The CambrianTapeats Sandstone is the lower geologic unit, about thick, at its maximum, of the 3-member Tonto Group. It is famous for being the highly-resistant mostly-horizontal unit above the 1,000 my, 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 it low cliff-forming cliff of Muav Limestone lie. The Tapeats Sandstone and the Tonto Platform follow the Colorado River, and surrounding tributary canyons, creeks, watercourses, or washes, in a dendritic fashion, much like the branches of a tree. The Tapeats unit is the bottom member of the Tonto Group, 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 early Devonian 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, of 1,000 million years. Besides the erosion unconformity, the Unkar Group of the basement rocks are also at an angular unconformity, being an 8-member sequence tilted at 45 degrees. The horizontal Tonto Platform has hiking trails that cross it from the South Rim to North Rim, Grand Canyon for instance. The extensive Tonto Trail lies on parts of the Tapeats Sandstone, and the platform on the south side of Granite Gorge.
Geologic sequence
The units of the Tonto Group:
3--Muav Limestone
2--Bright Angel Shale
1--Tapeats Sandstone
Gallery Tapeats Sandstone
Examples of the deposition unconformity of time, the Great Unconformity. The Tapeats approximately 200 ft thick.
