Bright Angel Shale
| Bright Angel Shale | |
|---|---|
| Stratigraphic range: Middle Cambrian[1][2] | |
 Muav Limestone-(dk or lt gray, brown, or orange-red subcliff at base of Redwall Limestone cliff) and Bright Angel Shale (greenish slope-former), resting on Tapeats Sandstone (short, dp brown vertical cliff) (Tapeats forms the "Tonto Platform"), inner canyon, Granite Gorge. (The 3 units–Muav, Bright Angel, and Tapeats, are easily seen below the red-stained Redwall Limestone (550 ft thick)) | |
| Type | Geological formation |
| Unit of | Tonto Group[3] |
| Underlies | Muav Limestone |
| Overlies | Tapeats Sandstone |
| Thickness | 500 feet (150 m) at its maximum |
| Lithology | |
| Primary | micaceous siltstone and shale |
| Other | sandstone and glauconitic sandstone |
| Location | |
| Region | northern Arizona, southeast California, southern Nevada, and southcentral Utah |
| Country | United States of America |
| Type section | |
| Named for | Bright Angel Canyon, Bright Angel quadrangle, Coconino Co., Arizona[4] |
| Named by | Noble (1914)[4] |
The Cambrian Bright Angel Shale is the middle layer of the three member Tonto Group geologic feature. The 3-rock Tonto section famously sits upon the Great Unconformity because of the highly resistant cliffs of the base layer, vertical Tapeats Sandstone cliffs.
The Bright Angel Shale is easily identified for two reasons. Its soft-greenish color stands out against the browns, reds, and whites of neighboring rock units. And secondly for its slope-forming character against mostly cliff-forming resistant rocks.
The Bright Angel Shale is about 500 feet (152 m) thick at its maximum.[5] It is a nonresistant slope-forming unit. The Bright Angel Shale consists of green and purple-red, siltstone and shale which is interbedded with red-brown to brown sandstone that is similar in lithology to the underlying Tapeats.[6] The Bright Angel Shale underlies and interfingers with Muav Limestone. The Bright Angel Shale is located in the lower elevations of the Grand Canyon, Arizona.[7] The Bright Angel Shale preserves fossils dating back to the Cambrian Period.[8]
The three units of the Tonto Group and the colorful Bright Angel Shale are easily identified as a geological sequence beneath the tall cliffs of the Redwall Limestone (the Redwall sits upon a short resistant cliff of Muav Limestone); the Tonto Group is also easily seen beside Granite Gorge of the Colorado River and the Vishnu Basement Rocks
The units of the Tonto Group:[5]
- Redwall Limestone
- Temple Butte Formation, Devonian-(409-363 Ma), channel deposits upon Muav Limestone
- Tonto Group (~544-505 Ma)
See also
- Geology of the Grand Canyon area
- List of fossiliferous stratigraphic units in Arizona
- Paleontology in Arizona
References
- ^ 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.
- ^ "Tonto Lexicon entry". National Geologic Map Database Lexicon. United States Geological Survey. n.d. Retrieved 2 June 2019.
- ^ a b L. F. Noble (1914). "The Shinumo quadrangle, Grand Canyon district, Arizona" (PDF). U.S. Geological Survey Bulletin. 549. doi:10.3133/B549. ISSN 8755-531X. Wikidata Q57659039.
- ^ a b Chronic, Halka (1983). Roadside Geology of Arizona. Seattle, Washington: The Mountaineers Books. ISBN 978-0-87842-147-3.
- ^ "Bright Angel Lexicon entry". National Geologic Map Database Lexicon. United States Geological Survey. n.d. Retrieved 2 June 2019.
- ^ Hampton, HM (1998). "Geologic Map of the Grand Canyon in the Vicinity of the South Rim Visitor Center". In Kamilli, Robert J.; Richard, Stephen M. (eds.). Geologic Highway Map of Arizona. Arizona Geological Society and Arizona Geological Survey. ISBN 978-1-8919-2400-2., 1 sheet, scale 1:62,500.
- ^ Various Contributors to the Paleobiology Database. "Fossilworks: Gateway to the Paleobiology Database". Archived from the original on 31 July 2014. Retrieved 8 July 2014.
{{cite web}}:|author=has generic name (help) - ^ "Muav Lexicon entry". National Geologic Map Database Lexicon. United States Geological Survey. n.d. Retrieved 2 June 2019.
- ^ "Tapeats Lexicon entry". National Geologic Map Database Lexicon. United States Geological Survey. n.d. Retrieved 2 June 2019.
Further reading
- Blakey, Ron and Wayne Ranney, Ancient Landscapes of the Colorado Plateau, Grand Canyon Association (publisher), 2008, 176 pages, ISBN 978-1934656037
- 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.
- Lucchitta, Ivo, Hiking Arizona's Geology, 2001, Mountaineers's Books, ISBN 0-89886-730-4
- 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.
- 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.