|Bright Angel Shale|
Stratigraphic range: Middle Cambrian
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))
|Unit of||Tonto Group|
|Thickness||500 feet (150 m) at its maximum|
|Primary||micaceous siltstone and shale|
|Other||sandstone and glauconitic sandstone|
|Region||northern Arizona, southeast California, southern Nevada, and southcentral Utah|
|Country||United States of America|
|Named for||Bright Angel Canyon, Bright Angel quadrangle, Coconino Co., Arizona|
|Named by||Noble (1914)|
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. 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. 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. The Bright Angel Shale preserves fossils dating back to the Cambrian Period.
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:
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The geology of the Grand Canyon area includes one of the most complete and studied sequences of rock on Earth. The nearly 40 major sedimentary rock layers exposed in the Grand Canyon and in the Grand Canyon National Park area range in age from about 200 million to nearly 2 billion years old. Most were deposited in warm, shallow seas and near ancient, long-gone sea shores in western North America. Both marine and terrestrial sediments are represented, including lithified sand dunes from an extinct desert. There are at least 14 known unconformities in the geologic record found in the Grand Canyon.
The Grand Staircase is an immense sequence of sedimentary rock layers that stretch south from Bryce Canyon National Park and Grand Staircase-Escalante National Monument, through Zion National Park, and into the Grand Canyon National Park.
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, or that observed by John Wesley Powell in the Grand Canyon in 1869. Both instances are exceptional examples of where the contacts between sedimentary strata and either sedimentary or crystalline strata of greatly different ages, origins, and structure represent periods of geologic time sufficiently long to raise great mountains and then erode them away.
The Sierra Ancha is a mountain range in Gila County, in central Arizona. It lies between Roosevelt Lake to the south, the Tonto Basin to the west, Cherry Creek to the east, and Pleasant Valley to the north. The range is one of several, including the Bradshaw Mountains, Mingus Mountain of the Black Hills, and the Mazatzal Mountains, which form a transitional zone between the lowland deserts of southern Arizona and the Colorado Plateau of northeastern Arizona. The highest point in the range is Aztec Peak, at an elevation of 2345 m (7694 ft).
The Cambrian Tonto Group is the three-member sequence of geologic formations that represent the basal section of Paleozoic rocks in the Grand Canyon. The group is about 1,250 feet (381 m) thick. The base unit, the Cambrian Tapeats Sandstone was deposited upon the erosion surface of the Vishnu Basement Rocks, which is found in Granite Gorge. The erosion resistant Tapeats Sandstone forms the platform, called Tonto Platform, that the two less erosion resistant upper layers, the Bright Angel Shale and Muav Limestone, rest upon.
The Cambrian Muav Limestone is the upper geologic unit of the 3-member Tonto Group. It is about 650 feet (198 m) 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 calcareous mudstone. The Muav is overlain in some areas by the Devonian Temple Butte Formation, but the major unit above are the vertical cliffs of Mississippian Redwall Limestone. The Muav is located in the lower elevations of the Grand Canyon, Arizona.
The Cambrian Tapeats Sandstone is the lower geologic unit, about 230 feet (70 m) thick, at its maximum, of the 3-member Tonto Group.
The Unkar Group is a sequence of strata of Proterozoic age that are subdivided into five geologic formations and exposed within the Grand Canyon, Arizona, Southwestern United States. The 5-unit Unkar Group is the basal member of the 8-member Grand Canyon Supergroup. The Unkar is about 1,600 to 2,200 m thick and composed, in ascending order, of the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. Units 4 & 5 are found mostly in the eastern region of Grand Canyon. Units 1 through 3 are found in central Grand Canyon. The Unkar Group accumulated approximately between 1250 and 1104 Ma. In ascending order, the Unkar Group is overlain by the Nankoweap Formation, about 113 to 150 m thick; the Chuar Group, about 1,900 m (6,200 ft) thick; and the Sixtymile Formation, about 60 m (200 ft) thick. These are all of the units of the Grand Canyon Supergroup. The Unkar Group makes up approximately half of the thickness of the 8-unit Supergroup.
Isis Temple, in the Grand Canyon, Arizona, USA, is a prominence below the North Rim, and adjacent Granite Gorge. The prominence lies north of the north bank of the west-flowing Colorado River, and is just north of Middle Granite Gorge. The Trinity Creek and canyon flow due-south at its west border; its north, and northeast border/flank is formed by Phantom Creek and canyon, a west tributary of Bright Angel Creek; the creeks intersect about 3 mi southeast, and 1 mi north of Granite Gorge. The peak of Isis Temple is only ~202 ft lower than Grand Canyon Village.
The Grand Canyon Supergroup is a Mesoproterozoic to Neoproterozoic sequence of sedimentary strata, mostly exposed in the eastern Grand Canyon of Arizona. This group is composed of the Unkar Group, Nankoweap Formation, Chuar Group and the Sixtymile Formation, which overlie Vishnu Basement Rocks. Several notable landmarks of the Grand Canyon, such as the "Isis Temple and Cheops Pyramid" and the "Apollo Temple," are surface manifestations of the Grand Canyon Supergroup.
The Hakatai Shale is a Mesoproterozoic rock formation that outcrops in the Grand Canyon, Coconino County, Arizona. It consists of colorful strata that exhibit colors that vary from purple to red to brilliant orange on outcrop. The colors are the result of the oxidation of iron-bearing minerals in the Hakatai Shale. It consists of lower and middle members that consist of bright-red, slope-forming, highly fractured, argillaceous mudstones and shale and an upper member composed of purple and red, cliff-forming, medium-grained sandstone. Its thickness, which apparently increases eastwards, varies from 137 to 300 m. In general, the Hakatai Shale and associated strata of the Unkar Group rocks dip northeast (10°-30°) toward normal faults that dip 60° or more toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area. In addition, thick, prominent, and dark-colored basaltic sills and dikes cut across the purple to red to brilliant orange strata of the Hakatai Shale.
The Late Mississippian-Early Pennsylvanian Surprise Canyon Formation is a conglomerate, sandstone, limestone and dark purple siltstone type formation, occurring as 'channel fill'. The Surprise Canyon Formation was deposited upon a mostly horizontal, much eroded Redwall Limestone surface, the Redwall originally deposited as marine-(oceanic) limestone, ; when the Surprise Canyon Formation was being deposited, the Redwall Limestone surface existed under karst topography conditions, in a "warm, and humid" (paleo)-environment. The karst topography created "sinks, caves, and underground channels", and created "deep ravines and stream valleys developed as caverns collapsed". Some valleys probably filled with " 'clayey red-orange soil' similar to that known in the Tropics today." Below the Redwall surface, solution-cavern deposits of the Surprise Canyon, have a Redwall Limestone geologic layer, now expressed above them.
The Bass Formation, also known as the Bass Limestone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The Bass Formation erodes as either cliffs or stair-stepped cliffs. In the case of the stair-stepped topography, resistant dolomite layers form risers and argillite layers form steep treads. In general, the Bass Formation in the Grand Canyon region and associated strata of the Unkar Group-rocks dip northeast (10°-30°) toward normal faults that dip 60+° toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area. In addition, thick, prominent, and dark-colored basaltic sills intrude across the Bass Formation.
The Shinumo Quartzite also known as the Shinumo Sandstone, is a Mesoproterozoic rock formation, which outcrops in the eastern Grand Canyon, Coconino County, Arizona. It is the 3rd member of the 5-unit Unkar Group. The Shinumo Quartzite consists of a series of massive, cliff-forming sandstones and sedimentary quartzites. Its cliffs contrast sharply with the stair-stepped topography of the underlying Hakatai Shale. Overlying the Shinumo, dark green to black, fissile, slope-forming shales of the Dox Formation create a well-defined notch. It and other formations of the Unkar Group occur as isolated fault-bound remnants along the main stem of the Colorado River and its tributaries in Grand Canyon. Typically, the Shinumo Quartzite and associated strata of the Unkar Group dip northeast (10°-30°) toward normal faults that dip 60+° toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area.
The Dox Formation, also known as the Dox Sandstone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The strata of the Dox Formation, except for some more resistant sandstone beds, are relatively susceptible to erosion and weathering. The lower member of the Dox Formation consists of silty-sandstone and sandstone, and some interbedded argillaceous beds, that form stair-stepped, cliff-slope topography. The bulk of the Dox Formation typically forms rounded and sloping hill topography that occupies an unusually broad section of the canyon.
The Vishnu Basement Rocks is the name recommended for all Early Proterozoic crystalline rocks exposed in the Grand Canyon region. They form the crystalline basement rocks that underlie the Bass Limestone of the Unkar Group of the Grand Canyon Supergroup and the Tapeats Sandstone of the Tonto Group. These basement rocks have also been called either the Vishnu Complex or Vishnu Metamorphic Complex. These Early Proterozoic crystalline rocks consist of metamorphic rocks that are collectively known as the Granite Gorge Metamorphic Suite; sections of the Vishnu Basement Rocks contain Early Paleoproterozoic granite, granitic pegmatite, aplite, and granodiorite that have intruded these metamorphic rocks, and also, intrusive Early Paleoproterozoic ultramafic rocks.
The Redwall Limestone is a resistant cliff-forming unit of Mississippian age that forms prominent, red-stained cliffs ranging in height from 500 feet (150 m) to 800 feet (240 m).
The Sixtymile Formation is the uppermost unit of the Grand Canyon Supergroup. It is a very thin accumulation of sandstone, siltstone, and breccia that is exposed in only four places in the Chuar Valley. These exposures occur atop Nankoweap Butte and within Awatubi and Sixtymile Canyons in the eastern Grand Canyon, Arizona. The maximum preserved thickness of the Sixtymile Formation is about 60 meters (200 ft). The actual depositional thickness of the Sixtymile Formation is unknown owing to erosion during the formation of the Great Unconformity and prior to deposition of the Tapeats Sandstone.
The Devonian Temple Butte Formation, also called Temple Butte Limestone, outcrops through most of the Grand Canyon of Arizona, USA; it also occurs in southeast Nevada. Within the eastern Grand Canyon, it consists of thin, discontinuous and relatively inconspicuous lenses that fill paleovalleys cut into the underlying Muav Limestone. Within these paleovalleys, it at most, is only about 100 feet (30 m) thick at its maximum. Within the central and western Grand Canyon, the exposures are continuous. However, they tend to merge with cliffs of the much thicker and overlying Redwall Limestone.
The geology of Arizona began to form in the Precambrian. Igneous and metamorphic crystalline basement rock may have been much older, but was overwritten during the Yavapai and Mazatzal orogenies in the Proterozoic. The Grenville orogeny to the east caused Arizona to fill with sediments, shedding into a shallow sea. Limestone formed in the sea was metamorphosed by mafic intrusions. The Great Unconformity is a famous gap in the stratigraphic record, as Arizona experienced 900 million years of terrestrial conditions, except in isolated basins. The region oscillated between terrestrial and shallow ocean conditions during the Paleozoic as multi-cellular life became common and three major orogenies to the east shed sediments before North America became part of the supercontinent Pangaea. The breakup of Pangaea was accompanied by the subduction of the Farallon Plate, which drove volcanism during the Nevadan orogeny and the Sevier orogeny in the Mesozoic, which covered much of Arizona in volcanic debris and sediments. The Mid-Tertiary ignimbrite flare-up created smaller mountain ranges with extensive ash and lava in the Cenozoic, followed by the sinking of the Farallon slab in the mantle throughout the past 14 million years, which has created the Basin and Range Province. Arizona has extensive mineralization in veins, due to hydrothermal fluids and is notable for copper-gold porphyry, lead, zinc, rare minerals formed from copper enrichment and evaporites among other resources.