Geology of the Grand Canyon area
2008/9 Schools Wikipedia Selection. Related subjects: Geology and geophysics
The geology of the Grand Canyon area exposes one of the most complete sequences of rock anywhere, representing a period of nearly 2 billion years of the Earth's history in that part of North America. The major sedimentary rock layers exposed in the Grand Canyon and in the Grand Canyon National Park area range in age from 200 million to nearly 2 billion years old. Most were deposited in warm, shallow seas and near ancient, long-gone sea shores. Both marine and terrestrial sediments are represented, including fossilized sand dunes from an extinct desert.
Uplift of the region started about 75 million years ago in the Laramide orogeny, a mountain-building event that is largely responsible for creating the Rocky Mountains to the east. Accelerated uplift started 17 million years ago when the Colorado Plateaus (on which the area is located) were being formed. In total these layers were uplifted an estimated 10,000 feet (3000 m) which enabled the ancestral Colorado River to cut its channel into the four plateaus that constitute this area.
The canyon, created by the Colorado River is 277 miles (446 km) long, ranges in width from 4 to 18 miles (6.4 to 29 km) and attains a depth of more than a mile (1.6 km). Nearly two billion years of the Earth's history have been exposed as the Colorado River and its tributaries cut their channels through layer after layer of rock while the Colorado Plateau was uplifted.
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 by 1.2 million years ago. Also about 2 million years ago volcanic activity started to deposit ash and lava over the area. At least 13 large lava flows dammed the Colorado River, forming huge lakes that were up to 2000 feet (600 m) deep and 100 miles (160 km) long. The nearly 40 identified rock layers and 14 major unconformities (gaps in the geologic record) of the Grand Canyon form one of the most studied sequences of rock in the world.
Deposition of sediments
Some important terms: A geologic formation is a rock unit that has one or more sediment beds, and a member is a minor unit in a formation. Groups are sets of formations that are related in significant ways, and a supergroup is a sequence of vertically related groups and lone formations. The various kinds of unconformities are gaps in the geologic record. Such gaps can be due to an absence of deposition or due to subsequent erosion removing the rock units.
Vishnu Group
The Vishnu Group had its beginnings about 2 billion years ago in Precambrian time when thousands of feet of ash, mud, sand, and silt were laid down in a shallow backarc basin similar to the modern Sea of Japan. During this time period the basin was between Laurentia (proto-North America/Europe) and an orogenic belt of mountains and volcanoes in an island arc not unlike today's Japan. From 1.84 to 1.65 billion years ago the Yavapai and Mojave provinces (island arcs) and then the Mazatzal province collided and accreted with the Wyoming craton of the proto-North American continent. This process of plate tectonics compressed and accreted marine sediments onto Laurentia. Essentially the island arcs slammed into the growing continent and the marine sediments in-between were squeezed together and uplifted out of the sea.
This is the metamorphic rock now exposed at the bottom of the canyon in the Inner Gorge. Geologists call this dark-colored, garnet-studded layer the Vishnu Schist. Combined with the other schists of this period, the Brahma and the Rama, this makes up the Vishnu Group (see 1a in figure 1). No identifiable fossils have been found in these strata, but lenses of marble now seen in these units were likely derived from colonies of primitive algae.
The Vishnu Group was intruded by blobs of magma rising from a subduction zone offshore as recently as 1.66 billion years ago. These plutons slowly cooled to form the Zoroaster Granite (seen as light-colored bands in the darker Vishnu Schist; see 1b in Figure 1). Some of this rock eventually was metamorphosed into gneiss. The intrusion of the granite occurred in three phases: two during the initial Vishnu metamorphism period, and a third around 1.5 billion years ago. This third phase was accompanied by large-scale geologic faulting, particularly along north-south faults that caused some rifting, and a possible partial breakup of the continent..
Studies of the sequence of rocks show that the Vishnu Group underwent at least two periods of orogeny (mountain-building). These orogenies created the 5 to 6 mile (8 to 10 km) high Mazatzal Mountains ( Yavapai-Mazatzal orogeny). This was a very high mountain range, possibly as high as or higher than the modern Himalaya. Then, for over 500 million years, erosion stripped much of the exposed sediments and the mountains away. This reduced this very high range to small hills a few tens to hundreds of feet (tens of meters) high, leaving a major angular unconformity. The once deeply buried mountain roots were all that remained of the Mazatzal Mountains as the sea reinvaded.
During the late Cretaceous or early Tertiary time the Farallon tectonic plate subducted under the west coast of the North American plate causing a compressional force across the region that resulted in an uplift and the formation of the Colorado Plateau.
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 (40 to 100 m) thick and grayish in colour. This is the oldest layer exposed in the Grand Canyon that contains fossils— 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 1000 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 colour 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 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. 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 (3700 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.
Tonto Group
When the ocean started to return to the area 550 million years ago in the Cambrian, it began to concurrently deposit the three formations of the Tonto Group as the shoreline moved eastward:
- Tapeats Sandstone (averages 545 million years old) – This formation is made of cliff-derived medium- to coarse-grained sand and conglomerate that was deposited on an ancient shore (see 3a in figure 1). 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, 250 to 300 feet (75 to 90 m) thick.
- Bright Angel Shale (averages 530 million years old) – Bright Angel is made of mudstone shale interbeded 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 (see 3b in figure 1). The colour of this formation is mostly various shades of green with some brownish-tan to gray parts. It is a slope-former, 325 to 400 feet (100 to 120 m) thick.
- Muav Limestone (averages 515 million years old) – The Muav is made of gray thin-bedded limestone that was deposited further offshore as calcium carbonate precipitates (see 3c in figure 1). It is fossil poor yet trilobites and brachiopods have been found in it. The western part of the canyon has a much thicker sequence of Muav than the eastern part. The Muav is a cliff-former, 250 to 375 feet (80 to 120 m) thick.
These three formations were laid down over a period of 30 million years from early to middle Cambrian time. Fossils of trilobites and burrowing worms are common in these formations. We know that the shoreline was transgressing (advancing onto land) 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 with the Tapeats Sandstone and Muav Limestone forming cliffs, and the Bright Angel Shale forming slopes. Unlike the Paleozoic 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 (barrier to groundwater seeping down), and thus collects and directs water through the overlying Muav Limestone to feed springs in the Inner Gorge.
Temple Butte, Redwall, and Surprise Canyon
The next two periods of geologic history, the Ordovician and the Silurian, are missing from the Grand Canyon geologic sequence. Geologists do not know if sediments were deposited in these periods and were later removed by erosion or if they were never deposited in the first place. Either way, this break in the geologic history of the area marks an unconformity of about 165 million years.
Geologists do know that deep channels were carved on the top of the Muav Limestone during this time. Streams were the likely cause but marine scour may be to blame. Either way, these depressions were filled with freshwater limestone about 350 million years ago in the Middle Devonian in a formation that geologists call the Temple Butte Limestone (see 4a in figure 1). Marble Canyon in the eastern part of the park displays these filled purplish-colored channels well. The Temple Butte Limestone is a cliff-former in the western part of the park where it is gray to cream-colored dolomite. Fossils of animals with backbones are found in this formation; bony plates from freshwater fish in the eastern part and numerous marine fish fossils in the western part. An unconformity marks the top of this formation. The Temple Butte is 250 to 375 feet (80 to 120 m) thick.
The next formation in the Grand Canyon geologic column is the cliff-forming Redwall Limestone, which is 450 to 525 feet (140 to 160 m) thick (see 4b in figure 1). The Redwall is composed of thick-bedded, dark brown to bluish gray limestone and dolomite with white chert nodules mixed in and was laid down in a retreating shallow tropical sea near the equator in early to middle Mississippian time (about 335 million years ago). Many fossilized crinoids, brachiopods, bryozoans, horn corals, nautiloids, and sponges, along with other marine organisms such as large and complex trilobites have been found in the Redwall. Caves and natural arches are also found. After this formation was deposited the Grand Canyon region was slowly uplifted, and part of the upper Redwall was eroded away in late Mississippian. The exposed surface of the Redwall gets its characteristic colour from rainwater dripping from the redbeds of the Supai and Hermit shale that lie above.
The Surprise Canyon Formation is a sedimentary layer of purplish-red shale that was laid down in discontinuous beds above the Redwall (see 4c in figure 1). It was created by evolving tidal estuaries in very late Mississippian and possibly in very earliest Pennsylvanian time. This formation, which only exists in isolated lenses up to 40 feet (12 m) thick, can only be reached by helicopter. It was unknown to science until the 1980s. An unconformity marks the top of the Surprise Canyon Formation and in most places this unconformity has entirely removed the Surprise Canyon and exposed the underlying Redwall.
Supai Group
The Supai Group was deposited in Pennsylvanian and early Permian time in swampy and riparian environments from clastic sediment mostly derived from the Ancestral Rocky Mountains (the average age of this group is 285 million years). The Supai in the western park of the canyon contains limestone, indicative of a warm, shallow sea, while the eastern part was likely a muddy river delta. This formation consists of red siltstones and shale capped by tan-colored sandstone beds that together reach a thickness of 600 to 700 feet (180 to 210 m). Shale in the early Permian formations in this group were oxidized to a bright red colour. Fossils include amphibian footprints, reptiles, and plentiful plant material in the eastern part and increasing numbers of marine fossils in the western part. The formations of the Supai Group are (from oldest to youngest; an unconformity is present at the top of each):
- Watahomigi (see 5a in figure 1): Slope-forming gray limestone with some red chert bands, sandstone, and purple siltstone that is 90 to 175 feet (30 to 50 m) thick.
- Manakacha (see 5b in figure 1): Cliff- and slope-forming pale red sandstone and red shale that is 200 to 275 feet (60 to 85 m) thick.
- Wescogame (see 5c in figure 1): Ledge- and slope-forming pale red sandstone and siltstone that is 100 to 225 feet (30 to 70 m) thick.
- Esplanade (see 5d in figure 1): Ledge- and cliff-forming pale red sandstone and siltstone that is 225 to 300 feet (70 to 90 m) thick.
An unconformity marks the top of the Supai Group.
Hermit, Coconino, Toroweap, and Kaibab
Like the Supai Group below it, the Hermit Shale was deposited in a swampy environment (see 6a in figure 1). The alternating thin-bedded iron oxide, mud and silt were deposited via freshwater streams in a semiarid environment an average of 265 million years ago. Fossils of winged insects, cone-bearing plants, and ferns are found in this formation as well as tracks of amphibians and reptiles. It is a soft, deep red shale and mudstone slope-former in the canyon that is 160 to 175 feet (49 to 53 m) thick. Slope development will periodically undermine the formations above and car- to house-sized blocks of that rock will cascade down onto the Tonto Platform. An unconformity marks the top of this formation.
The Coconino Sandstone formed as the area dried out and sand dunes made of pure quartz sand invaded a growing desert some 260 million years ago (see 6b in figure 1). Today, it is a 375 to 650 ft (115 to 200 m) thick golden white to cream-colored cliff-former near the canyon's rim. Eolian (wind-created) cross bedding patterns of the frosted, well-sorted and rounded sand can be seen in its fossilized sand dunes. Also fossilized are arthropod and early reptile tracks along with some burrows. An unconformity marks the top of this formation.
Next in the geologic column is the Toroweap Formation, 200 to 250 feet (60 to 75 m) thick (see 6c in figure 1). It consists of red and yellow sandstone and shaly gray limestone interbedded with gypsum that were deposited in a warm, shallow sea as its shoreline transgressed (invaded) and regressed (retreated) over the land (average age of the rock is about 250 million years). In modern times it is a ledge- and cliff-former that contains fossils of brachiopods, corals, and mollusks along with other animals and various terrestrial plants. The Toroweap is divided into the following three members:
- Seligman: Slope-forming yellowish to reddish sandstone and siltstone.
- Brady Canyon: Cliff-forming gray limestone with some chert.
- Wood Ranch: Slope-forming pale red and gray siltstone and dolomitic sandstone.
An unconformity marks the top of this formation.
One of the highest, and therefore youngest, formations seen in the Grand Canyon area is the massive Kaibab Limestone, 250 to 350 feet (80 to 110 m) thick (see 6d in figure 1). A prominent ledgy cliff-former, the Kaibab Limestone was laid down in middle Permian time an average of about 225 million years ago in the deeper parts of the same advancing warm, shallow sea that deposited the underlying Toroweap Formation. The Kaibab is typically made of sandy limestone sitting on top of a layer of sandstone, but in some places sandstone and shale are near or at the top. This is the cream to grayish-white rock that park visitors stand on while enjoying the spectacular vistas of the canyon from both rims (some call it "Grand Canyon's bathtub ring" due to its appearance). It is also the surface rock covering much of the Kaibab Plateau just north of the canyon and the Coconino Plateau immediately south. Shark teeth have been found in this formation as well abundant fossils of marine invertebrates such as brachiopods, corals, mollusks, sea lilies, and worms. An unconformity marks the top of this formation.
Mesozoic deposition
Uplift marked the start of the Mesozoic and streams started to incise the newly dry land. Broad, low valleys deposited sediment eroded from nearby uplands in Triassic time creating the once 1000 foot (300 m) thick Moenkopi Formation. The formation is made from sandstone and shale with gypsum layers in between. This easily eroded formation may have been deposited above the rim of the Grand Canyon. Moenkopi outcrops are found along the Colorado River in Marble Canyon, on Cedar Mountain (a mesa near the southeastern park border), and in Red Butte (located south of Grand Canyon Village). Remnants of the Shinarump Conglomerate, itself a member of the Chinle Formation, are above the Moenkopi Formation near the top of Red Butte but below a much younger lava flow.
Formations totaling over 5000 feet (1500 m) in thickness were deposited in the region in the Mesozoic and Cenozoic but were almost entirely removed from the Grand Canyon sequence by subsequent erosion (see below). For details on these layers see geology of the Zion and Kolob canyons area, and geology of the Bryce Canyon area. All these rock units together form a super sequence of rock known as the Grand Staircase.
Creation of the Grand Canyon
Uplift and nearby extension
The Laramide orogeny affected all of western North America by helping to build the Cordilleran Mountain Range (of which the Rocky Mountains are a major part). This major mountain-building event started near the end of the Mesozoic (around 75 million years ago) and lasted well into the early Cenozoic. A second period of uplift started 17 million years ago, creating the Colorado Plateaus (the Kaibab, Kanab, and Shivwits plateaus bound the northern part of the canyon and the Coconino bounds the southern part). However, for reasons poorly understood, the beds of the Colorado Plateaus remained mostly horizontal through both events even as they were uplifted an estimated 9000 feet (2700 m). One hypothesis suggests that the entire plateau shifted in a clockwise rotation during the uplift and this helped to maintain its stability. Before the uplift the plateau region was about 1000 feet (300 m) above sea level and bounded by high mountains to the south and west.
In middle Tertiary time (about 20 million years ago) tensional forces (crustal stretching) created and expanded faults in the area and caused some moderate volcanic activity. To the west, these forces created the Basin and Range province by forming long north-south-trending faults along which basins ( grabens) dropped down and mountain ranges ( horsts) were uplifted. The extreme western part of the park is intersected by one of these faults, the Grand Wash.
The Colorado River is born and cuts down
Continued uplift of the Colorado Plateaus created monoclines and also increased the elevation of its plateaus. This steepened the gradient of streams flowing in the Colorado Plateaus province. The ancestral Colorado River was a landlocked river until 5.3 million years ago (see below). Before that it had a series of temporary base levels (lowest points) in large lakes in the Colorado Plateaus in the early Tertiary and possibly the Basin and Range by the middle Tertiary. An alternate theory of the canyon's formation is that two canyons, one eroding headward from the west and another from the east met about six million years ago on the Kaibab Arch to form one continuous canyon.
The opening of an arm of the Gulf of California 5.3 million years ago changed the direction of nearby streams toward the sagging and rifting region. The upstream uplift and downstream sagging caused streams flowing into the gulf to run and downcut much faster. Soon (geologically speaking) headwater capture consolidated these streams into one major river and associated tributary channels—the modern Colorado drainage system. The most important consolidation occurred when a separate preexisting river that was carving a channel into the San Andreas Fault and out into the gulf likely captured the landlocked Colorado. Excavation of the eastern part of the Grand Canyon began previous to this but was greatly accelerated and expanded west afterward.
Ice ages during the Pleistocene brought a cooler and wetter pluvial climate to the region starting 2 to 3 million years ago. The added precipitation increased runoff and the erosive ability of streams (especially from spring melt water and flash floods in summer). With a greatly increased flow volume, steepened gradient, and lower base level, the Colorado cut faster than ever before and started to quickly excavate the Grand Canyon two million years before present, almost reaching the modern depth by 1.2 million years ago.
Volcanic activity dams the new canyon
During the Quaternary period, starting around 725,000 years ago, basaltic lava from the cinder cones in the Uinkaret volcanic field erupted from within and flowed into western Grand Canyon . The river was dammed multiple times from 725,000 to 100,000 years ago. While some believe that these lava dams were stable, lasting up to 20,000 years and forming large reservoirs , others think they failed quickly and catastrophically as massive floods . Lava flows traveled downriver 76 miles (121 km) from river mile 178 to 254.
Recent geology, human impact, and the future
The end of the Pleistocene ice ages and the start of the Holocene began to change the area's climate from a cool, wet pluvial one to dryer semi-arid conditions similar to that of today (although much of the rim then, as now, received enough precipitation to support large forests). With less water to cut, the erosive ability of the Colorado was greatly reduced (the rocks of the Inner Gorge are also relatively resistant to erosion). Mass wasting processes thus began to become relatively more important than they were before, creating steeper cliffs and further widening the Grand Canyon and its tributary canyon system.
In modern times, the building of the Glen Canyon Dam and other dams further upstream have regulated the flow of the Colorado River and have substantially reduced the amount of water and sediment it carries. This has diminished the river's ability to scour rocks, and the demand for water is so great that in most years the Colorado does not reach its delta in the Gulf of California.
The dam has also changed the character of the river water. Once both muddy and warm, with only bottom feeding fish, the river is now clear and cold and now supports planted trout. This in turn has changed the migration patterns of the bald eagle, which previously would transit the canyon to favorable fishing sites downstream, but now use the river as their seasonal feeding site.
About 45 earthquakes occurred in or near the Grand Canyon in the 1990s. Of these, five registered between 5.0 and 6.0 on the Richter Scale. Dozens of faults cross the canyon, with at least several active in the last 100 years.
The stream gradient of the Colorado River is still steep enough to suggest that the river could cut another 1200 to 2000 feet (400 to 600 m) assuming no additional uplift in the geologic future. This does not account for human impact, which would tend to slow the rate of erosion.