The

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

Bright Angel Shale is the middle layer of the three member

Tonto Group The Tonto Group is a name for an assemblage of related sedimentary strata, collectively known by geologists as a ''Group'', that comprises the basal sequence Paleozoic strata exposed in the sides of the Grand Canyon. As currently defined, the T ...

geologic feature. The 3-rock Tonto section famously sits upon the

Great Unconformity Of the many unconformities (gaps) observed in geological strata, the term Great Unconformity is frequently applied to either the unconformity observed by James Hutton in 1787 at Siccar Point in Scotland,Rance, H (1999''Historical Geology: The Pre ...

because of the highly resistant cliffs of the base layer, vertical

Tapeats Sandstone Except where underlain by the Sixtymile Formation, Cambrian Tapeats Sandstone is typically the lowest geologic unit, about thick, at its maximum, of the 5-member Tonto Group. It is famous for being the highly-resistant mostly-horizontal unit abo ...

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 thick at its maximum. 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

.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. The Bright Angel Shale underlies and interfingers with

Muav Limestone The Cambrian Muav Limestone is a geologic unit within the 5-member Tonto Group. It is about thick at its maximum. It is a resistant cliff-forming unit. The Muav consists of dark to light-gray, brown, and orange red limestone with dolomite and ...

. The Bright Angel Shale is located in the lower elevations of the

Grand Canyon The Grand Canyon (, yuf-x-yav, Wi:kaʼi:la, , Southern Paiute language: Paxa’uipi, ) is a steep-sided canyon carved by the Colorado River in Arizona, United States. The Grand Canyon is long, up to wide and attains a depth of over a m ...

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...

.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. 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 The Colorado River ( es, Río Colorado) is one of the principal rivers (along with the Rio Grande) in the Southwestern United States and northern Mexico. The river drains an expansive, arid drainage basin, watershed that encompasses parts of ...

and the

Vishnu Basement Rocks The Vishnu Basement Rocks is the name recommended for all Early Proterozoic crystalline rocks (metamorphic and igneous) exposed in the Grand Canyon region. They form the crystalline basement rocks that underlie the Bass Limestone of the Unkar G ...

The units of the Tonto Group: * 5 – Frenchman Mountain Dolostone * 4 – Muav Limestone * 3 – Bright Angel Shale * 2 – Tapeats Sandstone (start of transgression series) * 1 – Sixtymile Formation

References

* Blakey, Ron and Wayne Ranney, ''Ancient Landscapes of the Colorado Plateau,'' Grand Canyon Association (publisher), 2008, 176 pages, * 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, * 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

* Share, J. (2102a) [http://written-in-stone-seen-through-my-lens.blogspot.com/2012/02/great-unconformity-of-grand-canyon-part.html ''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

National Park Service, Grand Canyon National Park, Arizona. {{Chronostratigraphy of Nevada Shale formations of the United States Natural history of the Grand Canyon Geologic formations of Arizona Geologic formations of Nevada Cambrian Arizona Cambrian Nevada Cambrian System of North America Cambrian southern paleotropical deposits

See also

References

Further reading