Canyonlands geology
The geology of the Canyonlands is very varied. In the Canyonlands National Park in the US state of Utah , 12 rock formations are exposed, whose ages range from Pennsylvanian (Upper Carboniferous) to the Cretaceous period . The oldest and perhaps the most interesting formation consists of evaporites , which can be traced back to evaporating sea water. Advancing and retreating warm shallow seas left behind various fossil-rich limestone , sandstone and shale in the further course of the Paleozoic Era .
The detritus of the nearby mountain range of the Uncompahgre Mountains then mingled in Perm with the deposits of coastal dunes and sand bars. At the end of the Paleozoic Era and with the beginning of the Mesozoic Era, the sea withdrew forever from the Canyonlands region. A very flat landscape was dominated by alluvial plains and marshes in the Triassic . Further inland there was a dry climate. Extensive deserts covered large parts of North America . There was only a brief return of more humid climatic conditions, documented in river sediments competing with dune deposits.
In the late Upper Cretaceous the gradual uplifting of the Rocky Mountains began . This had serious consequences for the Canyonlands, for example the erosion rates were greatly increased. With the Pleistocene glaciation , the latter experienced a further acceleration. Only in the recent past has erosion started again much more slowly.
stratigraphy
Hermosa group
In early Pennsylvania , the Canyonlands were covered by an extensive sea. In the east a mountain range, the Uncompahgre Mountains, was raised . The immediate result was a deepening of the crust on its west side - the Paradox Basin , a typical foreland basin, was created . In this rapidly sinking basin, several thousand meters of evaporites (first anhydrite and gypsum , later rock salt and sylvine ) were excreted from the seawater during the Middle Pennsylvania due to the arid climate . The salt layers often interlock with siliciclastic deposits, which were washed out of the mountain range during periods of storm. Fresh seawater occasionally filled the pool again, but could never completely displace the pool water, which was oversaturated with salts (it only covered the denser pool water and remained limited to the surface). The precipitated evaporite layers later solidified to form the Paradox Formation , part of the Hermosa Group . During the later Pennsylvania, the salt layers under the compression of the increasing load began to deform plastically and to penetrate into higher altitudes. The main phase of this salt tectonics should have lasted until the end of the Jura . Satellite-based measurements, however, show that even to this day the salt and gypsum layers have not yet come to rest and continue to deform overlying sediment layers.
The Paradox Formation is up to 1520 meters thick in places. In the national park it is in the lower section of the Cataract Canyon as gypsum rock with black slate layers. Diapir-like ascent of the Paradox Formation may also explain the formation of the Upheaval Dome , even if the formation there is not open. However, the prevailing theory is still the impact of a meteorite .
Towards the end of the Pennsylvania, a warm shallow sea returned to the Canyonlands area. Lime sludge, sands and clays were now deposited on the mighty salt layers. These sediments became the fossil limestone , sandstone and shale clays of the gray- hued Honaker Trail Formation . Outcrops of the Honaker Trail formation are located on the valley floor of the deeply cut canyons in the national park, the best of course being on the Colorado River itself.
As a result, erosion set in and there was a layer gap in the geological records.
Cutler formation
In the early Permian there was a sea transgression from the west, the sediments of which now form the Elephant Canyon member of the Cutler Formation . The Elephant Canyon member interlocks to the east with the continental sediments of the Halgaito Shale member . Both members can be seen in Cataract Canyon and Elephant Canyon .
The Uncompahgre Mountains were exposed to severe erosion at the time. Large alluvial fans filled the transition area between the mountain range and the Paradox Basin . The resulting red sediments of the undivided Cutler Formation therefore mainly consist of iron-rich arkose sandstones. Marine sand bars and sand dunes close to the coast interlock with the red sediments, they later became white-colored, steep-walled Cedar-Mesa-Sandstone-Members . These two competing rock units are now open in the national park in a 6.4 to 8 kilometer wide band, which extends from the “Needles” to the “Maze” to the “Elaterite Basin”.
Brightly colored (red to brown), oxidized claystones were then deposited over the Cedar-Mesa-Sandstone-Member. They now form the Organ-Rock-Shale-Member , a relatively weather-prone rock unit that comes to light in the so-called “Land of Standing Rocks”.
Coastal sand dunes and marine sand bars returned again and left behind the sloping, steep-walled White-Rim-Sandstone-Member . It forms a prominent platform 365 meters below the top of the "Island in the Sky", which is what earned the latter its name. It is also exposed along the “White Rim Trail” and in the “Elaterite Basin” (as a fossilized former sandbar). The elaterite Basin takes its name from an outcropping here, dark brown, oily-tarry substance, Elaterit (elastic bitumen).
The Permian Sea retreated, long-lasting erosion began and a new layer gap emerged.
Moenkopi Formation and Chinle Formation
In the Triassic , red siliciclastic sediments were deposited on the leveled Paleozoic surface. The sediments came from rivers that drained an extensive, slightly sloping low plain to the west towards the open sea . Schlick, which was later to form the clay stones of the Moenkopi Formation , settled in marshes . Examples of this formation with ripple marks and dry cracks can be seen in the north and west of the national park.
The sea withdrew again and there was another phase of erosion. The brightly colored slate clays of the slope-forming chinle formation then settled on this erosion surface . Petrified wood is often found at the foot of the chinle formation in the Petrified Forrest Member .
Glen Canyon Group
The Glen Canyon Group includes the following formations (from young to old):
These rock units are best exposed in the western and northern parts of the national park.
The Triassic climate gradually became drier, creating sand dunes that buried dry valleys and associated alluvial plains. These dunes solidified into the sheer wall-forming and over a hundred meters thick, red-colored Wingate Sandstone . The sandstone wall often runs hundreds of kilometers without major interruptions and therefore often represents a traffic obstacle.
For a short time the climate became a little more humid again, so that now rivers paved their way through the dunes. They left behind red-brown to lavender-colored sandstones that alternated with siltstones and shale - the result is the slope-forming Kayenta formation .
The youngest formation from the Glen Canyon Group was created again under arid environmental conditions. At that time, a very dry, Sahara-like desert had formed in western North America, covering around 388,000 square kilometers. Slanted sand dunes reached enormous heights, especially in the nearby Zion National Park and in the Kolob Canyon (see also the geology of Zion National Park ). The slightly orange-colored Navajo Sandstone was created , which can form huge rock walls, rock towers and sometimes rock gates (such as the Millard Canyon Arch).
This was followed by renewed removal with the formation of a layer gap.
San Rafael group
On the erosion area of the Glen Canyon Group , mud flats, the later Carmel Formation, formed . The massive Entrada Sandstone , also a wall picture , was then deposited over the erosion-prone sediments of the Carmel Formation . Long-lasting erosion activity then removed a large part of the San Rafael group and any Cretaceous sediments.
Tectonic prominence
The Laramian orogeny began around 70 million years ago and continued well into the Tertiary . The result of this process was the gradual formation of the Rocky Mountains . The area of the Canyonlands was raised by around a thousand meters without the original stratification being disturbed. The layers remained in their original horizontal position. However, the slow lifting caused a pronounced fracturing of the rock structure, which in turn had a major influence on erosion patterns.
When the salt layers of the Paradox Formation were reached by the groundwater , the more soluble salts went into solution, only the gypsum remained. In the “Graben” area, things even got so far that higher-lying members of the stratum fell into the empty cavities created in this way.
During the Pleistocene glaciations , the rate of erosion increased enormously, so that the canyons were cleared more quickly. The widening and deepening of the canyons had accelerated particularly at the Green River and the Colorado River , as these rivers carried glacial melt water from the Rocky Mountains. These landscape-shaping processes continued in the Canyonlands into the Holocene (or now), but are now much slower due to the increased aridity.
literature
- Ann G. Harris, Esther Tuttle, Sherwood D. Tuttle: Geology of National Parks. 5th edition. Kendall / Hunt Publishing, Iowa 1997, ISBN 0-7872-5353-7 .
- M. Furuya, K. Mueller, J. Wahr: Active salt tectonics in the Needles District, Canyonlands (Utah) as detected by interferometric synthetic aperture radar and point target analysis: 1992-2002. In: Journal of Geophysical Research. Vol. 112, 2007, p. B06418, doi: 10.1029 / 2006JB004302