The Tapeats Sandstone

on the Mogollon Rim

The Tapeats sandstone is lower Cambrian in age, and belongs to the Tonto Group which includes the Muav Limestone, and Bright Angel Shale found together in the Grand Canyon. However, the Tapeats alone outcrops for great distances along the base of the Mogollon Rim. Appearing as a browinish to light tan in color, it contains almost shaley sandstones up to coarse conglomerate. Much crossbedding occurs in the Tapeats, because it is a tidal marine deposit, and includes the deposits right up to the shorline, but not beach dunes or eolian deposits. The material for the Tapeats was derived from the underlying Precambrian granites and metamorphic rocks found in the region. It is easy to see how a pounding marine surf can separate out the constituents of such basement rocks, and form beach sands and highly winnowed tidal deposits. The Tapeats is between 500 and 550 My old, and contains a few diagnostic trace fossils such as Correphioides and Diplocraterion over the Mogollon Rim, and some trillobite trackways in the Grand Canyon.


Resting unconformably on the 2 billion year old granite, the Tapeats is seen here as a darker layer on top of the ligheter tan igneous rock at the base. This photograph was taken on Highway 87, at the crossing of the East Fork of the Verde River.

Also taken at the Verde River outcrop, the overhanging Tapeats ledge seen here contained the trace fossil Skolithos, vertically burrowing worm like creatures that are index fossils to the lower Cambrian. The Tapeats here consists of very coarse to fine massive and cross bedded sandstones, with flat deposited layers from tidal deposits that contain the Skolithos. Dimples from sea anenomies have also been found here, forming in small groups.

Close up of the Tapeats at the same Verde River outcrop, showing crossbedding from underwater marine dunes that are interbedded with corse flat bedded conglomerates from shore line wave deposits.

This image shows the relationships over most of northern Arizona of The Precambrian through Devonian contacts. It was taken from Highway 87, and The East Fork of the Verde, just north of Payson. On the bottom, the pink 1.8 By precambrian granite forms a firm foundation for the sedimentary layers above. Lying nonconformably upon the granite is the Cambrian Tapeats Sandstone, with its lenslike lithology evident. At this location, few subareal crossbeds are present, if any, indidating a deposit that was completely within the tidal zone, complete with Skolithos trace fossils. The grey limestone ledges above the Tapeats are the Devonian Martin formation, which lies unconformably on the Tapeats.

Another view from East Fork of the Verde, a closer view of the Martin on top of the Tapeats. At this boundry exactly, we have seen arthrodire plates (primitive fish) occasionally, some actually formed into the sandstones upper inch or so. This has caused much confusion over the years as to the exact age of the Tapeats, which at one point was considered Devonian on the upper most layers. But how can devonian fossils be found actually pressed into the upper layers of solid cambrian rock? Reworking is the answer. The top few inches of the Tapeats was reworked back into sand during the devonian, and some fish material was preserved in it as it relithified.

Seen from Flowing Springs road, just off of Highway 87 north of the town of Payson, the grandeur of the Tapeats can be fully appreciated. Like a little grand canyon, the East Fork of the Verde cut into the surrounding rock, forming these elegant outcrops for geologists to enjoy! Note how the Devonian Martin formation, above the Tapeats quickly changes from a ledge forming limestone, to a slope forming shale above.

Just down the road from the above shot, the bold cliffs of the Devonian Martin Formation lies unconformably upon the Tapeats. The arthrodire plates lie in the flat ledge just above the Tapeats, at thier contacts.

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