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Tapeats Sandstone
Stratigraphic range: Early to Middle Cambrian
Grand Canyon-Mather point.jpg
Muav Limestone-(greenish, slope-forming) and Bright Angel Shale, resting on Tapeats and the Tonto Platform, inner canyon, Granite Gorge (the two units are easily seen below the red-stained Redwall Limestone) (550 ft thick)
Type Geological formation
Unit of Tonto Group
Underlies Bright Angel Shale
Overlies Vishnu Basement Rocks, Unkar Group, Nankoweap Formation, Chuar Group, and Sixtymile Formation
Thickness 230 feet (70 m)
Primary sandstone and conglomerate
Other conglomeratic sandstone
Region northern Arizona (Grand Canyon), central Arizona, southeast California, southern Nevada, and southeast Utah
Country United States of America
Type section
Named for Tapeats Creek
Named by Noble (1914)

The Cambrian Tapeats Sandstone is the lower geologic unit, about 230 feet (70 m) 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 (1 by), 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 (Inner Gorge) on the Colorado River, and because of its hardness, creates the Tonto Platform upon which the slope-forming, Bright Angel Shale, (extensive, and distinctively soft drab-greenish), and above which low cliffs 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-(conglomerate)-shale-limestone, all part of a paleo sea, initially adjacent to land, the source of the Tapeats rocks-(conglomerates) and sand; (a regressing sequence has the reverse order). 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 (1.0 billion). 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:

Gallery Tapeats Sandstone

Examples of the deposition unconformity of time (550-800 million yrs), the Great Unconformity. The Tapeats approximately 200 ft thick.

Popular Publications

  • Blakey, Ron and Wayne Ranney, Ancient Landscapes of the Colorado Plateau, Grand Canyon Association (publisher), 2008, 176 pages, ISBN: 978-1934656037
  • Chronic, Halka. Roadside Geology of Arizona, Mountain Press Publishing Co., 1983, 23rd printing, pp. 229–232, ISBN: 978-0-87842-147-3
  • Lucchitta, Ivo, Hiking Arizona's Geology, 2001, Mountaineers's Books, ISBN: 0-89886-730-4
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