Bright Angel Shale

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.

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))
TypeGeological formation
Unit ofTonto Group[3]
UnderliesMuav Limestone
OverliesTapeats Sandstone
Thickness500 feet (150 m) at its maximum
Lithology
Primarymicaceous siltstone and shale
Othersandstone and glauconitic sandstone
Location
Regionnorthern Arizona, southeast California, southern Nevada, and southcentral Utah
CountryUnited States of America
Type section
Named forBright Angel Canyon, Bright Angel quadrangle, Coconino Co., Arizona[4]
Named byNoble (1914)[4]

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]

Bright Angel Shale sites
Relative long distance photo of Isis Temple (adjacent prominence is Tiyo Point, of the North Rim)
The entire Tonto Group 3-unit sequence is visible above the Colorado River (side intersecting canyon to Granite Gorge)
(From Komo Point)-Sloping Supai Group redbeds upon Redwall Limestone cliffs and horizontal platforms on the Muav Limestone on greenish Bright Angel Shale

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]

See also

 Paleozoic portal

References
  1. Rose, E (2006) "Nonmarine aspects of the Cambrian Tonto Group of the Grand Canyon, USA, and broader implications." Palaeoworld. 15:223–241.
  2. 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.
  3. "Tonto Lexicon entry". National Geologic Map Database Lexicon. United States Geological Survey. n.d. Retrieved 2 June 2019.
  4. Levi F. Noble (1914), "The Shinumo quadrangle, Grand Canyon district, Arizona", U.S. Geological Survey Bulletin, 549, doi:10.3133/B549, Wikidata Q57659039
  5. Chronic, Halka (1983). Roadside Geology of Arizona. Seattle, Washington: The Mountaineers Books. ISBN 978-0-87842-147-3.
  6. "Bright Angel Lexicon entry". National Geologic Map Database Lexicon. United States Geological Survey. n.d. Retrieved 2 June 2019.
  7. 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.
  8. Various Contributors to the Paleobiology Database. "Fossilworks: Gateway to the Paleobiology Database". Archived from the original on 31 July 2014. Retrieved 8 July 2014.
  9. "Muav Lexicon entry". National Geologic Map Database Lexicon. United States Geological Survey. n.d. Retrieved 2 June 2019.
  10. "Tapeats Lexicon entry". National Geologic Map Database Lexicon. United States Geological Survey. n.d. Retrieved 2 June 2019.

Further reading
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