page_title: Geology of the Grand Canyon area
section_title: Grand Canyon Supergroup
hierarchical_section_title: Geology of the Grand Canyon area / Deposition of sediments / Grand Canyon Supergroup
caption_attribution_description: Sixtymile Formation in Grand Canyon
context_page_description: 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.
  Uplift of the region started about 75 million years ago during the Laramide orogeny; a mountain-building event that is largely responsible for creating the Rocky Mountains to the east. In total, the Colorado Plateau was uplifted an estimated 2 miles. The adjacent Basin and Range Province to the west started to form about 18 million years ago as the result of crustal stretching. A drainage system that flowed through what is today the eastern Grand Canyon emptied into the now lower Basin and Range province.
context_section_description: 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 that would ultimately fail to split the continent. Eventually, this sunken region of Laurentia was flooded with a shallow seaway that extended from at least present-day Lake Superior to Glacier National Park in Montana to the Grand Canyon and the Uinta Mountains. The resulting Grand Canyon Supergroup of sedimentary units is composed of nine varied geologic formations that were laid down from 1.2 billion and 740 million years ago in this sea. Good exposures of the supergroup can be seen in eastern Grand Canyon in the Inner Gorge and from Desert View, Lipan Point and Moran point.
  The oldest section of the supergroup is the Unkar Group. It accumulated in a variety of fluvial, deltaic, tidal, nearshore marine, and offshore marine environments. The first formation to be laid down in the Unkar Group was the Bass Formation. Fluvial gravels initially accumulated in shallow river valleys. They later lithified into a basal conglomerate that is known as the Hotauta Member of the Bass Formation. The Bass Formation was deposited in a shallow sea near the coast as a mix of limestone, sandstone, and shale. Diagenesis later altered the bulk of the limestone into dolomite. It is 120 to 340 feet (37 to 100 m) thick and grayish in color. Averaging 1250 million years old, this is the oldest layer exposed in the Grand Canyon that contains fossils—stromatolites. Hakatai Shale is made of thin beds of marginal-marine-derived mudstones, sandstones, and shale that, together, are 445 to 985 feet (136 to 300 m) thick. 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 is a resistant marine sedimentary quartzite that was eroded to form monadnocks that later became islands in Cambrian time. Those islands withstood wave action long enough to become re-buried by other sediments in the Cambrian Period. Dox Formation is over 3,000 feet (910 m) thick and is made of sandstone with some interbedded shale beds and mudstone that were deposited in fluvial and tidal environments. 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. At 1070 ± 70 million years old, the Cardenas Basalt is the youngest formation in the Unkar Group. It is made of layers of dark brown basaltic rocks that flowed as lava up to 1,000 feet (300 m) thick.
  Nankoweap Formation is around 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 Basalt. 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 were deposited in coastal and shallow sea environments about 1000 to 700 million years ago. The Galeros Formation is a mainly greenish formation composed of interbedded sandstone, limestone, and shale.  Fossilized stromatolites are found in the Galeros. The Kwagunt Formation 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. The Sixtymile Formation is made of tan-colored sandstone with some small sections of shale.
  About 800 million years ago the supergroup was tilted 15° 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. About 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 re
caption: Sixtymile Formation is the last rock unit in the Chuar Group
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