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 brownish 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 shoreline, 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 are between 500 and 550 My old and contain a few diagnostic trace fossils such as Correphioides and Diplocraterion over the Mogollon Rim, and some trillobite trackways in the Grand Canyon.


From Wikipedia

Tonto Group
When the ocean started to return to the area 550 million years ago in the Cambrian, it began to concurrently deposit the three formations of the Tonto Group as the shoreline moved eastward:

Tapeats Sandstone (averages 545 million years old) - This formation is made of cliff-derived medium- to coarse-grained sand and conglomerate that was deposited on an ancient shore (see 3a in figure 1). Ripple marks are common in the upper members of this dark brown thin-bedded layer. Fossils and imprint trails of trilobites and brachiopods have also been found in the Tapeats. Today it is a cliff-former, 250 to 300 feet thick.

Bright Angel Shale (averages 530 million years old) - Bright Angel is made of mudstone shale interbedded with small sections of sandstone and shaly limestone with a few thin beds of dolomite. It was mostly deposited as mud just offshore and contains brachiopod, trilobite, and worm fossils. The color of this formation is mostly various shades of green with some brownish-tan to gray parts. It is a slope-former, 325 to 400 feet thick.

Muav Limestone (averages 515 million years old) - The Muav is made of gray thin-bedded limestone that was deposited further offshore as calcium carbonate precipitates. It is fossil poor yet trilobites and brachiopods have been found in it. The western part of the canyon has a much thicker sequence of Muav than the eastern part. The Muav is a cliff-former, 250 to 375 feet thick.

These three formations were laid down over a period of 30 million years from early to middle Cambrian time. Fossils of trilobites and burrowing worms are common in these formations. We know that the shoreline was transgressing (advancing onto land) because finer-grade material was deposited on top of coarser-grained sediment. Today the Tonto Group makes up the Tonto Platform seen above and following the Colorado River with the Tapeats Sandstone and Muav Limestone forming cliffs, and the Bright Angel Shale forming slopes. Unlike the Proterozoic units below it, the Tonto Group's beds basically lie in their original horizontal position. The Bright Angel Shale in the group forms an aquiclude (barrier to groundwater seeping down), and thus collects and directs water through the overlying Muav Limestone to feed springs in the Inner Gorge.