From TheBestLinks.com
The primary geology of the Grand Canyon area is world renowned for presenting one of the most complete geologic columns, representing a period of 1400 million years of the Earth's history in that part of North America. The major sedimentary rock units exposed in the Grand Canyon range in age from 2000 million to 230 million years and were deposited in warm shallow seas and near-shore environments. These layers have undergone 5000 to 10,000 feet (1500 to 3000 m) of uplift starting about 65 million years ago which has increased the ability of the Colorado River to cut its channel into four plateaus of the Colorado Plateaus. The canyon did not start to form until about five to six million years ago when the Gulf of California opened up and thus lowered the river's base level (its lowest point). A million years ago volcanic activity deposited ash and lava over the area which at times even dammed the Colorado. The nearly 40 identified rock layers of Grand Canyon form one of the most studied geologic columns in the world.
The Grand Canyon from Navajo Point. The Colorado River is to the right and the North Rim can be seen to the left in the distance. Also visible is nearly every sedimentary layer described in this article.
Figure 1. A geologic cross section of the Grand Canyon. Black numbers correspond to subsection numbers in section 1 and white numbers are referred to in the text
Deposition of canyon wall sediments
Vishnu Group
About 2000 million years ago in Precambrian time, thousands of feet of ash, mud, sand, and silt were laid down in a shallow forarc basin behind an orogenic belt of mountains and volcanoes in an island arc. No fossils have been found in these strata.
1700 to 1600 million years ago the process of plate tectonics compressed and heated these marine sediments into 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. This combined with the other schists of this period, the Brahma and the Rama, make up the Vishnu Group (see 1a in Figure 1). This layer was later intruded by blobs of magma rising from a subduction zone offshore. 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.
100 million years later in Paleozoic time, an orogeny (mountain-building event) uplifted the region and created the 5-6 mile (8-9.5 km) high Mazatzal Mountains and tilted the beds 15°. For 200 million years erosion stripped much of the exposed sediments and the mountains away and thus left an angular unconformity in the area's geologic sequence. The roots were all that remained of the Mazatzal Mountains as the sea reinvaded the area. Today the Vishnu Schist and the Zoroaster Granite are exposed in the Inner Gorge of the canyon.
Grand Canyon Supergroup
The Grand Canyon Supergroup of sedimentary units is composed of nine varied formations and were laid down from 1200 million to 800 million years ago (a supergroup is a sequence of vertically related groups and formations). Rock outcroppings of the Grand Canyon Supergroup appear in parts of the Inner Gorge and in some of the deeper tributary canyons.
- Unkar Group - 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 - 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 (a formation is a rock unit that has one or more sediment beds and a member is a minor unit in the formation). The Bass Limestone formation was deposited in a shallow sea as a mix of limestone, sandstone, and shale. It is 120 to 340 feet (37 to 104 meters) thick. This is the oldest layer exposed in the Grand Canyon that contains fossils - stromatolites.
- Hakatai Shale - The Hakatai Shale is made of thin beds of non-marine derived mudstones, sandstones, and shales. This formation indicates a short-lived regression (retreat) of the seashore in the area.
- 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 - A shallow formation made of ocean-derived sandstone with some shale beds.
- Cardenas Lava - This is the youngest formation of the Unkar Group and is made of layers of basaltic rocks that flowed as lava up to 1000 feet (300 m) thick.
- Nankoweap Formation - This rock unit sits on top of the eroded surface of the Cardenas Lava and was deposited in a shallow sea.
- Chuar Group -
- Galeros -
- Kwagunt - evidence of algae
- Sixty Mile -
The supergroup was eventually tilted and block faulted due to tensional tectonic forces. 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 meters) of the supergroup were removed entirely - exposing the Vishnu Group below. This created what John Wesley Powell called the "Great Unconformity."
Tonto Group
When the ocean started to return to the area 550 million years ago it began to concurrently deposit the three formations of the Tonto Group as the shoreline moved eastward:
- the Tapeats Sandstone (made of shore deposited sand and cliff-derived rocks; see 3a in Figure 1),
- the Bright Angel Shale (made of mud deposited just off-shore and containing brachiopod, trilobite, and worm fossils; see 3b in Figure 1), and
- the Muav Limestone (made of calcium carbonate precipitates deposited further off-shore; see 3c in Figure 1),
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) due to the fact that 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 lie in pretty much their original horizontal position. The Bright Angel Shale in the group forms a 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 a major unconformity.
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 limestone 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 channels well. The Temple Butte Limestone is a cliff former in the western part of the park and has been largely changed over time into dolomite (which explains the relative lack of fossils in this formation). However, fossils of animals with backbones are found in this formation. An unconformity (gap in the geologic record) marks the end of this formation.
The next formation in the Grand Canyon geologic column is the Redwall Limestone which averages 500 feet (150 meters) thick (see 4b in Figure 1). The Redwall is composed of bluish gray limestone with chert nodules and was laid down in a retreating shallow tropical sea near the equator in early to middle Mississippian time (about 330 million years ago). Many fossilized crinoids, brachiopods, bryozoans, horn corals, nautiloids, and sponges, along with other marine organisms have been found in the Redwall. 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 color from rainwater dripping from the redbeds of the Sunpai and Hermit shale that lie above.
The Surprise Canyon Formation is a sedimentary layer of purple 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 that are up to a few hundred feet thick, can only be reached by helicopter and was unknown to science until the 1980s. An unconformity marks the end of the Surpise 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 swamppy and riparian environments. It consists of red siltstones and sandstones beds that together reach a thickness of 600 to 700 feet (180 to 210 meters). Shale in the early Permian formations in this group were oxidized to a bright red color. Fossils include amphibian footprints and a lot of plants. The formations of the Supai Group are (from oldest to youngest):
- Watahomigi (see 5a in Figure 1),
- Manakacha (see 5b in Figure 1),
- Wescogame (see 5c in Figure 1), and
- Esplanade (see 5d in Figure 1).
An unconformity marks the end of the Supai Group.
Hermit, Coconino, Toroweap, and Kaibab
Like the Supai Group below, the Hermit Shale was deposited in a swampy environment (see 6a in Figure 1). The iron oxide, mud and silt were deposited via freshwater streams in a semi-arid environment. Fossils of winged insects, cone-bearing plants, and ferns are found in this formation. It is a soft red mudstone slope former in the canyon.
The Coconino Sandstone formed as the area dried out and sand dunes invaded a growing desert (see 6b in Figure 1). Today it is a golden white cliff former near the canyon's rim that is 400 feet (120 meters) thick. Wind-created cross bedding patterns can be seen in its fossilized sand dunes. Also fossilized are arthropod and early reptile racks.
Next in the geologic column is the nearly 200 foot (60 meter) thick Toroweap Formation (see 6c in Figure 1). It consists of red and yellow sandstone and shaly limestone interbeded with gypsum that were deposited in a warm shallow sea as its shoreline transgressed (invaded) and regressed (retreated) over the land. In modern times it is a ledge and cliff former that contains fossils of brachiopods, corals, and mollusks along with various plants and other animals.
One of the highest, and therefore youngest, formations seen in the Grand Canyon is the massive 300 foot (90 meter) thick Kaibab Limestone (see 6d in Figure 1). A prominent cliff former, the Kaibab Limestone was laid down in middle Permian time in the deeper parts of the same advancing warm, shallow sea that deposited the underlying Toroweap Formation. This is the creamy white rock that park visitors stand on while enjoying the spectacular vistas of the canyon from both rims. Shark teeth have been found in this formation.
Uplift
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 Moenkopi Formation. The formation is made from sandstone and shale with gypsun 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. Other formations found in the region were deposited later in the Mesozoic and in the Cenozoic and may also have been removed from the Grand Canyon sequence. For details on these layers see Geology of the Zion Canyon area.
The Laramide orogeny at the end of the Mesozoic (around 65 million years ago) and the start of the Cenozoic effected all of western Larentia (proto-North America) by helping to build the Cordellian Mountain Range (of which the Rocky Mountains are a major part). However, the beds of the Colorado Plateaus remained mostly horizontal even as they were uplifted an estimated 9000 feet (2700 meters). Before the uplift the plateau region was about 1000 feet (300 meters) above sea level and bounded to 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 Fault.
Continued uplift of the Colorado Plateaus created monoclines and also increased the elevation of its plateaus. This steepened the gradient of streams flowing in this Colorado Plateaus province. The opening of an arm of the Gulf of California 5.3 million years also lowered the stream base level of the proto-Colorado River and reversed its direction toward the sagging and rifting region. These two forces in turn caused streams flowing into the gulf to flow and downcut into the Kaibab Plateau faster. Soon (geologically speaking) headwater capture consolidated these streams into one major river - the Colorado.
Erosion and Pleistocene volcanic activity
Ice ages during the Pleistocene brought a cooler and wetter pluvial climate to the region. The added precipitation increased runoff and the erosive ability of streams in the area (especially from spring meltwater 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 ago (reaching nearly its current depth 1.2 million years ago).
Cedar Mountain from Desert View
In later Pleistocene time, around one to two million years ago, basaltic lava covered part of the area and in places cascaded down side canyons, even damming the western part of the canyon between miles 178 and 188 in the Mount Trumbull area on occasion (volcanic activity was more common in that area). The river was damed in this way at least 13 times from 1.8 million to 400,000 years ago. Erosion eventually broke through each of these dams. The remains of some of them exist today as rapids such as Lava Falls.
Three of these lava dams were over 1000 feet (300 m) high, forming lakes similar to reservoirs such as Lake Mead or Lake Powell. Some of the lakes were over 100 miles (160 km) long and filled the lower portion of the Grand Canyon for many years before finally over-topping the dam and eroding much of it away. Cinder cones and the remnants of lava flows and dams are visible in the Toroweap area and from the river near Lava Falls.
The end of the Pleistocene and its 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 (although much of the rim does receive enough precipitation to support large forests). With less water to cut with, the erosive ability of the Colorado was decreased. Mass wasting processes thus began to become relatively more important than they were before, creating steeper cliffs.
Human impact, recent and future geology
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 greatly reduced the amount of water and sediment it carries. This has lessoned the river's ability to score rocks and the demand for water is so great that in most years the Colorado does not reach its outlet in the Gulf of California.
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 assuming no additional uplift in the geologic future (this does not account for human impact, which would tend to slow the rate of erosion).
References
Grand Canyon from below Grandview Point
- Geology of National Parks: Fifth Edition, Ann G. Harris, Esther Tuttle, Sherwood D., Tuttle (Iowa, Kendall/Hunt Publishing; 1997) ISBN 0-7872-5353-7
- Secrets in The Grand Canyon, Zion and Bryce Canyon National Parks: Third Edition, Lorraine Salem Tufts (North Palm Beach, Florida; National Photographic Collections; 1998) ISBN 0-9620255-3-4
- Grand Canyon The Continuing Story, Connie Rudd (KC Publishing, Inc.; 1990) ISBN 0-88714-046-7
- The Colorado River Super Guide Map of the Grand Canyon, Bronze Black (Flagstaff, Arizona; Dragon Creek Publishing; 2003)
- National Park Service: Grand Canyon National Park (http://www.nps.gov/grca/index.htm)
Related links
Top visited
0 of
0 links
[no links posted yet]
>> place link >>
Discussion
Last posted
0 of
0 messages
[no messages posted yet]
>> post message >>
Watch
You can
add this article to your own "watchlist" and receive e-mail notification about all changes in this page.
