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Except where underlain by the Sixtymile Formation, Cambrian Tapeats Sandstone is typically the lowest geologic unit, about 230 feet thick, at its maximum, of the 5-member Tonto Group. It is famous for being the highly-resistant mostly-horizontal unit above the Great Unconformity expressed areally in the Grand Canyon of Arizona; also in other areas of Arizona and adjacent Nevada.
The Tapeats Sandstone is the highly erosion-resistant unit laid upon the Vishnu Basement Rocks in the central, parts of east, and parts of west Grand Canyon, Arizona. The unit comprises the 'base horizontal unit' of the platform around Granite Gorge on the Colorado River, and because of its hardness, creates the Tonto Platform upon which the slope-forming, Bright Angel Shale, , and above which low cliffs of Muav Limestone lie.
The Tapeats unit is the basal unit of a typical marine transgression series of sandstone--shale-limestone, all part of a paleo sea, initially adjacent to land, the source of the Tapeats rocks- and sand;. The Tapeats Sea at the late Cambrian ceased to deposit more Muav Limestone, and a period of erosion ensued, a deposition unconformity.
The Tapeats Sandstone was laid upon the Vishnu Basement Rocks, after an unconformity of erosion, the Great Unconformity. Besides the erosion unconformity, the Grand Canyon Supergroup of the basement rocks are also at an angular unconformity, being an 7-member sequence tilted at 45 degrees.