Stratigraphic range: Early to Middle Cambrian
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)
|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|
|Region||northern Arizona (Grand Canyon), central Arizona, southeast California, southern Nevada, and southeast Utah|
|Country||United States of America|
|Named for||Tapeats Creek|
|Named by||Noble (1914)|
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 two slope-forming, highly erodable units, the Muav Limestone, and Bright Angel Shale 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.
Close-up of Tapeats Sandstone on the Great Unconformity
The units of the Tonto Group:
- Rose, E (2006) "Nonmarine aspects of the Cambrian Tonto Group of the Grand Canyon, USA, and broader implications." Palaeoworld. 15:223–241.
- Rose, E (2011) Modification of the nomenclature and a revised deposition model for the Cambrian Tonto Group of the Grand Canyon, Arizona. in JS Hollingsworth, FA Sundberg, and JR Foster, eds., pp 77-98, Cambrian Stratigraphy and Paleontology of Northern Arizona and Southern Nevada: Museum of Northern Arizona Bulletin 67, 321 p.
- Noble, LF (1914) The Shinumo quadrangle, Grand Canyon district, Arizona. Bulletin no. 549, US Geological Survey, Reston, Virginia.
- Chronic, H (1983) Roadside Geology of Arizona. The Mountaineers Books, Seattle, Washington. (softcover, ISBN 978-0-87842-147-3) p. 179.
- 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
- Anonymous (2011a) Tonto Group, Stratigraphy of the Parks of the Colorado Plateau. U.S. Geological Survey, Reston, Virginia.
- Anonymous (2011b) Tapeats Sandstone, Stratigraphy of the Parks of the Colorado Plateau. U.S. Geological Survey, Reston, Virginia.
- Anonymous (2011c) Bright Angel Shale, Stratigraphy of the Parks of the Colorado Plateau. U.S. Geological Survey, Reston, Virginia.
- Anonymous (2011c) Muav Limestone, Stratigraphy of the Parks of the Colorado Plateau. U.S. Geological Survey, Reston, Virginia.
- Brandriss, M. (2004) Angular unconformity between Proterozoic and Cambrian rocks, Grand Canyon, Arizona. GeoDIL, A Geoscience Digital Image Library, University of North Dakota, Grand Forks, North Dakota.
- Mathis, A., and C. Bowman (2007) The Grand Age of Rocks: The Numeric Ages for Rocks Exposed within Grand Canyon, Grand Canyon National Park, Arizona., National Park Service, Grand Canyon National Park, Arizona.
- Rowland, S. (nda) Frenchman Mountain Great Unconformity site. Department of Geoscience, University of Nevada, Las Vegas, Nevada.
- Rowland, S. (ndb) Geologic Map of Frenchman Mountain. Department of Geoscience, University of Nevada, Las Vegas, Nevada.
- Rowland, S. (ndc) Frenchman Mountain and the Great Unconformity. Department of Geoscience, University of Nevada, Las Vegas, Nevada.
- Share, J. (2102a) The Great Unconformity of the Grand Canyon and the Late Proterozoic-Cambrian Time Interval: Part I - Defining It.
- Share, J. (2102a) The Great Unconformity and the Late Proterozoic-Cambrian Time Interval: Part II - The Rifting of Rodinia and the "Snowball Earth" Glaciations That Followed.
- Timmons, M. K. Karlstrom, and C. Dehler (1999) Grand Canyon Supergroup Six Unconformities Make One Great Unconformity A Record of Supercontinent Assembly and Disassembly. Boatman's Quarterly Review. vol. 12, no. 1, pp. 29–32.
- Timmons, S. S. (2003) Learning to Read the Pages of a Book (Grand Canyon Geology Training Manual), National Park Service, Grand Canyon National Park, Arizona.
|Wikimedia Commons has media related to Tapeats Sandstone.|
Examples of the deposition unconformity of time (550-800 million yrs), the Great Unconformity. The Tapeats approximately 200 ft thick.
Gallery—Great Unconformity and angular unconformity
Examples of the angular unconformity below the Tapeats Sandstone, (approx. 15 degrees).