The Great Unconformity

GRAND CANYON - THE GREAT UNCONFORMITY

It is fitting that the Grand Canyon should contain some of the best exposures of The Great Unconformity -- the gap in the rock record between Cambrian times (~550 m.y. ago) and the pre-Cambrian (anything earlier). An unconformity is a surface in the rock record, in the stratigraphic column, representing a time from which no rocks are preserved. It could represent a time when no rocks were formed, or a time when rocks were formed but then eroded away. Small unconformities are ubiquitous in the rock record -- for instance, in the Coconino sandstone, every cross-bed surface represents a small unconformity. The Great Unconformity is important for three reasons:

it represents a long span of time -- 250 to 1200 million years in the Grand Canyon;
it is found nearly everywhere across the globe; and
it divides rocks with familiar fossils from those with no fossils or only fossil bacteria.


View from Ooh-ah point, west into Granite Gorge. The Great Unconformity (the red line) is the surface between the Tapeats sandstone (Cambrian age) and the metamorphic rocks of Granite Gorge. These are Karl's beat-up "happy" rocks. They were originally formed 1.85-1.65 billion years ago as several complex sets of sedimentary and volcanic rocks, were metamorphosed to schist and gneiss by burial and heating by granites at ~1.4 billion years, and then cooled and exhumed to form a flat, rocky plain at about 550 million years. That was when the ocean encroached and deposited beach sands as the Tapeats sandstone. For an unannotated image, click here. Credit.		Closeup of the Great Unconformity, Tapeats sandstone of Cambrian age (~550 m.y. old) deposited on top of preCambrian metamorphic rocks. They are commonly called 'Vishnu Schist,' but K. Karlstrom and other have shown that they are really a mess of many different original rock types of several different ages. Right of center is a vertical white band, which is a granitic intrusion that cut through the Vishnu rocks and was folded with them. Follow it upwards towards the Tapeats sandstone, where it just ends. This is a "cross-cutting relationship", which is nearly proof that the "cross-cut" rock unit is older. For an unannotated image, click here. Credit.

Just east of the scenes above, the Great Unconformity becomes deliciously complicated. A new set of rocks enters between the 'Vishnu' and the Tapeats -- the Unkar group of late preCambrian sedimentary rocks. In this scene are three of its formations: the Bass limestone, the Hakatai shale, and the Shinumo quartzite. These rocks were deposited at about ~1.1 billion years ago, ~600 million years after the Vishnu, and ~550 million before the Tapeats. Here, their western boundary is a fault (purple line with direction arrows), and they tilt down to the east (right). The Great Unconformity (red line) is above the Unkar Group, which is lies above a 'pretty good' unconformity (purple line) at the top of the Vishnu. To the west (left), the Tapeats is above the Great Unconformity. But, starting at the center mid-distance, the Tapeats thins to the east (right) and vanishes against the Unkar Group. The Shinumo quartzite pokes through the Tapeats and forms cliffs at the same elevation as the lower part of the Bright Angel shale (background center). In early Cambrian times, this Shinumo quartzite stood as a hill above the Tapeats sandy beach and the Bright Angel muds, and was finally buried in the mud as the sea level rose. For an unannotated image, click here. Credit.

Seen from Desert Tower, the Great Unconformity is the boundary between the flat-lying Tapeats sandstone (and overlying Paleozoic rocks) and tilted rocks of the Grand Canyon Supergroup (GCS). The Great Unconformity here, marked in red, cleanly cuts off the tops of the tilted GCS rocks, which were deposited between ~1,200 and ~800 million years ago. Here, the Great Unconformity is angular, meaning that there is a distinct angle between the layers of the older and younger rocks. The The Grand Canyon Supergroup includes rocks of the Unkar group (see above photo), the Nankoweap formation, and the Chuar group. Between this photo and the last, we saw all the formations of the Unkar group: the Bass limestone, the Hakatai shale, the Shinumo quartzite, the Dox formation (red sandstones and shales, similar to the Supai Group), and the Cardenas lavas (altered basalt flows). The Nankoweap formation is a distinctive striped set of shales and sandstones, which sit unconformably on the Cardenas lavas - possible fossil jellyfish have been collected from the Nankoweap. The Chuar group, which we saw only in the distance, includes rocks from many different near-shore environments. For an unannotated image, click here. Credit.

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