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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 (3,000 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), 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 2,000 feet (610 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.
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 (3,000 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), 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 2,000 feet (610 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.[1]
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..[2]
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).[3] 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.
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.[1]
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..[2]
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).[3] 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.[1] 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 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 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.[4]
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 15° and block faulted in the Grand Canyon Orogeny.[5][6] 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.[7] Good outcrops of the Grand Canyon Supergroup and the Great Unconformity can be seen in the upstream portion of the Inner Gorge.
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 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 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.[4]
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 15° and block faulted in the Grand Canyon Orogeny.[5][6] 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.[7] Good outcrops of the Grand Canyon Supergroup and the Great Unconformity can be seen in the upstream portion of the Inner Gorge.
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.
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.
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