|Grand Canyon Supergroup|
|Grand Canyon Supergroup||Help Topic||Metamorphic|
Grand Canyon Supergroup
In late Precambrian time, extension from a large tectonic plate or smaller plates moving away from Laurentia thinned its continental crust, forming large rift basins (this rifting ultimately failed to split the continent). Eventually, a region of Laurentia from at least present-day Lake Superior to Glacier National Park in Montana to the Grand Canyon and the Uinta Mountains was invaded by a shallow seaway. The resulting Grand Canyon Supergroup of sedimentary units is composed of nine varied formations that were laid down from 1250 million to 825 million years ago in this sea. The total thickness of the sediment and lava deposited was well over 2 miles (3 km). Rock outcroppings of the Grand Canyon Supergroup appear in parts of the Inner Gorge and in some of the deeper tributary canyons.
The oldest section of the supergroup is the Unkar Group (a group is a set of two or more formations that are related in notable ways). It was laid down in an offshore environment.
Bass Limestone (averages 1250 million years old) - Wave action eroded the land, creating a gravel that later lithified into a basal conglomerate. This formation is known as the Hotauta Member of the Bass Limestone. The Bass Limestone formation was deposited in a shallow sea near the coast as a mix of limestone, sandstone, and shale. It is 120 to 340 feet (37 to 100 m) thick and grayish in color. This is the oldest layer exposed in the Grand Canyon that contains fossilsâ€”stromatolites.
Hakatai Shale (averages 1200 million years old) - The Hakatai Shale is made of thin beds of non-marine-derived mudstones, sandstones, and shale. This formation indicates a short-lived regression (retreat) of the seashore in the area that left mud flats. Today it is very bright orange-red and gives the Red Canyon its name.
Shinumo Quartzite - This formation was a resistant marine sandstone that later formed islands in Cambrian time. Those islands withstood wave action long enough to become re-buried by other sediments in the Cambrian Period. It was later metamorphosed into quartzite.
Dox Sandstone (averages 1190 million years old) - A shallow formation made of ocean-derived sandstone with some interbedded shale beds and mudstone. Ripple marks and other features indicate it was close to the shore. Outcrops of this red to orange formation can be seen in the eastern parts of the canyon. Fossils of stromatolites and algae are found in this layer.
Cardenas Lava (1250 to 1100 million years old) - This is the youngest formation of the Unkar Group and is made of layers of dark brown basaltic rocks that flowed as lava up to 1,000 feet (300 m) thick.
The Nankoweap Formation averages 1050 million years old and is not part of a group. This rock unit is made of coarse-grained sandstone, and was deposited in a shallow sea on top of the eroded surface of the Cardenas Lava. The Nankoweap is only exposed in the eastern part of the canyon. A gap in the geologic record, an unconformity, follows the Nankoweap.
All formations in the Chuar Group (about 1000 to 825 million years old) were deposited in coastal and shallow sea environments.
Galeros Formation - A mainly greenish formation composed of interbedded sandstone, limestone, and shale with some shale ranging in color from red to purple. Fossilized stromatolites are found in the Galeros.
Kwagunt Formation - The Kwagunt consists of black shale and red to purple mudstone with some limestone. Isolated pockets of reddish sandstone are also found around Carbon Butte. Stromatolites are found in this layer.
Sixtymile Formation - Sixtymile is made of tan-colored sandstone with some small sections of shale.
About 800 million years ago the supergroup was tilted 15Â° and block faulted in the Grand Canyon Orogeny. Some of the block units moved down and others moved up while fault movement created north-south-trending fault-block mountain ranges. Some 100 million years of erosion took place that washed most of the Chuar Group away along with part of the Unkar Group (exposing the Shinumo Quartzite as previously explained). The mountain ranges were reduced to hills, and in some places, the whole 12,000 feet (3,700 m) of the supergroup were removed entirely, exposing the Vishnu Group below. This created what geologist John Wesley Powell called the Great Unconformity, itself one of the best examples of an exposed nonconformity (an unconformity with bedded rock units above igneous or metamorphic rocks) in the world. In all some 250 million years of the area's geologic history was lost in the Great Unconformity. Good outcrops of the Grand Canyon Supergroup and the Great Unconformity can be seen in the upstream portion of the Inner Gorge.
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