Geology of the Grand Canyon area
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. 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. The opening of the Gulf of California around 6 million years ago enabled a large river to cut its way northeast from the gulf. The new river captured the older drainage to form the ancestral Colorado River, which in turn started to form the Grand Canyon.
Wetter climates brought upon by ice ages starting 2 million years ago greatly increased excavation of the Grand Canyon, which was nearly as deep as it is now, 1.2 million years ago. Volcanic activity deposited lava over the area 1.8 million to 500,000 years ago. At least 13 lava dams blocked the Colorado River, forming lakes that were up to deep. The end of the last ice age and subsequent human activity has greatly reduced the ability of the Colorado River to excavate the canyon. Dams in particular have upset patterns of sediment transport and deposition. Controlled floods from Glen Canyon Dam upstream have been conducted to see if they have a restorative effect. Earthquakes and mass-wasting erosive events still affect the region.
Vishnu Basement Rocks
At about 2.5 and 1.8 billion years ago in Precambrian time, sand, mud, silt, and ash were laid down in a marine basin adjacent to an orogenic belt. From 1.8 to 1.6 billion years ago at least two island arcs collided with the proto-North American continent. This process of plate tectonics compressed and grafted the marine sediments in the basin onto the mainland and uplifted them out of the sea. Later, these rocks were buried under the surface and pressure-cooked into metamorphic rock. The resulting Granite Gorge Metamorphic Suite, which is part of the Vishnu Basement Rocks, consists of the metasedimentary Vishnu Schist and the metavolcanic Brahma and Rama Schists that were formed 1.75 billion to 1.73 billion years ago. This is the resistant rock now exposed at the bottom of the canyon in the Inner Gorge.As the volcanic islands collided with the mainland around 1.7 billion years ago, blobs of magma rose from the subduction zone and intruded the Granite Gorge Metamorphic Suite. These plutons slowly cooled to form the Zoroaster Granite; part of which would later be metamorphosed into gneiss. This rock unit can be seen as light-colored bands in the darker garnet-studded Vishnu Schist. The intrusion of the granite occurred in three phases: two during the initial Vishnu metamorphism period, and a third around 1.4 billion years ago. The third phase was accompanied by large-scale faulting, particularly along north–south faults, leading to a partial rifting of the continent. The collision expanded the continent from the Wyoming–Colorado border into Mexico and almost doubled the crust's thickness in the Grand Canyon region. Part of this thickening created the high ancestral Mazatzal Mountains.
Subsequent erosion lasting 300 million years stripped much of the exposed sediments and the mountains away. This reduced the very high mountains to small hills a few tens to hundreds of feet high. Geologist John Wesley Powell called this major gap in the geologic record, which is also seen in other parts of the world, the Great Unconformity. Other sediments may have been added but, if they ever existed, were completely removed by erosion. Such gaps in the geologic record are called unconformities by geologists. The Great Unconformity is one of the best examples of an exposed nonconformity, which is a type of unconformity that has bedded rock units above igneous or metamorphic rocks.
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 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 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 thick. This formation indicates a short-lived regression 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 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 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.
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. The mountain ranges were reduced to hills, and in some places, the whole of the supergroup were removed entirely, exposing the basement rocks below. Any rocks that were deposited on top of the Grand Canyon Supergroup in the Precambrian were completely removed. This created a major unconformity that represents 460 million years of lost geologic history in the area.
Tonto Group
During the Paleozoic era, the western part of what would become North America was near the equator and on a passive margin. The Cambrian Explosion of life took place over about 15 million years in this part of the world. Climate was warm and invertebrates, such as the trilobites, were abundant. An ocean started to return to the Grand Canyon area from the west about 550 million years ago. As its shoreline moved east, river profiles rose and fluvial sediments accumulated within tectonic basins and coastal plains at first as the Sixtymile Formation, a tan-colored sandstone with some small layers of shale. Later rising sea level resulted in the local accumulation of sediments in paleovalleys as the base of the Tapeat Sandstone. As sea level rose, the ocean flooded the coastal plain causing the concurrent deposition of the Tapeats Sandstone, Bright Angel Shale, Muav Limestone, and Frenchman Mountain Dolostone. Finally, the Frenchman Mountain Dolostone accumulated in beneath shallow seas.Tapeats Sandstone averages 525 million years old and is made of medium- to coarse-grained sand and conglomerate that was deposited on an ancient shore. Ripple marks are common in the upper members of this dark brown thin-bedded layer. Fossils and imprint trails of trilobites and brachiopods have also been found in the Tapeats. Today it is a cliff-former that is thick. Bright Angel Shale averages 515 million years old and is made of mudstone-derived shale that is interbedded with small sections of sandstone and shaly limestone with a few thin beds of dolomite. It was mostly deposited as mud just offshore and contains brachiopod, trilobite, and worm fossils. The color of this formation is mostly various shades of green with some brownish-tan to gray parts. It is a slope-former and is thick. Glauconite is responsible for the green coloration of the Bright Angel. Muav Limestone averages 505 million years old and is made of gray, thin-bedded limestone that was deposited farther offshore from calcium carbonate precipitates. The western part of the canyon has a much thicker sequence of Muav than the eastern part. The Muav is a cliff-former, thick.
These three formations were laid down over a period of 30 million years from early-to-middle Cambrian time. Trilobites followed by brachiopods are the most commonly reported fossils in this group but well-preserved fossils are relatively rare. We know that the shoreline was transgressing because finer grade material was deposited on top of coarser-grained sediment. Today, the Tonto Group makes up the Tonto Platform seen above and following the Colorado River; the Tapeats Sandstone and Muav Limestone form the platform's cliffs and the Bright Angel Shale forms its slopes. Unlike the Proterozoic units below it, the Tonto Group's beds basically lie in their original horizontal position. The Bright Angel Shale in the group forms an aquiclude, and thus collects and directs water through the overlying Muav Limestone to feed springs in the Inner Gorge.