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 exposed geology of the Death Valley area presents a diverse and complex set of at least 23 formations of sedimentary units, two major gaps in the geologic record called unconformities, and at least one distinct set of related formations geologists call a group. The oldest rocks in the area that now includes Death Valley National Park are extensively metamorphosed by intense heat and pressure and are at least 1700 million years old. These rocks were intruded by a mass of granite 1400 Ma and later uplifted and exposed to nearly 500 million years of erosion.
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 Mazatzal Mountains are a mountain range in south central Arizona, about 30–45 miles northeast of Phoenix and the Phoenix metropolitan area. The origin of the name remains obscure but one possibility is that it is from the Nahuatl language meaning "place of the deer". The crest of the Mazatzals forms the county line between Maricopa County and Gila County. SR 87, the Beeline Highway, traverses the Mazatzals on its way to Payson. The highest peak is Mazatzal Peak at 7,903 feet (2,409 m). They also include the Four Peaks, with elevation 7,659 ft, 2,334 m; a prominent mountain and landmark of the eastern Phoenix area.
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 Tonto groups consists of the Sixtymile Formation, Tapeats Sandstone, Bright Angel Shale, Muav Limestone, and Frenchman Mountain Dolostone. Historically, it included only the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone. Because these units are defined by lithology and three of them interfinger and intergrade laterally, they lack the simple layer cake geology as they are typically portrayed as having and geological mapping of them is complicated.
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. It is famous for being the highly-resistant mostly-horizontal unit above the 1,000 my, Great Unconformity expressed areally in the Grand Canyon of Arizona; also in other areas of Arizona and adjacent Nevada.
The Cardenas Basalt, also known as either the Cardenas Lava or Cardenas Lavas, is a rock formation that outcrops over an area of about 310 km2 (120 mi2) in the eastern Grand Canyon, Coconino County, Arizona. The lower part of the Cardenas Basalt forms granular talus slopes. Its upper part forms nearly continuous low cliffs that are parallel to the general course of the Colorado River. The most complete, readily accessible, and easily studied exposure of the Cardenas Basalt lies in Basalt Canyon. This is also its type locality.
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.
The Neoproterozoic Nankoweap Formation, is a thin sequence of distinctive red beds that consist of reddish brown and tan sandstones and subordinate siltstones and mudrocks that unconformably overlie basaltic lava flows of the Cardenas Basalt of the Unkar Group and underlie the sedimentary strata of the Galeros Formation of the Chuar Group. The Nankoweap Formation is slightly more than 100 m in thickness. It is informally subdivided into informal lower and upper members that are separated and enclosed by unconformities. Its lower (ferruginous) member is 0 to 15 m thick. The Grand Canyon Supergroup, of which the Nankoweap Formation is part, unconformably overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.
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. 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 with important exposures in the Grand Canyon, Coconino County, Arizona. It consists of colorful strata that exhibit colors varying from purple to red to brilliant orange. These 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 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 typically brightly-colored strata of the underlying slope-forming 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 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 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 Neoproterozoic Chuar Group consists of 5,250 feet (1,600 m) of fossiliferous, unmetamorphosed sedimentary strata that is composed of about 85% mudrock. The Group is the approximate upper half of the Grand Canyon Supergroup, overlain by the thin, in comparison, Sixtymile Formation, the top member of the multi-membered Grand Canyon Supergroup.
The Sixtymile Formation is a very thin accumulation of sandstone, siltstone, and breccia underlying the Tapeats Sandstone 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 prior to deposition of the Tapeats Sandstone.
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.
