Basic Stratigraphy of Barringer Meteor Crater

The rock layers of Northern Arizona that were disrupted by the impact are called sedimentary rocks. Sedimentary rocks are made of particles that have been transported by wind and water, or precipitated from water. Sedimentary rocks include, among other types: sandstones, shales, and limestone. The sediments that make up these rocks were originally deposited in horizontal layers.

The rock layers of northeastern Arizona that were affected by the impact event include:

Moenkopi Formation: The Moenkopi consists primarily of a thinly bedded brownish-red mudstone. The Moenkopi sediments were likely deposited on a coastal floodplain, similar to modern Louisiana. Rocks in the Moenkopi also contain ripple marks. When sand or silt settles in still water, it makes a featureless flat bed. But in a moving current, such as a stream, the sediment will form asymmetrical ripple marks.

(Above) An example of ripple marks in the Amole Arkose,
an unassociated rock unit in southern Arizona.

These ripples are sometimes preserved in sedimentary rocks and can tell geologists about the current direction of the ancient rivers and streams that deposited them. The Moenkopi is from the Triassic Period and was deposited over 200 million years ago.

Kaibab Formation: Predominately dolomite. Dolomite is a carbonate rock similar to limestone but with less calcium. The Kaibab also contains reddish mudstones and sandstones. The Kaibab represents a period of time (Permian) when the majority of Arizona was covered beneath a shallow sea.  Fossil shells are abundant in the Kaibab, as are the preserved burrows of marine organisms that lived and fed in the sea-bottom sediments. The Kaibab was deposited in a low-energy marine environment over 250 million years ago.

Toroweap Formation:  Predominately limestone, but with significant amounts of yellow sandstone and reddish mudstone. The limestone of the Toroweap formed on the floor of a shallow sea that migrated in from the west. The sandy portions represent a fluctuating ancient shoreline of western North America during the Permian, over 255 million years ago. The Toroweap sedimentary rocks are thin and do not appear in some parts of northern Arizona.

Coconino sandstone: This rock unit is primarily composed of an eolian (windblown) sandstone. The high-angle cross-bedding (fossilized sand dune slopes) in the Coconino suggests that these rocks are the remains of a huge dune field from over 265 million years ago (Permian). At that time, the region was covered by a desert similar to the modern Sahara.

(Above) Cross-bedding preserved in the shocked Coconino sandstone at Meteor Crater.

The graphic below (click to enlarge) is a colorized version of Gene Shoemaker's geologic map of Meteor Crater. The colors correspond to the rock units listed above (legend on right of map). The tans and greys represent young rocks and sediments (such as top soil), while the blues and greens are the very old (Paleozoic) rocks listed above (Moenkopi, Kaibab etc.). These older rock units are exposed near the crater rim, and would also be visible near the center of the crater were it not for slumping, erosion and the deposition of lake sediments (see below), that have covered the crater floor since the impact occurred thousands of years ago. These surficial (erosional) processes are what cover or destroy the remains of many of Earth's craters. Barringer crater is well preserved because of its young age and because the modern climate in northern Arizona is relatively arid.  Without much water, erosion occurs slowly.

Click here for a full-sized version of Gene Shoemaker's Meteor Crater geological map.

The diagram below shows the stratigraphy reshaped by the crater. Lake sediments found in the crater bottom indicate that the crater filled with water creating an impact-generated lake and a freshwater habitat. Conditions in northern Arizona are currently too arid for such a lake, but during the Pleistocene, 50,000 years ago, the climate was colder and wetter. The area labeled "breccia" consists primarily of angular pieces of sedimentary rocks that were shattered during the impact.

The  sequential layers of sedimentary rock described above are called strata (plural) and each stratum is older than the strata that rest on it and younger than the strata below.

Sometimes compressional or extensional forces caused by plate movements can cause these originally horizontal layers to tilt or fold. In some cases, these tectonic forces can even fold the rock layers back on themselves to such a degree that they are inverted or overturned. An impact event also has the potential to overturn strata locally. The high energy explosion can eject large amounts of material out of the crater, in some cases preserving stratigraphic relationships - but in an inverted sequence, as is the case in the rim of Meteor Crater (See diagram below). Notice that the normal undisturbed sequence has the Coconino (oldest) at the bottom, followed by the Toroweap, Kaibab and Moenkopi (youngest) as you move upwards. In the overturned rocks near the crater, this sequence is repeated above the Moenkopi, but in a reverse (overturned) order. The ejected and overturned material extends 1 to 2 km from the crater.