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Colorado Plateau

The Colorado Plateau, also called the Colorado Plateaus Province, is a physiographic region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. The province covers an area of 130,000 square miles (337,000 kmē) within western Colorado, northwestern New Mexico, southeastern Utah, and northern Arizona. About 90% of the area is drained by the Colorado River and its main tributaries; the Green, San Juan and Little Colorado.

Geography
The province is bounded by the Rocky Mountains, Uinta Mountains, Rio Grande Rift, Mogollon Rim and the Basin and Range. It is composed of six sections:
Uinta Basin Section
High Plateaus Section
Grand Canyon Section
Canyon Lands Section
Navajo Section
Datil Section


As the name implies, the High Plateaus Section is, on average, the highest section. North-south trending normal faults that include the Hurricane, Sevier, Grand Wash, and Paunsaugunt separate the section's component plateaus. This fault pattern is caused by the tensional forces pulling apart the adjacent Basin and Range province to the west, making this section transitional.

Occupying the southeast corner of the Colorado Plateau is the Datil Section. Thick sequences of mid-Tertiary to late-Cenozoic-aged lava covers this section.

Development of the province has in large part been influenced by structural features in its oldest rocks. Part of the Wasatch Line and its various faults form the western edge of the province. Faults that run parallel to the Wasatch Fault that lies along the Wastach Range form the boundaries between the plateaus in the High Plateaus Section. The Unita Basin, Uncompahgre Uplift, and the Paradox Basin were also created by movement along structural weaknesses in the region's oldest rock.

In Utah, the province includes several higher fault-separated plateaus:
Aawapa Plateau
Aquarius Plateau
Kaiparowits Plateau
Markagunt Plateau
Paunsaugunt Plateau
Sevier Plateau


The mostly flat-lying sedimentary rock units that make up these plateaus are found in component plateaus that are between 5000 feet (1500 m) to over 11,000 feet (3350 m) above sea level. A supersequence of these rocks are exposed in the various cliffs and canyons (including the Grand Canyon) that make up the Grand Staircase. Increasingly younger east-west trending escarpments of the Grand Staircase extend north of the Grand Canyon and are named for their color;
Chocolate Cliffs,
Vermillion Cliffs,
White Cliffs,
Gray Cliffs, and the
Pink Cliffs.

Within these rocks are abundant mineral resources that include uranium, coal, petroleum, and natural gas. Study of the area's unusually clear geologic history (which is laid bare due to the arid and semiarid conditions) has greatly advanced that science.

A rain shadow from the Sierra Nevada far to the west and the many ranges of the Basin and Range means that the Colorado Plateau receives 6 to 16 inches (15 to 40 cm) of annual precipitation. Higher areas receive more precipitation and are covered in forests of pine, fir, and spruce.

History

Anasazi lived in the region from around 2000 to 700 years ago.

U.S. Army Major John Wesley Powell was a geologist and one-armed American Civil War veteran who explored the area in 1869 and 1872. Using fragile boats and small groups of men the Powell Expeditions charted this largely unknown region of the United States for the federal government.

Construction of the Hoover Dam in the 1930s and the Glen Canyon Dam in the 1960s changed the character of the Colorado River. Dramatically reduced sediment load changed its color from reddish brown (Colorado is Spanish for "red colored") to mostly clear. The apparent green color is from algae on the riverbed's rocks, not from any significant amount of suspended material. The lack of sediment has also starved sand bars and beaches but an experimental 12 day long controlled flood from Glen Canyon Dam in 1996 showed substantial restoration. Similar floods are planned for every 5 to 10 years.

Geology

One of the most geologically intriguing features of the Colorado Plateau is its remarkable stability. Relatively little rock deformation such as faulting and folding has affected this high, thick crustal block within the last 600 million years or so. In contrast, provinces that have suffered severe deformation surround the plateau. Mountain building thrust up the Rocky Mountains to the north and east and tremendous, earth-stretching tension created the Basin and Range province to the west and south.

The Precambrian and Paleozoic history of the Colorado Plateaus is only known at its southern end where the Grand Canyon has exposed the 2000 million year old Vishnu Schist to the 600 million year old Kaibab Limestone. Mountain Ranges that existed 2000 million years ago were eroded away by 1700 million years ago. Their roots were covered by sediments some 1250 to 1070 million years ago, which in turn were uplifted and split into a range of fault-block mountains. Erosion greatly reduced this mountain range prior to the encroachment of a seaway along the passive western edge of the continent in the early Paleozoic.

A 12,000 to 15,000 foot (3700 to 4600 m) high extension of the Ancestral Rocky Mountains called the Uncompahgre Mountains were uplifted and the adjacent Paradox Basin subsided. Almost 4 miles (6.4 km) of sediment from the mountains and evaporites from the sea were deposited (see geology of Arches and Canyonlands area for detail).

Most of the formations were deposited in warm shallow seas and near-shore environments (such as beaches and swamps) as the seashore repeatedly advanced and retreated over the edge of a proto-North America (for detail, see geology of the Grand Canyon area). The province was probably on a continental margin throughout the late Precambrian and most of the Paleozoic era. Igneous rocks injected millions of years later form a marbled network through parts of the Colorado Plateau's darker metamorphic basement. By 600 million years ago North America had been leveled off to a remarkably smooth surface.

Throughout the Paleozoic Era, tropical seas periodically inundated the Colorado Plateau region. Thick layers of limestone, sandstone, siltstone, and shale were laid down in the shallow marine waters. During times when the seas retreated, stream deposits and dune sands were deposited or older layers were removed by erosion. Over 300 million years passed as layer upon layer of sediment accumulated.

Erosion-resistant sandstones of Mesozoic age result in bands of continuous cliffs, central Colorado Plateau

It was not until the upheavals that coincided with the formation of the supercontinent Pangea began about 250 million years ago that deposits of marine sediment waned and terrestrial deposits dominate. In late Paleozoic and much of the Mesozoic era the region was affected by a series of orogenies (mountain-building events) that deformed western North America and caused a great deal of uplift. Eruptions from volcanic mountain ranges to the west buried vast regions beneath ashy debris. Short-lived rivers, lakes, and inland seas left sedimentary records of their passage. Streams, ponds and lakes created formations such as the Chinle, Moenave, and Kayenta in the Mesozoic era. Later a vast desert formed the Navajo and Temple Cap formations and dry near-shore environment formed the Carmel (see geology of the Zion and Kolob canyons area for details).

The area was again covered by a warm shallow sea when the Cretaceous Seaway opened in late Mesozoic time. The Dakota Sandstone and the Tropic Shale were deposited in the warm shallow waters of this advancing and retreating seaway. Several other formations were also created but were mostly eroded following two major periods of uplift.

The Laramide orogeny closed the seaway and uplifted a large belt of crust from Montana to Mexico, with the Colorado Plateau region being the largest block. Thrust faults in Colorado are thought to have formed from a slight clockwise movement of the region, which acted as a rigid crustal block. The Colorado Plateaus Province was uplifted largely as a single block, possibly due to its relative thickness. This relative thickness may be why compressional forces from the orogeny were mostly transmitted through the province instead of compacting it. Pre-existing weaknesses in Precambrian rocks were reactivated by the compression. It was along these ancient faults and other deeply-buried structures that much of the province's relatively small and gently-inclined flexures (such as anticlines, synclines, and monoclines) formed.

Minor uplift events continued through the start of the Cenozoic era and was accompanied by some basaltic lava eruptions and mild deformation. The colorful Claron Formation that forms the delicate hoodoos of Bryce Amphitheater and Cedar Breaks was then laid down as sediments in cool streams and lakes (see geology of the Bryce Canyon area for details).

Tectonic activity resumed in Mid Cenozoic time and started to unevenly uplift and slightly tilt the Colorado Plateaus region and the region to the west some 20 million years ago (as much as 3 kilometers of uplift occurred). Streams had their gradient increased and they responded by downcutting faster. Headward erosion and mass wasting helped to erode cliffs back into their fault-bounded plateaus, widening the basins in-between. Some plateaus have been so severely reduced in size this way that they become mesas or even buttes. Monoclines form as a result of uplift bending the rock units. Eroded monoclines leave steeply tilted resistant rock called a hogback and the less steep version is a cuesta.

Great tension developed in the crust, probably related to changing plate motions far to the west. As the crust stretched, the Basin and Range province broke up into a multitude of down-dropped valleys and elongate mountains. Major faults, such as the Hurricane Fault, developed that separate the two regions. The dry climate was in large part a rainshadow effect resulting from the rise of the Sierra Nevada further west. Yet for some reason not fully understood, the neighboring Colorado Plateau was able to preserve its structural integrity and remained a single tectonic block. Eventually, the great block of Colorado Plateau crust rose a kilometer higher than the Basin and Range. As the land rose, the streams responded by cutting ever deeper stream channels. The most well-known of these streams, the Colorado River, began to carve the Grand Canyon less than 6 million years ago in response to sagging caused by the opening of the Gulf of California to the southwest.

The Pleistocene epoch brought periodic ice ages and a cooler, wetter climate. This increased erosion at higher elevations with the introduction of alpine glaciers while mid-elevations were attacked by frost wedging and lower areas by more vigorous stream scouring. Pluvial lakes also formed during this time. Glaciers and pluvial lakes disappeared and the climate warmed and became drier with the start of Holocene epoch.

Protected lands

This relatively high semi-arid province produces many distinctive erosional features such as arches, arroyos, canyons, cliffs, fins, natural bridges, pinnacles, hoodoos, and monoliths that, in various places and extents, have been protected. Also protected are areas of historic or cultural significance, such as the pueblos of the Anasazi culture. There are eight U.S. National Parks, fifteen U.S. National Monuments and dozens of wilderness areas in the province along with millions of acres in U.S. National Forests, many state parks, and other protected lands. In fact, this region has the highest concentration of parklands in North America.

Erosional features within Glen Canyon National Recreation Area. Lake Powell, in foreground, is not a natural lake but a reservoir impounded by Glen Canyon Dam.

National parks (from south to north clockwise):
Petrified Forest National Park
Grand Canyon National Park
Zion National Park
Bryce Canyon National Park
Capitol Reef National Park
Canyonlands National Park
Arches National Park
Mesa Verde National Park


National Monuments:
Sunset Crater National Monument
Wupatki National Monument
Colorado National Monument
Black Canyon of the Gunnison National Park
Grand Staircase-Escalante National Monument
Walnut Canyon National Monument
Canyon De Chelly National Monument
Cedar Breaks National Monument
El Malpais National Monument
El Morro National Monument
Hovenweep National Monument
Natural Bridges National Monument
Navajo National Monument
Rainbow Bridge National Monument
Vermilion Cliffs National Monument


Wilderness areas:
Kachina Peaks Wilderness
Strawberry Crater Wilderness
Kendrick Mountain Wilderness
Beaver Dam Mountains Wilderness
Pauite Wilderness
Grand Wash Cliffs Wilderness
Mount Logan Wilderness
Mount Trumbull Wilderness
Kanab Creek Wilderness
Cottonwood Point Wilderness
Paria Canyon-Vermilion Cliffs Wilderness
Saddle Mountain Wilderness
Mount Baldy Wilderness
Escudilla Wilderness
Black Ridge Canyons Wilderness
Flat Tops Wilderness
Uncompahgre Wilderness
Mount Sneffels Wilderness
Lizard Head Wilderness
Weminuche Wilderness
South San Juan Wilderness
Cebolla Wilderness
West Malpais Wilderness
Bisti-De-Na-Zin Wilderness
Pine Valley Mountain Wilderness
Ashdown Gorge Wilderness
Box Death Hollow Wilderness
Dark Canyon Wilderness
High Uintas Wilderness


Other notable protected areas include: Glen Canyon National Recreation Area, Dead Horse Point State Park, Kodachrome State Park, Goblin Valley State Park and Barringer Crater.

From Wikipedia, the free encyclopedia