have been released
from the tidal flat environment to the open ocean. The
termination of the Precambrian glaciation and the Cambrian
explosion were the consequences of this evolutionary change
of life.

Recent data by de Marais and other showed a reduction of the
carbonate-C/orgnaic C reduction by a factor of more than
half from 2.0 to 0.5 Ga before present, reaching a minimum
at about 0.8 Ga (Nature, March, 1993). According to the
relation obtained by Hollander and McKenzie, this
corresponds to a reduction of CO2 partial pressure. This
trend supports the postulate that the onset and retreat of
Precambrian glaciation and deglaciation were related to the
atmospheric greenhouse. Studies of Sr isotope values have
also indicated that the Sr-76/Sr-86 ratio reaches a maximum
at the beginning of the Cambrian. Terrestrial Sr should have
been preferentially stored with Ca in stromatolites. With
the destruction of the cyanobacteria biocoenosis the
terrestrial Ca Sr should have preferentially been release.
The increase flux could support the postulate that the
Precambrian seawater which was undersaturated with respect
to calcite became suptersaturated at the beginning of the
Cambrian. The increase flux of Ca and Sr to open ocean is
indicated by the maximum value of Sr-87/Sr-86 ration at the
time of Precambrian/Cambrian transition.

The hypothesis that the interaction of geosphere and
biosphere has influenced terrestrial climate postulates that
the early Tertiary warming was a long-term consequence of
the KT boundary event. The living biomass of marine plankton
is a sink of atmospheric CO2 and the Quaternary record
indicates reduced CO2 pressure and warmer climate during
interglacial epochs of reduced plankton production. Using
the same argument, we conclude that the early Tertiary
warming was related to a reduction of plankton biomass after
the KT boundary event. The cooling trend which started with
the evolution of marine planktons was interrupted at the KT
boundary and reversed during Paleocene and Eocene. Global
cooling resumed in Late Eocene when plankton-fertility, as
indicated by C-isotope data, had recovered sufficiently to
reduce atmospheric CO2 pressure, leading ultimately to
Quaternary glaciations.


Shoemaker E. M.*   Shoemaker C. S.
   The Crash of P/Shoemaker-Levy 9 Into Jupiter and its Implications for Comet
   Bombardment on Earth

Periodic Comet Shoemaker/Levy 9 will impact Jupiter in late
July 1994 [1,2]. The comet, which broke into more than 20
telescopically detectable fragments [3] when it passed
within the Roche lobe of Jupiter on July 8, 1992, is
captured in a highly eccentric orbit about Jupiter. The 21
recognized nuclei will be spread out in a train of the order
7 million km long at the time of impact, and the impacts
will be spread in time over about 5 1/2 days centered on
about July 21.2 UT [4]. In addition to the train of
recognized bright nuclei, the comet consists of "wings" of
unresolved bodies that are the source of a very broad
composite dust tail. The linear extent of the wings is about
an order of magnitude greater than that of the train of
recognized discrete nuclei. Collision of the wings will be
spread in time over several months. Thus the impact of P/S-L
9 with Jupiter will be an event of appreciable duration.

Sizes of the recognized nuclear fragments of P/S-L 9 are not
yet firmly established. Photometry from high-resolution
images acquired by the Hubble Space Telescope suggests that
the 11 largest nuclear fragments range from 2.5 to about 4.3
km in diameter, and that the precursor body, before breakup,
was about 8 km in diameter or larger [5]. A preliminary
dynamical solution for the development of the observed train
of nuclei by Scotti and Melosh [6] suggests that the
precursor body was only about 2 km in diameter. However, a
later solution by Chodas and Yeomans [4], based on orbits of
the individual nuclei, indicates that the precursor body was
about 9 km in diameter. We conclude that the precursor of
P/S-L 9 was of the order 9 km diameter and that the total
impact energy will be of the order 10^8 megatons TNT.

Observations of comets discovered shortly after escape from
Jovicentric orbit, plus the discovery of P/S-L 9, indicate
that the frequency of collision of objects orbiting Jupiter
with the estimated impact energy of P/S-L 9 is of the order
once per millenium [7]. Collisions with Jupiter of comets on
free heliocentric orbits are several times more frequent.
These collision rates are an order of magnitude higher than
those predicted with the use of Opik's equations, which fail
for very low velocity encounters with Jupiter.

The development of a train of cometary debris by tidal
breakup leading to multiple impacts on a planet has direct
relevance to the impact history of Earth. Both periodic and
long-period Sun-grazing comets are subject to tidal
disruption. A catastrophically disrupted periodic comet on a
free heliocentric orbit can be expected to form a compact
debris stream somewhat similar to but much narrower than an
ordinary meteoroid stream. If such a stream intersects
Earth's orbit at one of the nodes, Earth will be subject to
repeated impacts as a result of its annual passage through
the stream [8-10]. High-inclination periodic comets tend to
be driven to Sun-grazing orbits [11], and the formation of
compact debris streams should occur fairly frequently.
Breakup of very large (several hundred kilometers in
diameter) periodic comet nuclei should have occurred
occasionally during the last 0.5 Ga, leading to comet
showers with durations of the order of 10^5 yrs. If one such
breakup occurred when the distance to a node was near 1 AU,
an intense pulse of bombardment lasting a few decades may
have occurred. We postulate that just such an event happened
at K/T boundary time.

References: [1] Carusi A. et al. (1993) Paper presented at
Asteroids, Comets, Meteors 1993, Belgirate, Italy, IAU
Symposium 160, 9 pp. [2] Chodas P. W. and Yeomans D. K.
(1993) Bull. A.A.S., 25, 1042. [3] Jewitt D. et al. (1993)
Bull. A.A.S., 25, 1042. [4] Chodas P. W. and Yeomans D. K.
(1993) Paper presented at 25th Annual DPS Meeting, Boulder,
Colorado. [5] Weaver H. A. et al. (1993) Science, in press.
[6] Scotti J. V. and Melosh H. J. (1993) Nature, in press.
[7] Shoemaker E. M. et al. (1993) Bull. A.A.S., 25, 1042.
[8] Shoemaker E.M. (1991) Abstracts for the Internatational
Conference on Asteroids, Comets, Meteors 1991, 194, LPI,
Houston. [9] Shoemaker E. M. and Izett G. A. (1992) LPSC
XXIII, 1293-1294. [10] Shoemaker E. M. and Izett G. A.
(1992) International Conference on Large Meteorite Impacts
and Planetary Evolution, 66-68, LPI, Houston. [11] Bailey M.
E. et al. (1992) Astron. Astrophys., 257, 315-322.



                              POSTER PRESENTATIONS

Izett G. A.   Masaitis V. L.   Shoemaker E. M.   Dalrymple G. B.   Steiner M. B.
   Eocene Age of the Kamensk Buried Crater of Russia

The 25-km-diameter Kamensk buried impact crater of southern
Russia (48 degrees 20' N, 40 degrees 15' E) was long
considered to have an age close to that of the Cretaceous-
Tertiary (KT) boundary [1,2]. This age assignment seemingly
rested on secure biostratigraphic evidence. The uppermost
precrater rocks on the crater rim were reported to be Upper
Cretaceous marine marl, sandstone, claystone, and chalk
ranging in age from Cenomanian to Maastrichtian [l]. These
rocks also occur as clasts in the allogenic breccia
underlying the crater floor. The crater is filled with marl
with small reworked fragments of Upper Cretaceous and
Carboniferous rocks. The crater-filling marl was assigned to
the lower Paleocene on the basis of foraminfera considered
to be of early Paleocene age [1]. With this biostratigraphic
evidence in hand, it seemed possible that the Kamensk impact
might be related to the great Chicxulub impact event in
Mexico and possibly other impacts at KT boundary time.
Microstratigraphic evidence at the KT boundary in western
North America suggests that multiple impacts took place [3-
5]. If multiple large impacts did indeed occur at or near
the time of the KT boundary, the impacting bodies most
likely would have been derived either from the breakup of a
very large periodic comet or from a comet shower produced by
perturbation of the Oort cloud [4-9].

Impact glass recovered from drill core in the Kamensk impact
structure provided the opportunity to refine its age and to
determine if it could be of KT boundary age. A centimeter-
sized specimen of the Kamensk core was first demagnetized to
test whether the orientation could be recovered from viscous
remanent magnetization and whether the polarity of a
characteristic remanent magnetization could be determined.
The sample was then crushed, and shards of clear, yellowish-
brown glass that looked promising for ^40Ar-^39Ar dating
were hand picked.

To isotopically date the glass, we used a state-of-the-art
^40Ar-^39Ar laser-fusion analytical system in the USGS
laboratories in Menlo Park, California. Although this
technique has important advantages relative to the
conventional K-Ar method, ages of unknown samples are
relative to ages of neutron fluence-monitor minerals. We
calculated the age of the Kamensk glass using the age of an
intralaboratory neutron fluence-monitor mineral [sanidine
from the Taylor Creek Rhyolite (Oligocene) of New Mexico].
This sanidine has a ^40Ar-^39Ar age of 27.92 Ma relative to
a K-Ar age of 162.9 +- 0.8 Ma for a primary fluence-monitor
standard SB-3 biotite or of 513.9 Ma for an international
hornblende standard MMhb-1.

The ^40Ar-^39Ar analytical data for the Kamnesk glass shards
are shown in Table 1, and the weighted-mean total-fusion
^40Ar-^39Ar age of six glass shards is 49.15 +- 0.18 Ma. We
believe this age accurately reflects the time of origin of
the Kamensk structure, but final confirmation awaits ^40Ar-
^39Ar incremental-heating ages for the glass now in
progress. In view of the reported early Paleocene age of
foraminifera from postcrater fill marl, the ^40Ar-^39Ar
apparent age of the glass was a surprise. The tight
clustering of ages, however, gives us confidence in the
isotopic ages and indicates either that the reported age of
Paleocene foraminifera from the crater fill should be
reevaluated or that these forams have been reworked from
Paleocene beds on the crater rim that were previously
thought to postdate the crater.

Our ^40Ar-^39Ar age of 49.15 +- 0.18 Ma implies that the
Kamensk structure is early Eocene, not Paleocene or KT
boundary age. This isotopic age recalculated using a
reference age of 520.4 Ma for MMhb-1 is 49.84 +- 0.18 Ma,
which is identical (within experimental error) to the 50.5
+- 0.8 Ma age reported by Bottomley and York [10] for the
50-km-diameter Montagnais impact structure on the
continental shelf off Nova Scotia. This close match in age
for the two impact structures may only be fortuitous, but
alternatively, it might indicate that both impact structures
formed during nearly simultaneous impacts of large fragments
of a single disrupted comet.

It is also of interest to try to place Kamensk on the
standard paleontologic and magnetostratigraphic timescales.
If one uses the Harland et al. [11] or DNAG timescales, an
age of 49.84 Ma would fall in chron 21N, and fossil plankton
zones P10, CP12, and NP14. However, recent recalibration of
the Eocene time scale by Montanari et al. [12] suggests that
the standard timescale is off by several million years
through most of the Eocene. On the Montanari et al.
timescale, the age of the Kamensk structure falls in chron
22R and zones P9, CPll, and NP13. From stepwise alternating
field demagnetization measurements of the single specimen of
core, we believe that we have recovered the orientation of
the specimen and tentatively conclude that the
characteristic magnetization is reversed and consistent with
an age assignment to chron 22R.

References: [1] Masaitis V. L. et al. (1980) The Geology of
Astroblemes, Nedra, Leningrad, 231. [2] Grieve R. A. F.
(1982) GSA Spec. Paper, 190, 25-37. [3] Izett G. A. (1991)
Eos, 72, supplement, 278. [4] Shoemaker E. M. and Izett G.
A. (1992) LPSC XXIII, 1293-1294. [5] Shoemaker E. M. and
Izett G. A. (1992) International Conference on Large
Meteorite Impacts and Planetary Evolution, 66-68, LPI,
Houston. [6] Hut P.K. et al. (1987) Nature, 329, 118-126.
[7] Hut P. et al. (1991) LPSC XXII, 603-604. [8] Hut P. et
al. (1991) International Conference on Near-Earth Asteroids,
16, San Juan Capistrano Research Inst. [9] Shoemaker E. M.
(1991) Abstracts for the International Conference on
Asteroids, Comets, Meteors 1991, 199, LPI, Houston. [10]
Bottomley R. and York D. (1988) GRL, 15, 1409-1412. [11]
Harland W. B. et al. (1982) A Geologic Time Scale, Cambridge
Univ., 131. [12] Montanari A. et al., unpublished.


Koeberl C.   Shirey S. B.   Reimold W. U.
   Re-Os Isotope Systematics as a Diagnostic Tool for the Study of Impact
   Craters

1. Basics of Re-Os Isotopes for Impact Studies: The Re-Os
isotopic system is based on the Beta-decay of ^187Re to
^187Os (half-life = 42.3 +- 1.3 Ga). Re and Os behave
differently during partial melting of mantle rocks. Os is
highly compatible and remains in the residue but Re is
moderately incompatible and is therefore enriched in the
melt. Crustal rocks have therefore high Re, but low Os
concentrations. As a result of the high Re concentrations,
the abundance of ^187Os in crustal rocks increases
significantly with time. The growth of ^187Os from the decay
of ^187Re can be described by normalizing to an non-
radiogenic Os isotope:

^187Os/^188Os = (^187Os/^188Os)(sub)i +
^187Re/^188Os(e^lambda-t-1)

Present day mantle has a low ^187Os/^188Os ratio of about
0.13, and meteorites also have low ^187Os/^188Os ratios of
about 0.11 to 0.18 (^187Os/^186Os = 0.95-1.5) [1-3]. Because
in meteorites (and in the mantle) Os is much more abundant
than Re, the ^187Os/^188Os ratio changes only very slowly
with time. However, the crustal ^187Os/^188Os ratio
increases rapidly with time because Re abundances are
several orders of magnitude higher than Os abundances.
Isotopic ratios of crustal rocks depend on age and elemental
abundances. ^187Os/^188Os ratios of about 0.67 to 1.61 (with
an average of about 1.2) are thought to be representative of
currently eroding upper continental crust [4,5].

The fact that the absolute abundances of Os as well as the
^187Re/^188Os and ^187Os/^188Os ratios in meteorites are
distinctly different from those in old crustal target rocks,
is of potential use for the study of impact craters. Impact
melts, glasses, or breccias consist usually of predominantly
terrestrial target rocks, mixed with usually a very small
(<1%) meteoritic component. However, the difference in
absolute Os abundances between meteorites and crustal target
rocks favors detection of small amounts of a meteoritic
component. This concept has been proposed by Turekian [6]
for distinguishing between a cosmic and a terrestrial
component in KT boundary clays. Despite the potential
importance of such studies, a surprisingly small number of
studies has followed [7-12], probably in part because of
analytical difficulties. The first application of the Os
isotopic system to impact craters was attempted by [13], who
used AMS and found evidence for a meteoritic component in
melt rocks from the East Clearwater crater. Their method,
however, did not allow the measurement of all relevant Os
isotopes (or Re), required large amounts of sample, and
yielded relatively low precision.

Wider application for impact crater studies was made
possible by the recent development of the negative thermal
ionization technique (e.g.,[14]). This method allows the
determination of abundances and isotopic ratios of Os and Re
at the low abundance levels found in target rocks and in
impact-derived rocks while using relatively small amounts of
material. Limited sample quantity is a factor for many
impact glasses or impact melt rocks. The feasibility of the
method was recently show in studies of material from various
impact craters, as summarized below.

2. Examples from Different Craters Kalkkop Crater, South
Africa: The Kalkkop crater is a recently identified 640-m-
diameter impact crater [15]. A drill-core became available
in late 1992, yielding breccia and target rock samples. We
measured abundances and isotopic compositions of Re and Os
in two target rock samples (sandstone and shale), as well as
in four suevite breccia samples ([22], Table 1). Os
abundances and isotopic ratios in the target rocks are
typical for old continental crust. Two of the suevite
samples have Os contents about 10 times higher than values
obtained for the target rocks. These samples show low
^187Os/^188Os ratios of 0.215 and 0.333, respectively. Such
values are distinctly different from the target rock ratios
and incompatible with old continental crustal values. The
elevated Os content together with the near-meteoritic
^187Os/^188Os ratio demonstrate that the suevite contains
about up to 0.05% of an extraterrestrial component. Figure 1
shows the mixing relationship between target rocks and a
meteoritic component.

Saltpan Crater, South Africa: The Saltpan crater is a
220,000-year old, 1.13 km-diameter impact crater that was
formed in 2.05 Ga old granites from the Bushveld Complex.
The crater was drilled in 1988/89 to a depth of 200 m,
revealing the existence of a suevitic breccia with evidence
for shock metamorphism [16]. The Re-Os isotope
characteristics of basement granites and suevitic breccias
from the Saltpan crater were measured ([17]; Table 1). The
granitic target rocks have Os abundances of about 7.0 ppt
and a high ^187Os/^188Os ratio of 0.713. The breccias have
10x higher Os abundances and lower ^187Os/^188Os ratios of
about 0.205. The composition of the breccias is consistent
with derivation by mixing of a meteoritic component with
basement granites. Correction for indigenous concentrations
yields about 72 ppt Os, corresponding to about 0.015% of a
chondritic component in the bulk breccia.

Bosumtwi Crater, Ghana: This 11-km-diameter, 1.1 Ma old
crater is exposed in about 2 Ga old (Sm-Nd) metasediments,
and is the most likely source crater for the Ivory Coast
tektites, which are of the same age and have appropriate
chemical and isotopic characteristics. The Re-Os isotopic
characteristics of tektites, impact glasses, and target
rocks were studied [18]. The tektites have isotopic
characteristics very similar to meteorites, while the target
rocks show typical crustal values. A meteoritic component of
up to 0.6% was calculated for tektites [18].

Chicxulub Crater, Mexico: This >200-km-diameter multiring
impact structure is the best candidate for the KT boundary
impact crater [19]. Crystalline impact melt rocks and
breccias have been recovered from drill-cores within the
crater. A melt rock contained 25.2 ppb Os, and very low
^187Os/^188Os and ^187Re/^188Os ratios, which are
inconsistent with those of old continental crust [23].
Another melt rock has only 0.056 ppb Os and high ratios,
similar to continental crustal values. The terrestrial
mantle is an unlikely source for abundance and isotopic
signature of Os in the Chicxulub melt rock, which has all
chemical and isotopic characteristics of a continental
crustal rock [23]. An extraterrestrial source is the most
likely explanation for the observed values. We estimate a
maximum of 3% chondritic contribution, which is within the
range of meteoritic components reported for large craters
[20].

We also measured Re-Os isotopic characteristics of Haitian
KT boundary impact glass, which is indistinguishable from
Chicxulub melt rock in Rb-Sr and Sm-Nd isotopic
characteristics [21]. The glass contains 0.095 ppb Os, and
has a ^187Os/^188Os ratio of 0.251, which is lower than the
average crustal ratio and indicates contamination with
meteoritic material (CCC).

Work in Progress: Materials from a number of other
structures are currently being studied. Preliminary results
show the existence of meteoritic components for the Manson,
Gardnos, Aouelloul, and Vredefort structures.

3. Re-Os Isotopes as an Impact-Diagnostic Criterion: The
study of the Re-Os isotope characteristics is of
considerable importance for the investigation of impact
structures. It allows the determination and quantification
of a meteoritic component in impact-derived materials (in
comparison to target rocks) and may help to understand the
mixing between the bolide and target rocks. Furthermore,
such studies allow the confirmation of an impact origin for
doubtful structures. If properly applied, Os isotopes may be
almost as diagnostic a tool for the establishment of an
impact origin as shock metamorphism.

Acknowledgements: Supported by the Austrian FWF, Project
P9026-GEO (to CK) and NSF, Project EAR-9218847 (to SBS).

References: [1] Walker R. J. and Morgan J. W. (1989)
Science, 243, 519-522. [2] Morgan J. W. et al. (1992) EPSL,
108, 191-202. [3] Horan M. F. et al. (1992) Science, 255,
1118-1121. [4] Esser B. K. (1991) Ph.D. thesis, Yale. [5]
Esser B. K. and Turekian K. K. (1993) GCA, 57, 3093-3104.
[6] Turekian K. K. (1982) GSA, SP-190, 243-249. [7] Luck J.
M. and Turekian K. K. (1983) Science, 222, 613-615. [8]
Lichte F. E. et al. (1986) Nature, 322, 816-817. [9]
Krahenbuhl U. et al. (1988) Meteoritics, 23, 282. [10] Esser
B. K. and Turekian K. K. (1989) EOS, 70, 717. [11] Schmitt
R. A. (1990) LPS XXI, 1085-1086. [12] Meisel T. et al.
(1993) Meteoritics, 28, 396-397. [13] Fehn U. et al. (1986)
Nature, 323, 707-710. [14] Creaser R. A. et al. (1991) GCA,
55, 397-401. [15] Reimold W. U. et al. (1993) LPS XXIV,
1197-1198. [16] Reimold W. U. et al. (1992) Geology, 20,
1079-1082. [17] Koeberl C. et al. (1993) GCA, in press. [18]
Koeberl C. and Shirey S. B. (1993) Science, 261, 595-598.
[19] Sharpton V. L. et al. (1993) Science, 261. [20] Palme
H. (1982) GSA, SP-190, 223-233. [21] Blum J. D. et al.
(1993) Nature, 364, 325-327. [22] Koeberl C. et al. (1993)
GCA, in press. [23] Koeberl C. et al. (1993) GCA, in press.

Fig. 1. Re-Os isotope diagram for rocks from the Kalkkop
crater, showing that the breccias plot into a mixing field
defined by meteoritic and target rock compositions (after
[22]).

Table 1, which appears here in the hard copy, shows Re-Os
isotopic data for impact breccias, melts, glasses, and
target rocks.


Gayraud J.   Robin E.   Rocchia R.   Froget L.
   Preliminary Results Regarding the Formation Conditions of Metoric Spinels

Introduction: Meteoric spinels, which previously has been
termed cosmic spinels [1], are magnetic minerals formed when
a piece of extraterrestrial matter is melted and oxidized in
the Earth's atmosphere. Such minerals have no analog in
terrestrial rocks but are systematically found in cosmic
spherules [1,2], in the fusion crusts of meteorites [1,2]
and in various cosmic debris such as meteoroid ablation
droplets [3] and oceanic impact products [4]. We adopt here
the term "meteoric" rather than "cosmic" for two reasons:
(1) to avoid any confusion with spinels originally present
in meteoritic matter and (2) because their formation is
associated with the occurrence of meteors. The importance of
meteoric spinels in the geological record is twofold: they
are specific marker of meteoritic material and they contain
informations regarding the mode of accretion of this
material [1]. In order to interpret these informations it is
essential to understand the formation conditions of these
minerals. In terrestrial magmas, spinel formation is
controlled by the O fugacity, the temperature and the
initial composition of the crystallizing material [5,6]. In
the case of meteoric spinels, the kinetic of the reaction
(duration of the heating and cooling phases) may also play
an important role. The study of the effect of the O fugacity
on the composition of meteoric spinels is of a particular
interest as it provides information about the altitudes of
deceleration and oxidation of cosmic bodies in the
atmosphere and, hence, of their size. Here we report on the
variation with O fugacities of the elemental content and of
the Fe^3+/Fe (sub)total ratio of spinels synthetized from
meteoritic material. Applications of this study to spinels
found in Cretaceous-Tertiary boundary samples are discussed
in a companion abstract in this volume [7].

Experimental Technique: Synthetic spinels were obtained by
melting a few mg of chondritic material in a platinum
crucible enclosed in a quartz cell and placed in the coil of
a high frequency generator. The O pressure was stabilized in
a dynamical way by using a vacuum pump combined with a
leakage controlled system. The total air pressure was
measured with a manometer at high pressures (> 10^-3 atm)
and a Pirani gauge at low pressures (down to 3 x 10^-7 atm).
The O pressure measurements are accurate to +/-10%. The
temperature  was measured with an optical pyrometer
calibrated at the melting points of gold, palladium and
platinum. The absolute temperature measurements are
estimated to be accurate to +/-10 degrees C at temperatures
below 1400 degrees C and +/-20 degrees C at higher
temperatures. The starting materials for the runs were
powders prepared from 50 g of St. Severin LL-type chondrite
which was crushed and well-homogeneized in an agat mortar.
Each experiment was run for 1-2 minutes in order to simulate
a pulse-like heating event except for the lowest O pressure
(3 x 10^-7 atm) which was run for 60 minutes. The peak
temperature reached during the runs was fixed at 1335
degrees C, close to the melting point of chondrite. After
fusion, samples were mounted in epoxy and polished for
detailed scanning electron microscopy and electron
microprobe analysis. Spinels larger than 2 micrometers are
analyzed using an automatic routine. The elemental
contribution from the matrix was corrected from the Ca and
Si contents assuming that these elements are absent from the
spinel.

Results and Discussion: Analytical results for 11 runs are
given in the table and plotted in Figs. 1, 2, and 3 as a
function of O fugacities. As a main result, we observed a
marked change in the number, chemical composition and
Fe^3+/Fe (sub)total ratio of spinels between 2 x 10^-5 -10^-
4 atm. At O fugacities < 2 x 10^-5, few but large reduced
chromites are formed (Cr2O3 ~ 45-60%, Fe^3+/Fe (sub)total ~
20-50%) whereas at O fugacities > 10-4 atm, numerous
oxidized spinels of various morphologies (dendritic,
octahedral, cruciform...) and compositions are observed
(Cr2O3 ~ 3-10%, Fe^3+/Fe (sub)total ~ 70-90%). This change
in the crystallization or stability of spinel at low
pressures may be caused by a reaction between spinel, any
other crystallizing mineral (pyroxene) and liquid. It may
also be due to non-equilibrium conditions at low O
fugacities as the duration of the heating phase is short (1-
2 minutes). The maximum Fe^3+/Fe (sub)total ratio of spinel
at 0.21 atm of O fugacity (corresponding to 1 atm total
pressure) is ~ 90%. At this pressure a maximum number of
spinels is observed with a minimum Cr2O3 content. As the O
fugacity decreases, the number of spinels falls together
with their Fe^3+/Fe (sub)total ratio whereas their Cr2O3,
Al2O3 and TiO2 contents slightly increase. Note that the NiO
content of the spinel decreases with the O fugacity from ~
5% at 0.21 atm to < 1% at O pressure < 2 x 10^-5 atm.
Globally, the Fe^3+/Fe (sub)total and Cr2O3 variations at O
fugacities > 10^-4 atm are consistent with previous results
obtained by Hill and Roeder [6] on spinel crystallizing from
basaltic liquid (see Fig. 3). Unfortunatly, these authors
have no available data at O fugacities ranging from 10^-3 to
10^-6 atm and no direct comparison can be done.

Preliminary conclusions. (1) Formation of meteoric spinels
(Fe^3+/Fe (sub)total > 60%; NiO > 1% see reference [1])
requires O fugacities > 2 x 10^-5 atm. Therefore, meteoric
spinels cannot be formed in terrestrial magmas which evolve
under lower O fugacities (< 10^-6 atm) and are strongly
depleted in Ni. Even for magmas slightly enriched in Ni (~
0.2% in ultramafic rocks), the low O fugacity would prevent
the formation of Ni-rich spinels. (2) Fe3^+/Fe (sub)total
ratio and to a lesser extent Cr2O3, Al2O3 and TiO2 content
of spinels are relatively good indicators of the O fugacity.
It is possible to differentiate meteoric spinels that derive
from micrometeors which are decelerated at 40-80 km
corresponding to O pressures of 2 x 10^-6-10^-4 atm, from
spinels that derive from large meteors which are decelerated
deeper in the atmosphere corresponding to higher O
pressures. (3) Meteoric spinels with Fe^3+/Fe (sub)total >
90% were not reproduced in our experiments. It seems that O
pressures higher than the one existing at sea level (0.21
atm) is required to form spinels with higher Fe^3+/Fe
(sub)total ratio.

References: [1] Robin E. et al. (1992) EPSL, 108, 181-190.
[2] Robin E. et al. (1992) LPS XXIII, 1159-1160. [3] Jihanno
C. et al. (1988) Proc. LPSC 18th, 623-630. [4] Margolis S.
V. et al. (1991) Science, 251, 1594-1597. [5] Haggerty S. E.
(1976) Oxide Minerals, 3, (D. Rumble ed.), Mineral. Soc.
Am., 101-300. [6] Hill R. and Roeder P. (1974) J. Geol., 82,
709-729. [7] Robin E. et al., this volume.


McHone J. F. Jr.   Pitakpaivan K.   Bunopas S.   Angsuwathana P.   Supajanya T.
Wasson J. T.
   A Search for Tektite-Related Impact Structures in NE Thailand; An
   Examination of SPOT Satellite Images

The dip of the magnetic remanence indicates that layered
(Muong-Nong-Type) Australasian tektites formed as puddles of
melt with their layers roughly parallel to the Earth's
surface [1,2]. Wasson [3] concluded that, because silicate
melt can only retain its heat within the incandescent impact
plume, these melts were ejected from nearby small (km-size)
craters. Several deposits of layered Australasian tektites
have been found in the NE Thailand, with the largest in the
NE corner.

We obtained a set of seven high resolution, 1:50000-scale,
SPOT-1 and SPOT-2 satellite images covering parts of the
southeast corner of NE Thailand and examined them for
possible impact structures. Four adjacent images and
corresponding topographic maps provided location and
elevation controls for the most promising region
compromising more than 3000 km^2. Examination of these
yielded several sites that may be craters.

Layered tektites are found over a band 1200 km long
extending from Hainan Island in the NNE to central Cambodia
in the SSW. Wasson [3] noted that it appears to be
impossible for a single large (5-20 km radius) crater to
produce puddles of melt over such a large region and
suggested that a cometary projectile broke up in deep space
with the fragments producing many thousands of small craters
across the region where layered textiles have been found.
The microtektites and the tektites showing atmospheric
ablation features were launched from the largest one or more
craters whose formation deposited enough energy to produce a
plume that penetrated through the atmosphere.

Much of this 1200-km-long field of the layered tektites is
mountainous and has experienced appreciable erosion during
the 770 ka since the Australasian tektites were formed [4].
Other regions, particularly in the valley of the Mekong and
some other major rivers, have been covered by recent
sediments. Particularly promising for crater preservation is
the flat-lying Khorat Plateau of Northeast Thailand. The
southeast corner of Northeast Thailand is within the
layered-tektite band; in fact, large deposits of layered
tektites have recently been discovered near the county-seat
town of Buntharik. The existence of these tektites
demonstrates that erosion in the region has been minimal
during the past 770 ka. The bedrock is mainly sandstone;
there seems to be little salt or limestone in the
stratigraphic column, and thus there are no solution
features that would mimic small craters.

The locations of the most promising sites are summarized in
Table 1. All sites are on the four quadrangles 60381-60384
covering the region 105.0-105.5 degrees E and 14.5-15.0
degrees N. The Thai coordinates listed in Table 1 are km E
of Bangkok and N of the equator. The prime search site is
the first, which shows up prominently on aerial photos, the
SPOT image, and the 1:50000 topographic sheet. All bear
investigating; one, site 5, is only a few km from a region
where large layered tektites are being found today.

Site 1. This site was previously visited briefly in
reconnaisance by several of us during Feb 1989. Centered
about 5 km SSE of Amphoe Buntharik. The site is a
topographic depression about 0.7-km in radius surrounded by
a discontinuous, slightly elevated rim to the east. An
isolated hill in the west may represent a large ejecta
block. A N-S road from Buntharik to Kaeng Sawang traverses
the slightly elevated western margin of the depression.
Field work in 1989 showed sandstone outcropping on this
western margin. Unconsolidated sediments in the center of
the structure seem to be lake deposits.

Site 2. We list this site immediately after site 1 because
it is geographically near, but the available evidence is
ambiguous. Its center is only four kilometers east of site
1; a small airport, Sua Phra Camp, and the villages of Ban
Nan Noi and Ban Nong Saena occupy elevated ground
surrounding 3 sides of 0.8 radius, amphitheater-like
depression which opens westward into a small reservoir. The
satellite image of the area is dominated by bleached ground
indicating bare soil due possibly to grading or agriculture.

Site 3. About 17 km SE of Amphoe Det Udom, the village of
Ban Lup Lao is centered within a 3-km radius partial
bullseye pattern prominent by both the SPOT image and the
topographic map. Outcrops along the annular streams would be
suitable initial targets for investigation.

Site 4. This site is geographically near site 3 and also
seems to be a multiring structure. About 9 km ENE of Ban Lup
Lao, the village of Ban Bua Ngain is situated on a small
area of higher ground which may represent the center of a
bullseye feature nearly 3-km in radius. Trails radiate from
the village several kilometers in all directions indicating
generally gentle relief but the drainage pattern weakly
suggests possible concentric ring troughs at radial
distances of 0.8 and 3 km.

Site 5. A 1-km radius circular feature is centered some 3 km
North of Ban Pa Tia village and 2 km SE of Ban Nonk Kop.
This area is at the southeast end of the big Lam Dom Noi
Reservoir in a region of intense road building E and NE of
Buntharik. The structure is within several km of tektite
recovery sites near the village of Huai Sai.

Sites 6a-6e. Scattered in low lying regions throughout the
area are numerous elliptical or subcircular features having
radii smaller than about 0.3 km. Most appear to be basins
with subdued rims, especially along southern margins. Many
are elongated with longer axes running NNW to SSE. Several
are located along a 3 km stretch north of the road between
Wat Ban Mak Mai and Highway 2182. This is the area between
the Ban Lup Lao bullseye structure and Amphoe Det Udom.
Because these structures are quite small, it would be
remarkable if they are craters that have been preserved for
770 ka.

A field trip to NE Thailand by North American and Thai
scientists is planned for January 1994. The prime targets
for investigation are the first 5 listed in Table 1, which
appears in the hard copy. In addition we will survey the
occurrence of layered tektites in the region. A preliminary
survey in 1989 indicated that all fragments in the tektite-
bearing-laterite marker horizon in this region are layered.

If time permits, we will also survey some circular sites
about 200 km to the north where layered tektites have been
found NW of the city of Mukdahan. SPOT images of this region
have been requested.

We expect to receive SIR-C shuttle radar images of these
regions in mid-1994; these will allow additional sites to be
defined for investigation in subsequent field work.

References: [1] De Gasparis A. A. et al. (1975) Geology, 3,
605-607. [2] Barnes V. E. and Pitakpaivan K. (1962) Proc.
Nat. Acad. Sci., 48, 947-955. [3] Wasson J. T. (1991) EPSL,
102, 95-109. [4] Izett G. A. and Obradovich J. D. (1992) LPS
XXIII, 593-594.


Peng H.
   An Extraterrestrial Event at the Tertiary-Quaternary Boundary

During the North Pacific Manganese Noduls Investigation in
1983, we collected a large-diameter Core M14 (8 00.15N, 176
10.65W; 3991 m depth of water). This gravity core is 420 cm
long in total. It is composed of calcureous ooze at 0-320 cm
and siliceous ooze at 320-420 cm of depth. A great number of
microtektites were found from 270 cm to 300 cm (average
content is 15 microtektites per 100 gram sediment). Because
abundant microtektites are restricted to a 30-cm-thick zone
of deep-sea core, we called this zone the microtektite
layer. The age of sedimetary stratum containing
microtektites is 2.14-2.30 million years, corresponding from
the pliocene to pleistocene by the paleomagnetic and the
paleonotologic analysis[1].

Most microtektites appeared as light yellowish-green, small,
glassy spherules and their average size is 137 micrometers
in diameter. The individual grain is dumbbell shaped and its
size is 652 x 223 x 151 micrometers (Fig.1). The average
refractive index of microtektites determined by the oil
immersion method is 1.52.

The SiO2 contents of five microtektites are 69.13-70.54%,
Al2O3 is 15.89-18.95%, FeO is 3.45-4.96%, MgO is 2.33-3.62%,
CaO is 1.93-2.60%, and K2O is 1.24-2.78%. These oxide
contents are similar to the North American Microtektites.
But Core M14 microtektites have unique properties, i.e.,
their Na2O contents are only 0.11-0.18%, which is lower than
North American Microtektites and other microtektites; their
TiO2 contents reached 1.08-1.34%, which are higher than
North American Microtektites and other microtektites.

Core M14 microtektites also have high siderophile element
content. For example, Ni content reached 1380 ppm, Co
reached 21.7 ppm, Os reached 22.5 ppm, Ir reached 0.026 ppm.
This property of high concentrating siderophile element
indicates that their parent materials come from outside the
Earth. We also found that Core M14 microtektite layer is
apparently associated with the beginning of the Reunion
geomagnetic event, and is probably associated with the
Matuyama/Gauss geomagnetic reversal boundary in the Core M14
(Fig.2). As you can see, the declination is from about 170
decreased extremly to 0 and the inclination is from about +7
decreased extremly to -15 at 270 cm of depth (corresponding
age is 2.14 million years ago). In addition, three specific
Radiolarians that belong to the Tertiary are extinguished
near the microtektite layer. They are Pterocanium
prismatium, Ommatartus antepenultimus, and Lamprocyrtis
heteroporos [1].

Based on above related various data, we can give the
following two primary conclusions: (1) The Core M14
microtektites are products of a major meteorite or asteroid
impact 2.14-2.30 million years ago. (2) The age of 2.14-2.30
million years is an important geological age; perhaps it is
just a correct age for dividing the Tertiary/Quaternary
boundary. This result is quite close to previous data. For
example, Kyte et al. found high noble metals and meteoritic
particle concentrations occur in upper pliocen sediments
(2.3 Ma B.P.) from the Antarctic Ocean [2]; Yuan Baoying et
al. found the microtektite layer in clay between the Wuchen
loess and Paleosol (2.4 Ma B.P.) from Luochuan county in
Shanxi province in China [3]; and Wu Xihao et al. found the
microspherule layer in the Paleosol, which is associated
with the Matuyama/Gauss geomagnetic reversal [4].

References. [1] Wang Y. et al. (1986) in Comprehensive
Investigation Report on Manganese Nodules in the Northern
Central Pacific, 64-69. [2] Kyte F. T. et al. (1981) Nature,
292, 417-420. [3] Yuan B. et al. (1989), Kexue Tongbao (in
Chinese), No. 22, 1725-1728. [4] Wu X. et al. (1991) Kexue
Tongbao (in Chinese), No. 23, 1799-1802.

Fig. 1. Microscape and SEM photographs of Microtektites from
Core M14. (A) Microscope photograph of spherical
microtektites; (B) A largest dumbbell microtektite (652 x
223 x 151 micrometers in size); (C and D) SEM photographs of
microtektites with circular, flat-topped, elevated area at
top.

Fig. 2. Correlation between the microtektite layer and
Paleomagnetic Stratigraphy in the Core M14 collected from
the North Pacific. Note the declination and inclination from
270 cm to 320 cm in depth (corresponding age from 2.14
million years to 2.48 million years) are showing extremly
variation.


Schultz P. H.
   Visualizing the Nature and Consequences of the Chicxulub Impactor: Clues
   from Venus

Little doubt remains that a major impact occurred at the end
of the Cretaceous and that the Chixulub site is the smoking
gun [1-3]. Although its outer dimension has grown from less
than 200 km [1,2] to almost 300 km [4], the limit of crustal
disruption does not necessarily correspond to the scale of
excavation nor is it simply proportional to impactor size.
Impact craters on other planets clearly document post-
formation crater enlargement through collapse of the rim [5]
with concentnc patterns of structural failure extending well
beyond the excavation cavity [6,7]. The well-preserved
cratering record on Venus provided by the Magellan mission
provides a critically needed dataset not only to assess the
effect of terrestrial-scale gravity on crater formation but
to appreciate both the lithospheric and atmospheric response
to impact cratering [8]. On the basis of such comparisons,
it is proposed that the Chixulub crater was formed by a 10-
15 km diameter impactor striking at an angle close to 30
degrees from the southeast. This type of collision would
maximize the atmospheric and biospheric response to an
impact, for the northern hemisphere.

Venus Cratering Record: Radar imaging from Magellan combined
with other planetary data allows testing models of crater
growth and the effect of impact angle [8]. Five conclusions
are relevant for understanding the Chixulub structure.
First, central peak morphology preserves a signature of
impactor direction and angle as determined independently by
ejecta deposit asymmetry. On Venus (and other planets),
central peaks become offset uprange with decreasing impact
angle (from horizontal) and typically become breached
downrange. This observation is significant since it suggests
that information about the impactor may not be completely
lost during crater modification. Second, central peak size
comprises a greater fraction of crater size with decreasing
angle of impact. This result is inconsistent with models
ascribing the scale of central relief solely to energy or
gravity effects. For a given impactor size, however, crater
size decreases as impact angle decreases. The Magellan data
then can be understood if central peak size corresponds to a
characteristic pressure limit indicative of the terminal
deceleration of the impactor [5,9,10]. Hence, impactor size
largely controls central peak (uplift) diameter while crater
size reflects the combined effects of size and velocity (on
a given planet). Third, enhanced crater enlargement occurs
for craters larger than about 80 km on Venus. Fourth,
oblique impacts result in greatest depths uprange and
shallower excavation downrange. This asymmetry results in
greater uprange collapse, thereby circularizing the outer
crater boundanes during modification. And fifth, the
atmospheric response to impacts becomes increasingly offset
downrange on Venus as the dense atmosphere decelerates and
captures a significant portion of the impactor and target
products.

Chixulub Impactor: Geophysical data reveal several
intriguing asymmetries in the interior ring structure of
Chixulub similar to craters on Venus and other planets.
First, the central Chixulub gravity--field becomes poorly
expressed to the northwest [2,4] yet preserves a preexisting
linear gravity anomaly [4]. As a result, the central
structure appears to be breached, open to the northwest.
Second, two gravity lows occur to either side of the
breached central anomaly about 100 km from the center and
subtending an angle of about 30 degrees to either side of
the NW-trending axis of symmetry. Third, the center of
curvature of the 180 km semicircular break in the Bouguer
gravity anomaly to the southeast is not coincident with the
center of curvature of either the inner 20 mgal central high
nor the concentric gravity lows. And fourth, the central 70
km wide magnetic anomaly is slightly elongate in a
northwest-southeast direction [2].

Cratering efficiency decreases with increasing scale due to
the effects of gravity [11,12]. At very large scales, the
early compression stage during penetration comprises a
significant fraction (10-20%) of the final transient
excavation cavity. Hypervelocity impacts in laboratory and
computational experiments reveal that lateral detachment of
the shock during penetation physically corresponds to a
transition zone from downward to lateral growth. The lateral
dimensions (x) of this zone was found to be expressed by an
interior pit in a wide range of nonporous media, as shown in
equation (1).

Equation (1) appears here.

where impactor velocity (v), radius (r), and density (delta
(sub)p) are scaled to target sound speed (c (sub)t) and
density (delta (sub)t). This dimension is independent of
impact angle. The limit of shock damage, however, extends to
much greater distances, thereby promoting collapse and
enlargement, whether by residual seismic energy [13] or the
continued dynamic motion of the gravity-limited flow field
[14,15].

This perspective on scaling combined with planetary analogs
allows interpreting the geophysical data for Chixulub in a
different working hypothesis. If the 60-km-diameter central
gravity anomaly defines the rebound limit established by the
penetration zone (x), then equation (1) indicates that the
impactor dimensions range from 10 to 15 km for velocities of
30 and 15 km/s, respectively. The pair of offset gravity
lows to the northwest closely resemble the downrange effects
of projectile failure and ricochet found on the planets and
in laboratory experiments. On this basis, Chixulub appears
to have been formed by an object impacting at about 30
degrees (from the horizontal) heading northwest. The well-
defined southeastern boundary marks the enhanced collapse of
the uprange rim. The proposed trajectory is consistent with
the distribution pattern of fragments of shocked quartz
[16], the size of "tektites" [17], and enhanced Ir levels
[18]. Moreover, it is consistent with the paleontological
record of the greatest biospheric stress occurring in the
northern hemisphere [19].

Consequences: Specifying the impactor size and trajectory
allows exploring possible consequences, largely preserved on
Venus [8]. Images reveal spatial and temporal decoupling of
the vapor/melt cloud from the late-stage ejecta deposition
around oblique impacts, consistent with phenomena observed
in laboratory experiments [20]. The combined deceleration
and lateral containment of the downrange-moving vapor cloud
interferes with ejecta emplacement and creates a parabolic
zone of turbulent deposition. Craters larger than about 40
km in diameter (40-70 km) exhibit a downrange disruption
zone about 1.6 x larger than the crater diameter. Allowing
for the difference in atmospheric density between Venus and
the Earth, the downrange vapor cloud from Chixulub would
have created a 650 km wide firestorm engulfing and boiling
upper meters of the Gulf of Mexico in its path. The
unrestricted expansion in the upper atmosphere, however,
eventually envelopes most of the northern hemisphere.
Entrained shocked target material would be rapidly driven
downrange. This mix of dispersed melt and clastic debris
arrives well in advance of debris from a separate more
hemisphencal cloud rising above the impact and later
ballistic ejecta. If 10% of the impactor energy comprises
this downrange firestorm, then scaling relations indicate
that its basal limit would take about 5 minutes while the
excavation crater finishes within a minute.

The downrange (northwest) ricochet/melt fractions from the
impactor should result in low-angle hypervelocity impacts
fanning out over the Gulf. Consequently, tsunamis will be
generated not only by formation of the crater but also by
downrange ricochet and later ejecta. At greater distances
downrange, ejecta deposits should exhibit staged and complex
emplacement sequences. Calculations further indicate the
generation of an intense hurricane-like storm as the
downrange firestorm equilibrates.

References: [1] Penfield G. T. and Camargo Z. A. A. (1981)
Mtg. Soc. Explor. Geophys. Abstr., 51, 37. [2] Hildelbrand
A. R. et al. (1991) Geology, 19, 867-871. [3] Sharpton V .L.
et al. (1992) Nature, 359, 819. [4] Sharpton V. L. et al.
(1993) Science, 261, 1564-1567. [5] Schultz P. H. (1988) in
Mercury (F. Vilas and C. Chapman, eds.), 274-335, U. Arizona
Press, Tucson. [6] Melosh H. J. and McKinnon W. B. (1978)
GRL, 5, 985-988. [7] Wichman R. and Schultz P. H. (1989)
JGR, 94, 17333-17357. [8] Schultz P. H. (1992) JGR, 97,
1618316248. [9] Schultz P. H. and Gault D. E. (1992) 1992
Sudbury Conferences, LPI Contrib. No. 790, LPI, Houston, 64-
65. [10] Schultz P. H. and Gault D. E. (1993) LPS XXIV,
1257-1258. [11] Gault D. E. and Wedekind J. (1977) in Impact
and Explosion Cratering (D. Roddy et al., eds.), 1231-1244,
Pergamon, New York. [12] Holsapple K. A. and Schmidt R. M.
(1982) JGR, 97, 1849-1870. [13] Melosh H. J. (1982) JGR, 87,
371-380. [14] Schultz P. H. et al. (1981) in Multi-Ring
Impact Basins (P. Schultz and R. B. Merril, eds.), 181-195.
Pergamon. [15] O'Keefe J. A. and Ahrens T. A. (1992) 1992
Sudbury Conference, LPI Contribution 790, LPI, 55-56. [16]
Bohor B. F. (1990) in GSA Spec. Paper, 247 (V. L. Sharpton
and P. Ward, eds.), 335-342. [17] Smit J. et al. (1992) in
LPS 22, 87-100. [18] Alvarez W. et al. (1982) in GSA Spec.
Paper, 190 (L. Silver and P. Schultz, eds.), 305-316. [19]
Spicer R. A. (1989) Phil. Trans. R Soc. Lond. Ser. B., 325,
291-305. [20] Schultz P. H. and Gault D. E. (1990) GSA Spec.
Paper, 247 (V. L. Sharpton and P. Ward, eds.), 239-262.


Boslough M. B.   Chael E. P.   Trucano T. G.   Kipp M. E.   Crawford D. A.
   Axial Focusing of Impact Energy in the Earth's Interior: Proof-of-Principle
   Tests of a New Hypothesis

Along with the appreciation of the importance of impact
events on the Earth's evolution, there has been increasing
speculation that energetic collisions have been responsible
for processes as varied as continental flood basalt
eruptions, mantle plumes, continental rifting, and
geomagnetic pole reversals. The link between impacts and
such geophysical processes was first discussed by Seyfert
and Sirkin [1], who suggested that impact-induced mantle
plumes could be a mechanism for initiating the breakup of
plates. Burek and Wanke [2] listed correlations between
known Cenozoic impacts and geomagnetic field reversals,
unconformity ages, shifts in paleotemperatures, and tectonic
episodes. They suggested that major impacts could generate
shock-induced phase transitions in the upper mantle,
disrupting a delicately balanced stability down to the core-
mantle boundary. Rampino and Strothers [3] proposed a quasi-
periodic correlation between mass extinctions and major
continental flood basalt volcanism over the last 250 million
years and attempted to explain it in terms of episodic
showers of impacting comets. Connections between impacts and
the internal workings of the Earth are supported by
correlations of the ages of tektites from strewn fields with
geomagnetic field reversals [4], and by a reversal
associated with sediments deposited immediately after the
impact that formed the Ries Crater [5].

A causal link between major impact events and global
processes would probably require a significant change in the
thermal state of the Earth's interior, presumably brought
about by coupling of impact energy. One possible mechanism
for such energy coupling from the surface to the deep
interior would be through focusing due to axial symmetry.
Antipodal focusing of surface and body waves from
earthquakes is a well-known phenomenon [6], which has
previously been exploited by seismologists in studies of the
Earth's deep interior [7,8]. Antipodal focusing from impacts
on the Moon, Mercury, and icy satellites has also been
invoked by planetary scientists to explain unusual surface
features opposite some of the large impact structures on
these bodies [9,10]. For example, "disrupted" terrains have
been observed antipodal to the Caloris impact basin on
Mercury and the Imbrium basin on the Moon. Very recently
there have been speculations that antipodal focusing of
impact energy within the mantle may lead to flood basalt and
hotspot activity [11,12] but there has not yet been an
attempt at a rigorous model.

We have proposed a new hypothesis and performed preliminary
proof-of-principle tests for the coupling of energy from
major impacts to the mantle by axial focusing of seismic
waves. Because of the axial symmetry of the explosive
source, the phases and amplitudes are dependent only on ray
parameter (or takeoff angle) and are independent of
azimuthal angle. For a symmetric and homogeneous Earth, all
the seismic energy radiated by the impact at a given takeoff
angle will be refocused (minus attenuation) on the axis of
symmetry, regardless of the number of reflections and
refractions it has experienced. Mantle material near the
axis of symmetry will experience more strain cycles with
much greater amplitude than elsewhere and will therefore
experience more irreversible heating. The situation is very
different than for a giant earthquake, which in addition to
having less energy, has an asymmetric focal mechanism and a
larger area. It should be noted that our hypothesis is
fundamentally different than those proposed by many others
(e.g., [13-16]), which involve melting and excavation at the
impact location. Problems with models of this type have been
pointed out by Loper [17].

We are using two independent proof-of-principle approaches.
The first makes use of seismic simulations, which are being
performed with a realistic Earth model to determine the
degree of focusing along the axis and to estimate the volume
of material, if any, that experiences significant
irreversible heating. The second involves 2-D hydrodynamic
code simulations to determine the stress history, internal
energy, and temperature rise as a function of radius along
the axis.

For the preliminary seismic modeling the impact was
represented as a vertical point force applied at the Earth's
surface as a delta function in time. The impactor was
assumed to yield an impulse of approximately 3 x 10^24 dyne
sec, as an estimate of a KT-sized impact. Synthetic
displacement and strain records were generated for such a
source by summing the normal modes of the elastic Earth
model 1066A of Gilbert and Dziewonski [18]. We used the
attenuation profile of the model PREM [19] to determine Q
values for the 1066A modes. We assumed a vertically directed
point source so we included only spheroidal modes in the
synthetic seismogram calculations. Toroidal modes are not
excited by a vertical point force. All spheroidal modes with
periods greater that 45 seconds were summed for the
synthetics.

The synthetic signals yield estimates of the peak strains at
any location in or on the Earth. Figure 1 shows the
difference in strain histories on the Earth's surface at two
different angular distances from the source. This figure
demonstrates the gross effects of antipodal focusing.
Strains at the surface near the antipode (angular distance =
180 degrees) are orders of magnitude higher than those over
most of the rest of the Earth's surface. Figure 2 plots the
peak strains as a function of depth beneath the antipode of
the impact, from the surface to the core-mantle boundary. It
can be seen that peak strain amplitude varies by two orders
of magnitude and is largest at the top. This would imply
that most of the energy is focused at shallow depths;
however it may be an artifact of ignoring shorter period
modes. This issue will be resolved by adding more modes, and
by doing body wave calculations. Figure 3 demonstrates that
focused arrivals have much greater amplitudes than direct
arrivals at the core-mantle boundary. At the beginning of
Fig. 3(a), the direct arrival is shown for a point directly
beneath the impact, followed by a long high-amplitude
focused trace. For comparison, the strain history at the
same level on the antipodal axis is shown in Fig. 3(b). It
can be seen to have a larger peak amplitude.

In addition to seismic modeling, we are in the process of
performing detailed calculations of a hypothetical impact
and subsequent wave propagation through the Earth with the
CTH [20] strong shock hydrodynamics code. CTH is a 1-, 2-,
and 3-dimensional multimaterial elastic-plastic Eulerian
hydrodynamics code. CTH accurately computes the very large
material deformations, material ejection, and strong shock
generation created by the initial KT impact. For these
calculations we model the Earth as a central gravitationally
stable, stratified body, with a variety of interior
structure, including inner and outer core and mantle. The
materials are treated as elastic-plastic. Also, the
equations of state associated with CTH allow
thermodynamically consistent phase transitions, so that
vaporization at the initial impact site can be properly
modeled, as well as shock-wave modification in the Earth's
interior due to solid-solid phase transitions.
Alternatively, we have also modeled the initial impact site
as a general energy source, similar in strategy to the
seismic model and to the work of Watts et al. [10].

CTH can be used to study the long-term evolution of the
resulting wave structures through the interior of the Earth,
with some caveats related to accuracy. CTH is an artificial
viscosity code, and numerically widens shock
discontinuities. Generally speaking, at very long ranges and
times the numerical dissipation in the code will cause
inaccuracies in the almost linear wave propagation that is
expected to occur. In particular, the numerical dissipation
will attenuate stress waves at great propagation distances.
In addition, resolution of these low amplitude waves as they
propagate through regions of large hydrostatic compression
in the Earth's interior must be maintained. While we have
still studied the overall response of the antipodal region
to the KT impact, the resulting amplitudes and strain due to
the long-term wave interactions can be in error.
Nonetheless, this approach has also been pursued by Watts et
al. [10], and has led to useful investigation of the
response of the antipodal region. Our numerical resolution
is significantly finer than that of Watts et al. [10].
Ultimately, we will improve the accuracy of our numerical
modeling by linking earlier time CTH results to the more
accurate seismological analyses described earlier.

Acknowledgments: This work was performed at Sandia National
Laboratories supported by the US Department of Energy under
contract DE-AC04-76DP00789 with funding under the LDRD
program.

References: [1] Seyfert C. K. and Sirkin L. A. (1979) Earth
History and Plate Tectonics, Harper and Row. [2] Burek P. J.
and Wanke H. (1988) Phys. Earth Planet. Inter., 50, 183-194.
[3] Rampino M. R. and Strothers R. B. (1988) Science, 241,
663-668. [4] Glass B. P. et al. (1979) Proc. LPSC 10th, 25-
37, Pergamon. [5] Pohl J. (1977) Geol. Bavarica, 75, 329-
348. [6] Gutenberg B. and Richter C. (1934) Gerlands Beitr.
Geophys., 43, 56. [7] Rial J. A. (1979) Ph.D. Thesis,
Caltech. [8] Chael (1983) Ph.D. Thesis, Caltech. [9] Schultz
P. H. and Gault D. E. (1975) The Moon, 12, 159-177. [10]
Watts A. W. et al. (1991) Icarus, 93, 159-168. [11] Hagstrum
J. T. and Turrin B. D. (1991) EOS, 72, 44, 516. [12] Rampino
M. R. and Caldeira K. (1992) GRL, 19, 2011-2014. [13] Green
D. H. (1972) EPSL, 15, 263-270. [14] Alt D. et al. (1988) J.
Geol., 96, 647-662. [15] Oberbeck V. R. et al. (1992) J.
Geol., 101, 1-19. [16] Negi J. G. et al. (1993) Phys. Earth
Planet. Int., 76, 189-197. [17] Loper D. E. (1991)
Tectonophys., 187, 373-384. [18] Gilbert F. and Dziewonski
A. M. (1975) Phil. Trans. Roy. Soc. Lond. A, 278, 187-269.
[19] Dziewonski A. M. and Anderson D. L. (1981) Phys. Earth
Planet. Inter., 25, 297-356. [20] McGlaun J. M. et al.
(1989) Sandia National Laboratories Report SAND89-0607.

Fig. 1. Synthetic strain histories at and near impact
antipode, plotted on same scale.

Fig. 2. Peak strain vs. depth along antipodal axis.

Fig. 3. Synthetic strain histories just above the core-
mantle boundary (CMB) on the impact axis (a) beneath the
impact and (b) antipodal to the impact.


Rampino M. R.
   A General Theory of Impacts and Mass Extinctions and the Consequences of
   Large-Body Impacts on the Earth

The theory that large-body impacts are the primary cause of
mass extinctions of life on the Earth now has a sound
theoretical and observational foundation. A convergence of
evidence suggests that the biosphere may be a sensitive
detector of large impact events, which result in the
recorded global mass extinction pulses. The astronomically
observed flux of asteroids and comets in the neighborhood of
the Earth, and the threshold impact size calculated to
produce a global environmental catastrophe, can be used to
predict a time history of large impact events and related
mass extinctions of life that agrees well with the record of
~24 extinction events in the last 540 m.y. [1].

The search for impact signatures at extinction boundaries in
the geologic record has produced six cases of diagnostic
evidence of impacts (e.g., layers with high Ir, shocked
minerals, microtektites), and at least six cases of elevated
Ir of the amplitude expected from collision of relatively
low Ir objects, such as comets. Recent detailed studies of
the extinction boundaries show evidence of biological,
isotopic, and geochemical signatures of sudden environmental
crisis and abrupt mass mortality similar to those seen at
the Cretaceous/Tertiary (KT) boundary [1]. A comparison of
extinction rates and severity with the record of impact
events provides a straightforward relationship--the greater
the energy of the impact, the greater the mass extinction.
The largest known craters have a significant correlation
with the extinctions, including Manicouagan (~100 km wide),
close to the end-Triassic; Doulon in China (~80 km) near the
Jurassic/Cretaceous boundary, Popigai (~100 km) near the
Eocene/Oligocene transition, and Chicxulub (>-200 km) at the
KT boundary. A lower limit seems to exist, as impacts
producing craters <-50-60 km wide are not associated with
global extinction pulses above background levels of
extinction. A newly proposed impact structures on the
Falkland Plateau (~350 km) may correlate with the end-
Permian extinction (~250 m.y.), the most severe in the last
540 m.y. [2].

The formation and subsequent breakup of supercontinents and
the coming and going of Ice Ages are among the major events
in Earth history. Furthermore, within the plate tectonic
paradigm they have been closely linked through changes in
ocean-floor creation rates, sea level, volcanism, and
atmospheric composition (CO2 content) [3]. These would seem
to depend strictly upon internal Earth processes (e.g.,
mantle convection, rising deep mantle plumes). However,
recent evidence points to a possible role for asteroid and
comet impacts in supercontinent breakup episodes, and in
explaining several paradoxes pertaining to past ice ages.

Large impacts create magnitude >-earthquakes, and such
quakes could result in deep fracturing near impact sites, as
well as in regions at great distances from the sites, where
seismic waves are focused [4]. Impact-related fracturing
could lead to cracking of the lithosphere, mantle melting,
vast outpourings of lava (flood basalt eruptions), and to
the development of hotspots. These might be most likely to
occur in areas where broad upwellings or plumes in the
mantle produced stresses that could be released by the
impact-induced fracturing [3].

Correlation and possible cause-and-effect relationships
among large-body impacts, extinction events, and continental
flood basalt eruptions have been suggested [5,6]. Our
previous work showed an apparent correlation between the
ages of rapidly erupted continental flood basalts (CFBs) and
extinction boundaries over the past 250 m.y. Use of new and
more reliable ^40Ar/^39Ar and U-Pb age determinations on
flood basalts improves the correlation.

In the hard copy Table 1 appears here.

This correlation is significant at the ~99% level [7]. We
believe that these data support possible cause-and-effect
relationships among impacts, extinctions, and flood basalt
eruptions.

The breakup of supercontinents, such as Pangaea, are
attributed to upwelling of hot plumes in the Earth's mantle,
which eventually break through to the surface. The breakup
events are often marked by flood basalts. The discrete ages
of the flood basalt eruptions show that the breakup of
Pangaea (and its southern region of Gondwanaland) occurred
in a number of phases.

In the hard copy Table 2 appears here.

A clue to the possible association of supercontinent breakup
and a large impact comes from events that took place at the
end of the Permian Period (245-250 m.y.), a time marked by
the most severe mass extinction of life in the geologic
record (about 96% of species became extinct). A large,
roughly circular region of anomalous crustal deformation
occurred around the present Falkland Plateau at the same
time (248 m.y.). Geophysical and geological information
allow the identification of one and possibly two circular
basins (about 350 and 200 km in diameter) of possible Late
Permian age on the Falkland Plateau--suggesting impacts of
planetesimals 10 to 20 km in diameter [2]. The great impact
may have provided a locus for the subsequent breakup of
Gondwanaland, providing radial and concentric lines of
weakness in the lithosphere, and possibly localizing the
development of mantle convection patterns. This model may
explain the similarity in timing of impactor showers, flood
basalt eruptions, extinctions, hotspot outbreaks, and
reorganizations of plate motion [3].

Major episodes of glaciation are recorded by diamictites
interpreted as tillites in the Early Proterozoic (about 2.3
b.y.), Late Proterozoic (~850-600 m.y.), Late Ordovician
(~435 m.y.), Late Devonian (~370 m.y.) and Late Paleozoic
(~320 to ~250 m.y.). This leads to several climatic
paradoxes--Late Proterozoic glacial deposits are found at
all paleolatitudes; the early glaciations occur at times
when atmospheric CO2 is predicted to have been high; and
some glacial deposits are associated with warm climate
indicators [8,9].

Numerous published studies have revealed that many
recognized tillite sequences were actually the products of
massive debris flows and large-scale "rainout" of coarse
sediment. As a result, the tillites are now generally
reinterpreted as glaciomarine debris-flow deposits, but this
interpretation does not readily explain the scale and
textures of the diamictites [8,9]. Recently, three groups
have presented evidence that some inferred glacial sequences
have many of the essential characteristics of ballistic
debris-flow ejecta created by large impacts [8-10] and may
contain evidence of shock [9]. Might some deposits
interpreted as glacial in origin be ejecta of large impact
events? If so, significant revisions in the long-term
climate history of the Earth will be required.

References: [1] Rampino M. R. and Haggerty B. M. in Hazards
due to Comets and Asteroids, Univ. of Arizona (in press).
[2] Rampino M. R. (1992) Eos, Trans. Amer. Geophys. Union,
73, 336. [3] Rampino M. R. and Caldeira K. (1993) EPSL, 114,
215-227. [4] Rampino M. R. and Caldeira K. (1992) GRL, 19,
2011-2014. [5] Rampino M. R. and Stothers R. B. (1988)
Science, 241, 663-668. [6] Stothers R. B. and Rampino M. R.
(1990) GSA Spec. Paper, 247, 9-18. [7] Stothers R. B. (1993)
GRL, 20, 1399-1402. [8] Oberbeck V. R. et al. (1993) J.
Geol., 101, 1-19. [9] Rampino M. R., J. Geol. (in press).
[10] Sears J. W. and Alt D. (1992) Intern. Basement Tect.
Assoc. Pub., 5, 385-392.



Matese J. J.   Whitman P. G.   Innanen K. A.   Valtonen M. J.
   Modulating Terrestrial Impacts from Oort Cloud Comets by the Adiabatically
   Changing Galactic Tides

Time modulation of the flux of new Jupiter-dominated Oort
cloud comets is the subject of interest here. The major
perturbation of these comets during the present epoch is due
to the tidal field of the relatively smooth distribution of
matter in the Galactic disk. A secondary source of the near-
parabolic comet flux are stars penetrating the inner Oort
cloud and providing impulses that create brief comet
showers. Substantial stellar-induced showers occur ~ every
100 Myr. Less frequent (but stronger) impulses due to giant
molecular clouds can also perturb comets from the inner
cloud. These occur on timescales of ~500 Myr. In contrast to
these infrequent stochastic shower phenomena is the
continuously varying tidal-induced flux due to the Galaxy.
As the sun orbits the Galactic center it undergoes quasi-
harmonic (~64 Myr) motion about the Galactic midplane, which
is superimposed on the small eccentricity, near-Keplerian
motion in the plane having epicycle period ~150 Myr. In the
process the Galactic tidal field on the Sun/cloud system
will vary, causing a modulation of the observable Oort cloud
flux. We have created a model of the Galactic matter
distribution as it affects the solar motion over a time
interval ranging from 300 Myr in the past to 100 Myr into
the future. As constraints on the disk's compact dark matter
component we require consistency with (1) the observed
Galactic rotation curve, (2) today's flux distribution of
new comets, (3) the studies of K-giant distributions, and
(4) the periodicity found in the terrestrial cratering
record. The adiabatically varying galactic tidal torque is
then determined and used to predict the time dependence of
the flux. We find that a model in which ~half of the disk
matter is compact is consistent with these constraints.
Under such circumstances the peak-to-trough flux variation
will be ~5 to 1 with a full width of 9 Myr. This variability
will manifest in the terrestrial cratering record and is
consistent with the observed cratering periodicity, if over
half of the impacts on earth are caused by comets or
asteroids that originate in the outer Oort cloud.

Analysis: The Galaxy is modeled using a modified CBIP
potential with a nucleus, a bulge, a halo, and a disk. The
analysis is similar to that described in Innanen et al. [1].
A central issue here is the question of local dark disk
matter. Bahcall et al [2] have analyzed the distribution of
K giants and have concluded that there is "significant but
not overwhelming evidence for disk dark matter." We are
guided by their ISM-like model. Two of the three disk
components of the disk are fixed as having scale heights of
325 pc and 125 pc, mass fractions of 4:1 respectively, and
represent old stellar and atomic gas contributions. The
third component is compact and simultaneously includes
contributions of young stars, dust, molecular H, and the
dark matter. We then vary three disk parameters, the total
disk mass, the fraction of disk matter that is compact, and
the scale height of the compact component.

The disk mass is tightly constrained by a present solar
velocity, which we fix at theta = 225 km s^-1. The scale
height and mass fraction of the compact component are
primarily limited by the K-giant studies and the period
requirement. The equations of motion of the Sun about the
galactic center are solved both backwards and forwards in
time. As initial conditions we adopt Z(sub)circle = 10 pc
and R(sub)circle = 8 kpc along with a Z component of
velocity of 7.5 km s^-1. The scale height and mass fraction
of the compact component is then varied (keeping the total
mass fixed) and the solar motion is determined. In Fig. 1 we
show the parameter range that reproduces the required plane
crossing period. For further illustrations we choose the
specific parameter set for the compact disk component to
have scale height 50 pc and mass fraction 0.50. It
corresponds to total local disk midplane density of 0.29
M(sub)circle pc^-3 and yields a mean plane crossing period
of 32 Myr. This is within the 1 sigma range of values (dark
matter mass fraction 0.24-0.74) given in [2] for their ISM
model. In Fig. 2 we illustrate the trajectory of the Sun as
it orbits the galaxy. The time interval considered is from
100 Ma the future to 300 Ma in the past.

Results: It has been demonstrated that the galactic tidal
field dominates random stellar encounters in making Oort
cloud comets observable (Heisler and Tremaine [3]; Matese
and Whitman [4,5]). Matese and Whitman describe how the time
evolution of all orbital elements can be analytically
determined if the disk density rho(sub)circle(t) can be
taken as adiabatically changing. The results are expressible
in terms of Jacobi elliptic functions. Of the elements
omega, Omega, i, e, and a only the last is taken to be
invariant in the adiabatic limit. Since a typical Oort comet
has a semimajor axis ~30000$ AU (period ~5 Myr) this
approximation is adequate here. Our model of the Oort cloud
includes the following assumptions.

The distribution of comet angular momenta and perihelia
directions is taken to be random. For the in situ comet
energy flux (shown in Fig. 3) we adopt the predictions of
Bailey [6] where x = 10^6 AU/a is a dimensionless original
energy and the nominal cloud boundary is a = 10^5 AU. A
Monte Carlo analysis proceeds in a manner detailed in [4,5]
in which trajectories are evolved one period into the future
and new orbit parameter values are determined. The notion of
a tidal efficiency, which measures the ability of the
galactic tide to overcome the loss cylinder barrier and
change the perihelion distance was described in [5]. We use
the tidal efficiency, at a time retarded by half an orbital
period, along with in situ flux density to predict the flux
of Oort comets whose perihelia leap the loss cylinder
barrier (15 AU) to the zone of Jupiter dominance in a single
orbit due to tides.

Figure 3 also shows todays predicted Earth-crossing as well
as Jupiter-dominated flux. The results are scaled to the
observed flux of new comets today. For comparison we further
show the observed distribution [7] of class I comets that
are broadened by measurement errors and nongravitational
forces. The time-dependent Jupiter-dominated current is
displayed along with the Galactic density at the solar
location in Fig. 4. We have subjected the results of Fig. 4
to a conventional regression analysis to determine the width
of the current peaks. The standard deviations are 4.0-4.5
Myr.

In Table 1 we list the epochs of peak Oort cloud currently
entering the solar system as well as the maximum and minimum
rates. One observes that the cycle interval, tau, changes,
i.e., the "periodic" oscillations have in fact a variable
period. Further, the phase of the nearest cycle peak is 0.6
Myr in the future with the adopted parameters. This is
consistent with the terrestrial cratering record (Rampino
and Stothers [8]; Shoemaker and Wolfe [9]). There is ample
evidence from the dating of impact structures and impact
glass that suggests that the nearest flux maximum may not
have occurred as yet. The predictions are in good agreement
with the dating of impact glasses, microspherules, and the
Ir anomalies that have peaks at 1, 35, and 65 Ma [9].

Shoemaker and Wolfe state that the cratering record appears
to be periodic but consider it to be a statistical fluke as
they can find no mechanism for the periodicity. In
particular they reject the Galactic oscillations model of
Rampino and Stothers [8] since it requires the impulsive
perturbation of inner Oort cloud comets by molecular clouds.
As the plane crossing epochs are too close to the crater
peak times of 2, 32, 65, and 99 Ma to be due to rare
stochastic cloud impulses, the Rampino-Stothers mechanism
was rejected as a clock for this phenomena. These objections
are obviated in the present model since modulation of outer
Oort cloud comet flux is the physical mechanism for the
periodicity.

In comparing these results with the numerical experiments on
impact periodicity, we need to estimate the total dispersion
expected from a strictly periodic sequence, i.e., we should
convolve the standard error in the mean cycle interval (~1
Myr) with the standard deviation of the modulation about the
cycle peak (~ 4 Myr) and the dating uncertainties of the
most accurately dated craters (~ 4 Myr). If terrestrial
cratering impact probabilities were modulated as described
here with dating uncertainties of this order, we would
expect a net dispersion about a strictly periodic signal to
be ~ 5-6 Myr. Were such a signal to have a reasonable
probability of manifesting itself in the cratering record
the steady state main-belt asteroidal cratering rate would
have to be <50% of the total.

It is noted that the actual Galactic disk matter compactness
and the past solar motion are not well known. Should it be
verified that (1) over half the disk matter is compact and
of scale height ~ 50 pc, and (2) over half of terrestrial
cratering is due to active, dormant, or extinct comets
having an origin in the outer Oort cloud, then the solar
oscillation cycle will manifest itself in the cratering
record at the marginally significant level observed. These
results do not require the dark matter to be in the form of
massive objects capable of impulsing the inner Oort cloud.
Detectable modulation will exist even in the absence of
infrequent molecular cloud showers. The observed periodicity
in terrestrial cratering would then no longer be rejectable
as a statistical fluke and we would be approaching a flux
maximum from the outer Oort cloud within the next ~1 Myr. We
could then say that today we are at the peak of a continuous
cometary drizzle, if not a shower.

References: [1] Innanen K. A. et al. (1978) Astrophys. Space
Sci., 57, 511-515. [2] Bahcall J. N. et al. (1992) The
Astrophysical Journal, 389, 234-250. [3] Heisler J. and
Tremaine S. (1986) Icarus, 65, 13-26. [4] Matese J. J. and
Whitman P. G. (1989) Icarus, 82, 389-401. [5] Matese J. J.
and Whitman P. G. (1992) Celest. Mech. and Dyn. Astron., 54,
13-36. [6] Bailey M. E. (1986) Mon. Not. R. Astron. Soc.,
218, 1-30. [7] Marsden B. G. (1989) Catalogue of Cometary
Orbits, 6th ed. [8] Rampino M. R. and Stothers R. B. (1986)
The Galaxy and the Solar System, 241-260. [9] Shoemaker E.
M. and Wolfe R. F. (1986) The Galaxy and the Solar System,
338-386.

Fig. 1. Parameter range of the disk compact matter. Solid
curves border parameters for Galactic midplane crossing
intervals of 30-32 Myr. Dashed curves illustrate the ratio
of maximum to minimum disk density per oscillation cycle.

Fig. 2. Solar trajectory from 300 Ma in the past to 100 Ma
in the future.

Fig. 3. Theoretical Oort cloud flux for today's density. The
observed flux of new class I comets is shown also.

Fig. 4. Time dependence of the Jupiter-dominated current of
comets from the outer Oort cloud due to the adiabatically
changing Galactic tides. Note the small retardation between
the current and the density.


Carlson S.   Culler T.   Muller R. A.   Tetreault M.   Perlmutter S.
   In Search of Nemesis

The Nemesis model [1] was proposed in 1984 to account for a
reported periodicity in mass extinctions [2]. In this model
a companion star (probably a red dwarf) orbiting the Sun at
a distance of about 3 lightyears triggers a comet shower
once per orbit. This theory contained the first proposal
that apparently extended extinctions could in fact be a
series of stepwise events in a comet shower; see ref [3].

Since the original proposal, both the periodicity and self-
consistency of the Nemesis theory has been strongly debated.
In detailed Monte Carlo simulations, Hut [4] concluded that
the orbit was stable for about 1 Gyr, consistent with the
original model [1]. However others argued that the orbit was
unstable. Clube and Napier [5] argued that the effects of
giant molecular clouds (GMCs) dominate and decrease the
lifetime to about 0.1 Gyr. However Morris and Muller [6]
pointed out that most GMCs are too diffuse to have such an
effect and that gravitational scattering from randomly
passing stars dominates. Other authors assumed that a 1 Gyr
year lifetime was too short to be compatible with the nearly
5 Gyr lifetime of the solar system; however the original
paper [1] argued that the lifetime at formation was
approximately 5 Gyr, and has been decreasing linearly ever
since; see, for example, Hut [4]. For more details on this
debate, see Perlmutter et al. [7].

What to Look for--Red Dwarfs: The parallax of all stars of
visual magnitude greater than about 6.5 has been already
measured. If Nemesis is a main sequence star 1 parsec away,
this requires Nemesis's mass to be less than about 0.4 solar
masses. If it were less than about 0.05 solar masses it's
gravity would be too weak to trigger a comet storm [1,3]. If
Nemesis is on the main sequence, this mass range requires it
to be a red dwarf.

A red dwarf companion would likely have been missed by
standard astronomical surveys [8]. Nearby stars are usually
found because they are bright or have high proper motion.
However, Nemesis's proper motion would now be 0.01
arcsec/yr, and if it is a red dwarf its magnitude is about
10--too dim to attract attention.

Unfortunately, standard four color photometry does not
distinguish between red dwarfs and giants. So although
surveys such as the Dearborn Red Star Catalogue list stars
by magnitude and spectral type, they do not identify the
dwarfs. We therefore must scrutinize every star of the
correct spectral type and magnitude.

Our candidate list is a hybrid--candidate red stars are
identified in the astrometrically poor Dearborn Red Star
Catalogue and their positions are corrected using the Hubble
Guide Star Catalogue. When errors in the Dearborn catalogue
make it impossible to identify the corresponding Hubble star
we split the fields so that we have one centering on each
possible candidate. We are currently scrutinizing 3098
fields, which we believe contain all possible red dwarf
candidates in the Northern Hemisphere.

Since our last report [7] the analysis and database software
has been completely rebuilt to take advantage of updated
hardware, to make the data more accessible and to implement
improved methods of data analysis. The software is now
completed and we are eliminating stars every clear night.

Finding Nemesis: A star at 3 lightyears would display a
parallactic motion of 2.5 arcseconds over 6 months. We are
collecting digitized images of 6 x 6 arcminutes fields
centered on each of the candidate stars. Images of each star
are taken several months apart and aligned using the field
stars. The candidate's apparent motion is then measured. The
plate scale of our CCD is 0.7 arcseconds per pixel and we
can routinely locate the centriod of a candidate to within
0.2 pixels. Thus, Nemesis's peak to peak displacement should
be discernible at about the 18 sigma level.

Figure 1 shows two candidates that have been eliminated. The
data mark the candidate's motion in pixels (zero is defined
as the location at which it was first observed) on different
nights. Also plotted (squares) are the locations at which a
star at 1 parsec would be expected to be on the same nights.
The dashed lines show which measurements go with which
squares. Figure 1a summarizes four measurements. Figure 1b
shows that we can easily eliminate candidates with only two
observations.

Figure 2 summarizes the data taken as of October 1993.
Measured along the semi-major axis a star executes a motion
given by Pmajor = -(1/d) sin(wt + d), where d is the
distance to the object in parsecs, w = 2 pi /365 days, t is
the time between observations in days. The phase angle d is
set by the date of the first observation. The motion along
the semi-minor axis is given by Pminor = [(1/d)
Isin(L)I]cos(wt + d) where L is the heliocentric latitude of
the candidate. The plots show the observed motion for all
the candidates projected along the semi-major and semi-minor
axies of their respective parallax ellipses vs. the relevant
trigonometric factors. Observations of a nearby star should
fall on a straight line through the origin. For 2a the slope
will be equal to the distance in parsecs. For 2b it is the
distance in parsecs divided by sin(L).

At present we are limited only by available observing time.
When this abstract was written we were eliminating typically
10 candidates every clear night.

References: [1] Davis M. et al. (1984) Nature, 308, 715-717.
[2] Raup D. M. and Sepkoski J. J. (1984) Proceedings of the
National Academy of Science, 81, 801-805. [3] Muller R. A.
(1985) The search for extraterrestrial life; Recent
developments: Dordrecht, Holland, D. Reidel Publishing Co.,
233-243. [4] Hut P. (1984) Nature, 311, 638-640. [5] Clube
S. V. M. and Napier W. M. (1984) Nature, 311, 635-636. [6]
Morris D. E. and Muller R. A. (1986) Icarus, 65, 1-12. [7]
Perlmutter S. et. al. (1990) GSA Special Paper, 247, 87-91.
[8] van de Kamp P. (1982) Binary and multiple stars as
tracers of stellar evolution, Dordrecht, Reidel, 81-103.


Ryder G.
   The KT Boundary:  Historical Context, Counter-Revolutions, and Strawmen

Some authors have recently downplayed both the abruptness of
the Cretaceous-Tertiary transition and its importance.
Interest in the boundary is attributed to: (1) extinction of
dinosaurs, (2) rise of mammals, (3) controversy related to
impact, and 4) closeness to the present. Thus Stanley [1]
states that "...these factors have led us to overrate the
terminal Cretaceous crisis". In fact, this boundary has been
recognized as both major in importance and abruptness since
the early decades of the 19th century, well before dinosaurs
or Ir were even concepts. I contend that the Cretaceous-
Tertiary boundary extinctions represent the most clear
example of a mass extinction event, more so than the
Permian-Triassic extinctions, because the latter took place
over a period of several millions of years and cannot even
be postulated as a (single) event.

Many of those who are at best sceptical and commonly devoted
opponents of the impact-extinction hypothesis for the
Cretaceous-Tertiary boundary changes have set up strawmen
and imputed to that hypothesis attributes neither inherent
nor generally included by impact proponents. Such
imputations are that nearly all of the uppermost
Maastrictian extinctions are impact-caused, that virtually
everything was instantaneously wiped out worldwide, and that
extraterrestrial causes would produce more random
extinction. Whereas some (particularly some paleontologists)
have demanded exact burdens of proof of "impactors", they
have not demanded them of themselves; for instance,
"impactors" alone are criticized for not explaining the
survival of crocodiles and turtles. I attempt first to put
the Cretaceous-Tertiary boundary in a historical context,
and then to limit what the impact hypothesis actually does
demand.

The recognition of a major and abrupt change that is the
Cretaceous-Tertiary boundary took place (under catastrophic
thinking, admittedly) at about 1810 with the studies in the
Paris Basin by Cuvier and Brongniart. This preceded
dinosaurs, Ir, thin sections, isotopes, radioactivity, and
natural selection. In 1813 D'Halloy distinguished
"cretaceous" from "mastozooic" as the upper two of four
divisions of rocks; no Permian-Triassic boundary was then
recognized, but subsumed in the second group from the (now)
Paleozoic through Jurassic (yet D'Halloy was familiar with
the rocks that Murchison later included in the Permian).

Phillips, in recognizing major and significant divisions and
labelling them as Paleozoic, Mesozoic, and Cenozoic around
1840, followed D'Halloy's distinction of Permian rocks from
Triassic rocks (around 1830), and recognized the critical
significance of the Mesozoic-Cenozoic change. Murchison's
Permian originally included rocks now recognized as
Triassic, so the Permian-Triassic boundary was not obvious
to everyone in the rocks of Europe at around 1840. Under the
then-recently-established uniformitarian (in all its
combined forms) principles, transitions were expected.
Boundaries were in consequence expected to be arbitrarily
assigned, and this was (and remains) the case for the
Permian-Triassic. The expected passage beds for the
Mesozoic-Cenozoic boundary were never found. That they did
not appear seems to have been an embarassment for Lyell and
perhaps even more so later for Darwin. Lyell had subdivided
the Cenozoic, yet noted (under uniformitarian thought) that
the change from Cretaceous to (then) Eocene was equivalent
to the entire Tertiary. While the relevant strata in Britain
are indeed missing (though only a few million years worth,
not sixty) Lyell was familiar with European strata, where
more complete successions occur. Although Darwin was
familiar with this faunal change, his only mention of it
concerns the abrupt extinction of the ammonites. The immense
faunal break without significant lithological change has
been used to infer the presence of a sedimentological break
even with no evidence e.g., north-east Mexico [2], a clear
demonstration of the power of uniformitarianism.
Nonetheless, the geologically sudden extinctions at the end
of the Mesozoic have been generally regarded as real and
demonstrably more abrupt than those at the beginning [e.g.,
3] even if some sort of uniformitarian and terrestrial
causes were sought.

There appears to have been little difficulty anywhere in
pinning down the Cretaceous-Tertiary boundary with precision
since the mid-1800's. An (arbitrary?) question as to whether
the Danian should be included with the Cretaceous was
resolved. The boundary has since become more tightly defined
and recognizable, not less, particularly using microfossils.
The recognition of a global Ir-rich layer (whatever origin
is invoked) at the paleontologically-defined boundary has
led to its use as a marker bed, and the more precise
correlation of some marine and non-marine records.
Terrestrial disruptions took place geologically
simultaneously with marine changes, and both ammonite and
dinosaur fragments persist to the boundary. Such precision
and correlation is not possible for the Permian-Triassic
boundary, for which correlation of marine and non-marine
sequences has proven elusive, and equivocal even among
marine sequences. The Cretaceous-Tertiary boundary
extinction is not a poor relative of the Permian-Triassic
mass extinction. It was quite different, far more abrupt and
clear (i.e., rate was much faster) and affected many more
ecological niches, even if the total number of orders,
families, or genera that became extinct was smaller. They
are not really comparable. There are mass extinctions and
there are mass extinctions.

The presence of an Ir-rich layer, shocked quartz, and the
identification of perhaps the largest Phanerozoic impact
crater with a precisely correct stratigraphic and
radiometric age in the Yucatan, have convinced most workers
that there was a boundary impact event. There remain
considerable differences in opinion of the significance, if
any, of that event in extinctions. An important historical
point is that the abruptness of extinctions led to the
analysis for Ir at a particular horizon, and that
measurement led to the inference of an impact. The
consequent hypothesis that the impact in some way caused the
extinctions is not in itself dependent on a specific
mechanism(s) although such were reasonably suggested. Many
counter-revolutionary papers over the last decade however
give the impression that it is those who invoke an impact
who have required a particular paleontological
significance.Yet it was never the case that an impact was
inferred and that then there was a search for associated
extinctions. It is ironical that some paleontologists chose
to downplay the significance of the boundary after the
impact was inferred. Rather than evaluate the record in the
light of an impact, they chose to construct straw men.

Some paleontologists have made an effort to show that
(stepped) extinctions were taking place up to the boundary;
they follow with the specious conclusion that the impact had
no significant effect. (Identification of relevant "stepped
extinction" requires exceptionally fine stratigraphic
resolution, and the phrase also incorporates some
significant genetic implications of its own). The impact
hypothesis however, in no way requires that all or even most
of the extinctions resulted from the impact. It does demand
that those which define the change from Mesozoic to Cenozoic
took place fairly rapidly as a result of (and therefore not
before) the impact. This demand is fulfilled in the drastic
microfaunal changes, the floral changes, and the extinction
of ammonites, belemnites, dinosaurs, and probably marine
reptiles right at the boundary. That inoceramids did not
survive as far as the boundary, or that some foraminifera
went extinct earlier, is (or may be) correct, but does not
deny the catastrophic extinction. Neither does the
persistence of genera, or even species, through the
boundary, even if they went extinct shortly after. While
there were considerable geologically immediate effects of an
impact, longer-term effects seem unavoidable. Thus Hallam's
[4] statement "..exact contemporaneity required by the
impact hypothesis..." is misleading.

Impact sceptics have also turned to demonstrating the case
for selective extinctions, particularly but not only
latitude-dependent ones, and concluding that these are more
compatible with a non-impact scenario [e.g., 5,6]. Such a
conclusion depends upon a knowledge of extinction mechanisms
in both gradual and abrupt situations, the very things
researchers are currently attempting to understand. To
suggest that gradual climatic changes and sea-level changes
caused abrupt extinctions is to divorce the direct mechanism
from the indirect one, with the same problems of
interpretation and association as the impact scenario. There
is no reason to suggest that changes created by an impact at
the end of the Cretaceous would not have affected tropical
more than high-latitude environments.

The abruptness and importance of the Cretaceous-Tertiary
boundary has been recognized from the birth of stratigraphy,
and the fundamental change from Mesozoic to Cenozoic has
come to be more tightly constrained over 180 years. Its
significance is not merely a product of dinosaur-fascination
and impact fantasy. It is a much more abrupt boundary than
that of the Paleozoic-Mesozoic. Those who are sceptical of
the impact hypothesis should not demand more of it than is
inherent in it; too many strawmen have led to confusion on
what are the relevant issues.

[1] Stanley S. M. (1987) Extinction, Scientific American
Library, 242. [2] Hay W. M. (1960) Proc. 21st Int. Geol.
Congress, Copenhagen, 5, 70-77. [3] Bramlette M. N. (1965)
Science, 148, 1696-1699. [4] Hallam A. (1989) Great
Geological Controversies, Oxford Science Publications, 244.
[5] Keller G. et al. (1993) GSA Bull., 105, 979-997. [6]
MacLeod N. and Keller G. (1993) GSA Abst. with Progs., 25,
6, A-296.


Glen W.
   Science Observed: The Mass-Extinction Debates

The upheaval triggered in 1980 by the Alvarez-Berkeley group
impact hypothesis transformed the literature of mass
extinctions from an unfocused, sporadic collection of papers
that virtually ignored extraterrestrial causes and treated
endogenous ones only sparingly better, to an integrated,
diverse body of literature. Research programs organized
seemingly overnight spawned collaborative teams whose
members (often from distant, isolated disciplines)
redirected their careers in order to address the
captivating, high-stakes issues.

The initial, generally skeptical cool reception of the
impact hypothesis might have been predicted for any of a
number of reasons: Such an instantaneous catastrophe
contravened Earth science's reigning philosophy of
uniformitarianism; it was formulated from a form of evidence
(siderophile element anomalies) alien to the community
charged with its appraisal; it advanced a causal mechanism
that was improbable in terms of canonical knowledge; and it
was proffered mainly by specialists alien to Earth and
biological science, especially paleobiology.

Early on it became clear that irrespective of which causal
hypothesis was chosen, the chosen one became the strongest
predictor of how the chooser would select and apply
standards in assessing evidence bearing on all such
hypotheses. Less strong correlation also appeared between
disciplinary specialty and choice of hypothesis, and between
disciplinary specialty and the assessment of evidence. Such
correlations varied with the level of specialization; the
most robust correlations appeared in the most restricted
subspecialties--the weakest in the broadest areas of science
practice. The gestalt or mindset seemingly engendered by the
embrace of an extinction hypothesis overrode, or was
stronger than the intellectual predispositions attributable
to disciplinary specialty.

The great majority of paleontologists rejected outright or
eschewed impact theory at its advent, and most who later
came to believe in an impact event(s) at the KT boundary
still deny impact(s) as the main extinction cause. Many
paleontologists regarded the duration, severity, and other
aspects of the KT mass extinction in terms of the fate of
their own fossil taxa at the extinction boundary;
specialists in severely affected fossil groups were most
often among those who spoke of a connection between impact
and the mass extinction. Certain subspecialty communities
within paleontology differed markedly in their opinions on
the cause of mass extinction: for example, vertebrate
paleontologists were almost unanimously opposed to impact-
as-extinction-cause; in contrast, micropaleontologists--
especially those treating planktonic calcareous forms--were
most often oppositely inclined.

Most cosmo- and geochemists, planetary geologists, impacting
specialists, and students of Earth-crossing comets and
asteroids were immediately sympathetic to impact theory in
its entirety. Study of impacting and closely related topics
appeared to foster sympathy for the impact hypothesis;
however, most volcanologists (volcanic specialists) did not
accept the volcanist hypothesis-- which was not conceived by
volcanologists--but no unusual proportion of volcanologists
favored the alternative impact hypothesis.

Published authors and published supporters of alternatives
to the impact hypothesis of mass extinction, in all cases
examined save one, were opposed to the impact hypothesis at
its advent. Irrespective of their discipline, or however
poorly informed, scientists rarely failed to embrace one of
the mass extinction hypotheses.

The use of obsolete data and/or the omission of contrary
evidence almost always punctuated published and oral
arguments, and opposing views were never treated at equal
length. The gestalts (mindsets) or cognitive frames of
members of the opposing theoretical camps seemingly
precluded mutually congruent viewpoints on any of the
important debated issues or the assessment of evidence. The
widely held view that such adversaries suffer an
"incommensurability of viewpoint" seemed understated.

Impactors (proponents of impact as extinction cause) and
volcanists (proponents of volcanism as extinction cause)
commonly used different standards of appraisal and weighted
the same evidence differently. Application of standards and
weighting of evidence varied widely even among those in the
same theoretical camp. The impactors argued mainly from
canonical standards and claimed that their hypothesis--
backed by empirical evidence--facilitated clear predictions
with implicit directions for testing. The prediction of
impact-generated, global, ballistic transport of impact
products raised expectations for many that impact evidence
in addition to iridium would be found at KT boundary sites--
this provided important impetus for the rapid formation of
diversely comprised research teams. Almost always, the
members of the same collaborative team subscribed to the
same extinction hypothesis.

Impact's opponents focused on the great range of variation
in the character of the boundary interval around the world,
emphasizing that the geographic variations indicated a cause
that was neither global nor instantaneous. It was a prime
mission of impact's opponents to demonstrate a lack of the
ubiquitous, globally uniform effects claimed for an impact.
Unlike the orthodox evidence that was advanced for the
radical impact hypothesis, impact's active opponents--mainly
the volcanists--sought to undermine a wide range of
suppositions and vagaries that lay long hidden in
established principles and methods; the volcanists searched
for the weaknesses in the orthodox standards and thus
prompted much research at unprecedented levels of
refinement. The call for higher resolution and greater
detail required the development of new, or the acquisition
of theretofore unused methods, techniques, and instruments
in a number of disciplines; such needs were often fulfilled
through the formation of appropriately composed
collaborative teams.

In the mid 1980s the postulate of stepped or multiple
extinctions near the K-T boundary evolved through a series
of recognizable stages. The evidence for extinction steps
was initially viewed as an anomaly in terms of the single-
impact hypothesis and, as such, was more or less dismissed
early on by the impactors. But it then became an
increasingly serious problem as supporting data accrued and
stepped extinctions approached the status of a normative
assumption. Ultimately, with further affirmative evidence,
the stepped-extinction idea and all it came to imply in
terms of impugning single-impact theory, evolved into a
standard of appraisal that drove the reassessment of the
nature of extinctions at the KT boundary. At that point the
newly born standard of multiple extinction steps--which had
begun life as an anomaly in terms of the single-impact
hypothesis--evolved into the primary forcing function that
forced the reformulation of the single-impact hypothesis
into a hypothesis of multiple impacts spanning enough time
to accommodate the extinction steps.

Leadership of the various factions engaged in the debates
was, in all cases, clearly in the hands of only one or very
few senior leaders who exercised magisterial authority. Such
doyens were most frequently sought for their opinions on
debated issues by both their own communities and the media;
that was reflected in both the publications of science and
the public. The rapid pace of the mass extinction debates,
with which only few could keep abreast, seemed to add to
general reliance on the magister. Such two-step
communication, from the world to the magister and then from
the magister to the world has been documented in other
studies of conflicted ideas.

Closure has not been reached on any of the many issues
reticulated in these debates, but most Earth scientists are
now convinced of at least one impact at the KT boundary and
many are inclined to think in terms of multiple impacts,
either instantaneous or spread over one to three million
years; however, far from all who subscribe to impact(s)--
especially among paleontologists--view impact(s) as the
chief cause of the mass extinction(s).

Research programs organized in the past decade to address
the many issues that have arisen in the course of this
upheaval continue to generate new publications at a
surprising rate (more than 2500 have already appeared).
Modeling of impacts and mantle plumes and their effects on a
number of internal, crustal, and biospheric processes grow
increasingly sophisticated with different research
approaches showing promising convergence in their
conclusions. The search for impact sites, which includes the
reexamination of many long-enigmatic structures, and the
remapping and dating of great flood basalt bodies is being
actively pursued. Mass extinction horizons throughout the
Phanerozoic are being scrutinized with methods, techniques
and instruments that did not exist just a decade ago, and
the broad character of the debates has forced unprecedented
interpenetrations of long isolated subdisciplines. The
debates continue, sustaining the opportunity to study the
workings of science during a time of conflict over several
related theories that span multiple disciplines. The
intensity, fast pace, and disciplinary diversity of this
upheaval has opened windows on the workings of science where
only peepholes had been expected. These conclusions and
others, and a historical overview of the debates are
detailed by me and a dozen other scientists and social
studies of science scholars in [1].

References: [1] (1994) The Mass-Extinction Debates: How
Science Works in a Crisis, Stanford Univ.


Alvarez W.
   Trajectories of Ballistic Impact Ejecta on a Rotating Earth

On an airless, slowly rotating planetary body like the Moon,
ejecta particles from an impact follow simple ballistic
trajectories. If we ignore gaseous interactions in the
fireball, ejecta particles follow elliptical orbits with the
center of the planetary body at one focus until they
encounter the surface at the point of reimpact. The partial
elliptical orbit of the ejecta particle lies in a plane in
inertial (galactic) coordinates. Because of the slow
rotation rate (for example, 360 degrees/28 days for the
Moon), the intersection of the orbital plane and the surface
remains nearly a great circle during the flight time of the
ejecta. For this reason, lunar rays, representing
concentrations of ejecta with the same azimuth but different
velocities and/or ejection angles, lie essentially along
great circles.

Ejecta from airless but more rapidly rotating bodies will
follow more complicated, curving trajectories, when plotted
in the co-ordinate frame of the rotating planet or viewed as
rays on the planetary surface. The curvature of trajectories
of ejecta particles can be treated as a manifestation of the
Coriolis effect, with the particles being accelerated by
Coriolis pseudoforces. However, it is more straightforward
to calculate the elliptical orbit in inertial space and then
determine how far the planet rotates beneath the orbiting
ejecta particle before reimpact.

The Earth's eastward rotation affects ballistic ejecta in
two ways: (1) The eastward velocity component increases the
velocity of eastbound ejecta and reduces the velocity of
westbound ejecta; and (2) the Earth turns underneath
inflight ejecta, so that although the latitude of reimpact
is not changed, the longitude is displaced westward, with
the displacement increasing as a function of the time the
ejecta remains aloft.

Low-velocity ejecta reimpacts so quickly that the second
effect is small. However, at ejection velocities above about
9 km/s and approaching escape velocity (11.2 km/s), the
semimajor axis and the related height of apogee increase
rapidly, and the time aloft may be measured in hours. In
such cases, effect (2) becomes important. For example,
ejecta particles sent eastward at an elevation angle of 45
degrees from an equatorial impact at a velocity of about 10
km/s reach an apogee of >6 Earth radii and stay aloft for
about 8 hr, during which time the Earth turns far enough for
the ejecta to reimpact close to the site from which they
were launched. Under similar conditions, but at a velocity
of 10.5 km/s, the ejecta reach an apogee of 16 Earth radii
and stay aloft for 34 hr. During that time the ejecta, seen
from the Earth, make a complete rotation around the Earth,
backward! At still higher velocities (but below escape
velocity), the ejecta make multiple retrograde rotations.

Calculations still in progress suggest that because of this
effect there may be a forbidden zone beyond about 90 degrees
east of the impact site, which cannot be directly reached by
eastbound ejecta. If so, this will make a prediction that
can be tested by mapping the size and column density of
ejecta (spherules and shocked quartz) in the KT boundary
layer.

Interactions between the atmospheric density gradient and
the expanding fireball of a large impact can accelerate
ejecta to velocities approaching and probably exceeding
escape velocity. Thus the effects described above are
relevant. The time-aloft difference between low- and high-
velocity ejecta may explain the double KT layer in the
western interior U.S. (a layer of altered melt ejecta
overlain by a layer of shocked quartz, both of which appear
to derive from Chicxulub). However, there are serious
problems in explaining the failure of the shocked quartz (1)
to melt and (2) to disperse, if it is interpreted as fast,
high-trajectory ejecta.

The Earth's atmosphere complicates the calculation of ejecta
trajectories in at least four ways: (1) At early stages in
the evolution of the fireball, temperatures are sufficient
to form a plasma, and electromagnetic interactions within
the plasma distort the simple inertial trajectories that
ejecta would otherwise follow. (2) Even after expansion of
the fireball cools it to below plasma temperatures, thermal
encounters in the fireball gases continue to distort the
ballistic trajectories. (3) Low-velocity portions of the
fireball may fall back close to the impact site, forming a
base surge that expands radially on top of the normal
atmosphere. (4) As ejecta particles settle through the
atmosphere they will be carried laterally by winds for
distances inversely related to the particle size.

The calculation of ballistic trajectories is an essential
step in understanding ejecta distributions, but must be
augmented by a fuller understanding of fireball-atmosphere
dynamics under conditions that are difficult to calculate
and have not been observed. The observational limitation may
be partially overcome when the fragments of Comet Shoemaker-
Levy impact Jupiter in July 1994.





                          Saturday, February 12, 1994
                                PUBLIC LECTURES
                           7:00 PM     UH Auditorium


Russell D. A.*
   Dinosaurs Within the History of Multicellular Organisms

Since the appearance of multicellular organisms about 600
Myr ago [1], various descendent groups separated into
temporally overlapping assemblages ("evolutionary faunas")
and their diversifications often approximated exponential
functions of time [2-11]. Exponentiality suggests that the
diversifications were quasi-autocatalytic or in part
biologically driven. Beginning (presumably) with a single
species 3800 Myr ago, there are now about 10 million species
living on our planet [12]. These points, fitted to an
exponential curve, imply that half of the modern number of
species was present on our planet during the dinosaurian
era.

Although the identification of "progressive" trends in the
history of life has long been controversial [13], some have
argued that living systems became more complex [14] and that
locomotor and feeding efficiency and activity levels have
increased in animals through geologic time [15]. These
"improvements" could be linked to the relative competitive
fitness of members of the more modern "evolutionary faunas"
[16- 17], and evidently enhanced probabilities of surviving
mass extinctions as well [5]. Among backboned animals, an
increase in activity levels (metabolic rates) over the past
150 Myr is reflected in the projected food requirements (per
unit time and body mass) of typical dinosaurian and modern
mammalian herbivores [18]. The mass of the central nervous
system relative to that of the body in backboned animals has
also increased in a quasi-exponential manner through
geologic time. Brain size in dinosaurs conformed to the
general trend [19-20].

In a dynamic sense, the relative fitness conferred by
"progressive" attributes should be reflected in the
competitive success of organisms possessing them. Organisms
that are known to have extended their distributions are
characterized by their relatively superior competitive,
defensive, and reproductive performances, and their
origination within areas of relatively greater diversity
[3,5,21-24]. Thus, the success of Asian placental mammals in
invading Australia may be facilitated by the generally lower
life-time fecundities, metabolic rates and smaller brains of
indigenous Australian mammals [23, 25-26]. Dinosaurs would
in all probability be even less able to resist modern Asian
invaders than are living Australian mammals.

These patterns imply that the primitive metazoan biosphere
was characterized by a low diversity of sluggish animals
capable of only limited and stereotyped behavior and the
modern ("advanced") biosphere by a high diversity of
animals, many of which are very active and capable of a
large variety of behavioral responses. The biosphere during
the dinosaurian era probably lay near the midpoint of these
two extremes.

Biodiversity is known to be linked to various physical
variables over broad areas of the Earth's surface, including
area [27-28], temperature [29-30], solar radiation,
atmospheric warmth, availability of water and plant fodder
[28], and atmospheric CO2 levels [31]. As well as being
autocatalytic, diversity trends should reflect broad-scale
temporal changes in the physical environment of the
planetary surface. Indeed, downward "spikes" in diversity
caused by mass extinctions were immediately followed by
logistic rebounds, revealing the operation of non-biogenic
factors modulating diversity [4,7]. When exponential curves
are fitted to points at the extremes of family-group
diversity trends (e.g., marine animals [8, Fig. 4],
terrestrial plants [9, Fig. 6B] and terrestrial vertebrates
[11, Fig. 9.4B]), it is evident that the trends often
broadly departed from exponentiality (departures are larger
with linear fits). Brief "hyperexponential bursts" of
diversity due to biological innovation [4], and long-term
compensatory interactions between a warming Sun and
declining greenhouse gases [32] are probably unrelated to
these departures.

In the case of marine animals, an early interval of maximal
diversification coincided with the absence of polar ice, an
expansion of shallow seas over the continents [33] and very
high atmospheric CO2 levels [34] (the latter limit the
growth of marine phytoplankton today [35]). The succeeding
negative trend coincided with a reduction in the area of
epicontinental seas, return of high latitude glaciation, and
rapid fall in CO2 levels. It is perhaps relevant that marine
organisms with calcareous skeletons, which are preserved in
abundance in the fossil record, become less diverse in
progressively cooler waters [36]. During the negative marine
interval, complex ecosystems were becoming well established
on land. Near its end, animals of terrestrial origin were
invading the seas, suggesting that plant productivity and
organismal diversity had become relatively greater on land,
as they are today [7,37].

In terrestrial environments, plant diversity stagnated over
a long period during which the diversity of marine
invertebrates was again increasing. This period coincided
with a time when most of the land areas of the Earth were
united into a giant supercontinent and arid environments
were extreme and widespread [33]. Plant diversity and
productivity were evidently concentrated in higher latitudes
instead of the broader equatorial region [38-40]. The
general pattern of diversification in terrestrial
vertebrates resembled that in terrestrial plants in
conformity with their dependence upon plant productivity.
The most recent phase of maximal diversification, in which
dinosaurs participated [41], began during the latter part of
the dinosaurian era [41] and continued to the present. The
trend in brain size appears to reflect diversity of
terrestrial organisms, increasing little through much of the
dinosaurian era [19]. It thus appears to be a function of
the diversity of inter-organismal reactions.

Insect diversification does not show the prominent
departures from exponentiality present in the three previous
cases. One relatively minor negative departure coincides
with the arid interval encompassing most of the dinosaurian
era, and another with the origin of small insectivorous
birds [42]. Dinosaurs belonged to a biosphere that was much
simpler than that of the present; similarities to relatively
hyperdiverse, hyperactive, and hypercerebral modern
terrestrial vertebrates should not be overemphasized. During
early Mesozoic time growth in diversity in terrestrial
environments appears to have been constrained by heat and
aridity. However, with the return of more mesic conditions
in late Mesozoic time, dinosaurs increased in diversity,
activity and behavioural complexity, and decreased in size,
thereby paralleling birds and mammals. The mass extinction
produced by the terminal Mesozoic bolide impact did not
greatly impede the general trend toward greater diversity
[6-7]. However, dinosaurs were apparently exterminated as a
result of their dependance on living plant tissues [43].
Acid rains generated by the impact were evidently more
injurious to plant growth than thermal excursions or a brief
interval of darkness [44]. The relative importance of
selective pressures toward greater organismal
diversification, metabolic rates and behavioral complexity
through geologic time suggests the utility of broad
interests, industriousness, and thought to our own species.

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Horner J.*
   To Be Determined

No abstract available.