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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 Group of the Grand Canyon Supergroup and the Tapeats Sandstone of the Tonto Group .
In 1963 Glen Canyon Dam and other dams farther upstream started to regulate the flow of the Colorado River through Grand Canyon. Pre-dam but still historic flows of the Colorado through Grand Canyon ranged from 700 to 100,000 cubic feet (20 to 2,832 m 3 ) per second with at least one late 19th century flood of 300,000 cubic feet (8,500 m 3 ...
The Grand Canyon Supergroup is a Mesoproterozoic to a Neoproterozoic sequence of sedimentary strata, partially exposed in the eastern Grand Canyon of Arizona. This group comprises the Unkar Group , Nankoweap Formation , Chuar Group and the Sixtymile Formation , which overlie Vishnu Basement Rocks .
The age and exhumation history of the underlying Vishnu Basement Rocks constrain the age of the Unkar Group. Radiometric dating of these basement rocks demonstrate that these basement rocks underwent metamorphism and deformation at mid-crustal depths of about 20–25 km (12–16 mi) between about 1840 and 1660 Ma ago.
Except where underlain by the Sixtymile Formation, the Tapeats Sandstone is the Cambrian geologic formation that is the basal geologic unit of the Tonto Group.Typically, it is also the basal geologic formation of the Phanerozoic strata exposed in the Grand Canyon, Arizona, and parts of northern Arizona, central Arizona, southeast California, southern Nevada, and southeast Utah.
The basement rock of the crust can be 32–48 kilometres (20–30 mi) thick or more. The basement rock can be located under layers of sedimentary rock, or be visible at the surface. Basement rock is visible, for example, at the bottom of the Grand Canyon, consisting of 1.7- to 2-billion-year-old granite (Zoroaster Granite) and schist (Vishnu ...
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The Grand Canyon Supergroup, of which the Unkar Group is the lowermost part, overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks. [ 3 ] [ 5 ] [ 6 ] The Cardenas Basalt has also been called the Rama Formation .