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The '''Bright Angel Shale''' is one of five [[geological formation]]s that comprise the [[Cambrian]] [[Tonto Group]]. It and the other formations of the Tonto Group [[outcrop]] in the Grand Canyon, Arizona, and parts of northern [[Arizona]], central Arizona, southeast [[California]], southern [[Nevada]], and southeast [[Utah]]. The Bright Angel Shale consists of often fossiliferous, green and red-brown, micaceous, fissile shale (mudstone) and siltstone with some thicker beds of brown to tan sandstone and dolostone. It ranges in thickness from {{convert|57 to |
The '''Bright Angel Shale''' is one of five [[geological formation]]s that comprise the [[Cambrian]] [[Tonto Group]]. It and the other formations of the Tonto Group [[outcrop]] in the Grand Canyon, Arizona, and parts of northern [[Arizona]], central Arizona, southeast [[California]], southern [[Nevada]], and southeast [[Utah]]. The Bright Angel Shale consists of often fossiliferous, green and red-brown, micaceous, fissile shale (mudstone) and siltstone with some thicker beds of brown to tan sandstone and dolostone. It ranges in thickness from {{convert|57 to 450|ft|m|sp=us}}. Typically, its thin-bedded shales and sandstones are interbedded in cm-scale cycles. They also exhibit abundant sedimentary structures that include current, oscillation, and interference ripples. The Bright Angel Shale also gradually grades downward into the underlying Tapeats Sandstone. It also complexly interfingers with the overlying Muav Limestone. These chracters make defining the upper and lower contacts the Bright Angel Shale often difficult to define. Typically, its thin-bedded shales and sandstones erode into green and red-brown slopes that rise from the Tonto Platform up to cliffs formed by limestones of the overlying Muav Limestone and dolomites of the Frenchman Mountain Dolostone.<ref name="MiddletonOthers2003a">Middleton, L.T. and Elliott, D.K., 2003. ''Tonto Group'', in Beus, S. S., and Morales, M., eds. ''Grand Canyon geology'' Museum of Northern Arizona Press, Flagstaff, Arizona. pp. 90–106.</ref><ref name="ConnorsOthers2020a">Connors, T.B., Tweet, J.S., and Santucci, V.L., 2020. ''Stratigraphy of Grand Canyon National Park''. In: Santucci, V.L., Tweet, J.S., ed., pp. 54–74, ''Grand Canyon National Park: Centennial Paleontological Resource Inventory (Non-sensitive Version) ''. Natural Resource Report NPS/GRCA/NRR—2020/2103. National Park Service, Fort Collins, Colorado, 603 pp.</ref> |
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Revision as of 20:51, 6 June 2023
| Bright Angel Shale | |
|---|---|
| Stratigraphic range: Middle Cambrian[1] | |
 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 feet (170 m) thick) | |
| Type | Geological formation |
| Unit of | Tonto Group [2] |
| Underlies | Muav Limestone |
| Overlies | Tapeats Sandstone |
| Thickness | 450 feet (140 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[3] |
| Named by | Noble (1914)[3] |
The Bright Angel Shale is one of five geological formations that comprise the Cambrian Tonto Group. It and the other formations of the Tonto Group outcrop in the Grand Canyon, Arizona, and parts of northern Arizona, central Arizona, southeast California, southern Nevada, and southeast Utah. The Bright Angel Shale consists of often fossiliferous, green and red-brown, micaceous, fissile shale (mudstone) and siltstone with some thicker beds of brown to tan sandstone and dolostone. It ranges in thickness from 57 to 450 feet (17 to 137 m). Typically, its thin-bedded shales and sandstones are interbedded in cm-scale cycles. They also exhibit abundant sedimentary structures that include current, oscillation, and interference ripples. The Bright Angel Shale also gradually grades downward into the underlying Tapeats Sandstone. It also complexly interfingers with the overlying Muav Limestone. These chracters make defining the upper and lower contacts the Bright Angel Shale often difficult to define. Typically, its thin-bedded shales and sandstones erode into green and red-brown slopes that rise from the Tonto Platform up to cliffs formed by limestones of the overlying Muav Limestone and dolomites of the Frenchman Mountain Dolostone.[2][4]
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 (white)-greenish (slopes) of Bright Angel Shale
The 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
See also
Wikimedia Commons has media related to Bright Angel Shale.
- Geology of the Grand Canyon area
- List of fossiliferous stratigraphic units in Arizona
- Paleontology in Arizona
References
- ^ Karlstrom, K.E., Mohr, M.T., Schmitz, M.D., Sundberg, F.A., Rowland, S.M., Blakey, R., Foster, J.R., Crossey, L.J., Dehler, C.M. and Hagadorn, J.W., 2020. Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian time scale. Geology, 48(5), pp. 425–430.
- ^ a b Middleton, L.T. and Elliott, D.K., 2003. Tonto Group, in Beus, S. S., and Morales, M., eds. Grand Canyon geology Museum of Northern Arizona Press, Flagstaff, Arizona. pp. 90–106.
- ^ 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.
- ^ Connors, T.B., Tweet, J.S., and Santucci, V.L., 2020. Stratigraphy of Grand Canyon National Park. In: Santucci, V.L., Tweet, J.S., ed., pp. 54–74, Grand Canyon National Park: Centennial Paleontological Resource Inventory (Non-sensitive Version) . Natural Resource Report NPS/GRCA/NRR—2020/2103. National Park Service, Fort Collins, Colorado, 603 pp.
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.