Sevier orogeny

The Sevier orogeny was a mountain-building event that affected western North America from Canada to the north to Mexico to the south.

The Sevier orogeny was the result of convergent boundary tectonic activity between approximately 140 million years (Ma) ago and 50 Ma. The Sevier River area of central Utah is the namesake of this event. This orogeny was produced by the subduction of the oceanic Farallon Plate underneath the continental North American Plate. Crustal thickening that led to mountain building was caused by a combination of compressive forces and conductive heating initiated by subduction in the Sevier region which caused folding and thrusting.[1]

SunRiver
An example of Thin-skinned thrusting in Montana. Note the white Madison Limestone repeated, with one example in the foreground (that pinches out with distance) and another to the upper right corner and top of the picture.

Extent

The mountains that were formed as a result were located in western Utah and eastern Nevada. The size, shape, and depth of the thrust faults created in the Sevier event are determined by seismic studies and deep well data because they are mostly still buried by overlying rock and sediment.

The Sevier and Laramide orogenies ended when subduction along the western edge of North America was overcome by western extension of the North American Plate to start the Basin and Range Orogeny. The well known and familiar Basin and Range faults cut the older Sevier thrust faults.[2] The Sevier orogeny was preceded by several other mountain-building events including the Nevadan orogeny, the Sonoman orogeny, and the Antler orogeny, and partially overlapped in time and space with the Laramide orogeny.

Sevier or Laramide?

Since the Sevier and Laramide orogenies occurred at similar times and places, they are sometimes confused.[3] In general the Sevier orogeny defines a more western compressional event that took advantage of weak bedding planes in overlying Paleozoic and Mesozoic sedimentary rock. As the crust was shortened, pressure was transferred eastward along the weak sedimentary layers, producing “thin-skinnedthrust faults that generally get younger to the east. In contrast, the Laramide orogeny produced “basement-cored” uplifts that often took advantage of pre-existing faults that formed during rifting in the Late Precambrian during the breakup of the supercontinent Rodinia or during the Ancestral Rocky Mountains orogeny.[3]

Geologic structures

BasinrangeINDEX
Map from U.S.G.S. showing the Basin and Range Province in the United States. Basin and Range includes the western portion of Utah, essentially all of Nevada - the central heart of the Great Basin itself - bits of southern Oregon and Idaho, southern Arizona, New Mexico and far west Texas, and the eastern fringe and southeastern desert region of California. It also extends into Baja California and other areas of northwestern Mexico.

The Sevier orogenic belt consisted of a series of thin plates along gently dipping west thrust sheets and moving from west to east.[4] These thin skinned thrusts moved late Precambrian to Mesozoic age rock of the Cordilleran passive margin east. The Sevier meets the Laramide orogenic belt on its eastern side.[5] The Sevier and Laramide combination is similar to the modern day Andean margin in Chile. They are comparable because the younger Laramide faults and structures were a geometric response to the shallow dipping Sevier thrusts.[6]

The location of the eastern edge of the Sevier orogeny was determined by conglomerates largely made up of boulders that would have been shed from the eastern and steepest edge of the rising mountains. Such conglomerates can be seen throughout Utah in Echo Canyon, the Red Narrows in Spanish Fork Canyon, and in Leamington Canyon near Delta, Utah. Today Sevier faults at the surface have been broken up and tilted steeply from their original gently dipping positions due to the extension of the Basin and Range faulting. The earliest thrusts of the Sevier are located furthest west with each newer thrust cutting the older thrust. This pattern caused the older thrusts to ride on top of the younger thrusts as they moved eastward. The Paris-Willard thrust in Utah was determined to be the oldest thrust in the series using this pattern. The youngest thrust is the Hogback in Wyoming.[2]

The Sevier thrust belt in Utah can be divided in two, north of Salt Lake City and South of Salt Lake City. The thrusts to the north are much better understood because oil and gas are often associated with them. The northern portion runs through present day Utah, Idaho, and Wyoming. The southern portion stops around Las Vegas. The total crustal shortening of the northern portion was roughly 60 miles.[2]

TransZ
This is a diagram showing how transverse zones often connect thrust faults in a fold and thrust belt.

The Sevier belt left behind many distinctive geologic features in the Wyoming and Utah region, namely recesses and salients. Transverse zones can accompany thrust faults connecting the segments of the belt. One such zone is the Charleston transverse zone linking the Provo salient to the southern arm of the Uinta/Cottonwood arch. Although the Uinta/Cottonwood arch is a Laramide structure the Sevier helped the arch form. Another important zone is the Mount Raymond transverse zone connecting the Wyoming salient and the northern arm of the arch.[7]

While continental margins are typically the most deformed in orogenic events, the interior of continental plates can also deform. In the Sevier-Laramide orogenic events evidence for interior plate deformation includes folds, cleavage and joint fabrics, distorted fossils, persistent faulting, and calcite twinning.[6]

Sevierorogeny
This is a cross section of the Sevier fold and thrust belt along with major geologic features that accompanied the orogeny.

How and when

The Sevier fold and thrust belt was active between late Jurassic through Eocene time.[8] The actual age of initiation of the belt is not entirely agreed upon by researchers.[8] The beginning of deformation in the earliest stages of the orogeny started about 120-80 Ma (millions of years ago) with the formation and continuation of a magmatic arc and foreland fold-thrust belt.[1]

However, data from the southern portion of the belt shows contraction in southern Nevada and southeastern California beginning about 200 to 92 Ma largely based on intrusions and the formation of the Lavinia Wash conglomerate sequence due to mountain building and erosion.[8] This deformation continued and intensified around 105 to 100 Ma caused by the continued subduction of the Farallon plate beneath the North American plate.

Deformation spread eastward starting around 80 to 75 Ma. At this time the elevated crust ran into the Colorado Plateau. The collision resulted in lateral spreading of deformation and led to a weakened lithosphere and crustal thickening.[9] Metamorphism due to the crustal heating and thickening is prevalent between 90 and 70 Ma in the present Great Basin region.[9]

Studies

Transverse zones and the Uinta recess

Parallel thrust faults and folds make up a fold-thrust belt on a regional scale. At the local scale segments of the belt are connected by transverse zones. The Charleston transverse zone mentioned earlier runs perpendicular to the thrust faults within the Sevier belt. It has been debated among geologists if this transverse zone developed during the Sevier orogeny or the Uinta/Cottonwood arch formation during the Laramide orogeny.[5] Mapping Sevier thrusting in the Basin and Range Province suggests Sevier structures curve around the Uinta/Cottonwood arch defining the Uinta recess. Looking closely at Sevier faults in American Fork Canyon indicate that these faults are the oldest in the Charleston transverse zone suggested by cross cutting relationships observed in the area.[7]

The Basin and Range Province extending across Nevada, into western Utah, and south into Mexico now consists of N-S normal faulting due to crustal extension. If these normal faults show any extension in late Eocene to early Miocene, this could be evidence the Sevier orogenic event collapsing after deactivation.[5] Thickening of the crust due to Sevier and Laramide faulting is thought to have led to current Basin and Range extension throughout the Cenozoic.[1] This could have caused the Charleston thrust fault to reactivate as an extensional fault. The Charleston transverse zone contained high angle faults which suggests it initiated as a response to connecting the low angle thrust faults of the Sevier. The Charleston transverse zone outlines a main sidewall ramp that would have been part of the Sevier belt.[5]

To the north of the Uinta/Cottonwood arch during the Sevier orogeny there was a basement high area gently dipping to the north identified by isopach maps. Thus sediment thickened quickly to the south. To the north strata changed gradually throughout the thrust and a gradual curve developed around the Wyoming salient and to the south around the Provo salient. The Charleston and Mount Raymond transverse zones formed the Uinta recess indicating the recess was initiated during the Sevier orogeny.[7]

The results were interpreted to support the Charleston transverse zone forming during the Sevier orogeny to accommodate geometric changes along strike of the thrusts. The zone served as a linking tool of the various segments of the orogeny. The transverse zone varied throughout the region in terms of depth and displacement. The zone was later tilted and was reactivated through crustal extension.[5] Results also support the Uinta recess forming during the Sevier orogeny due to similar geometric crustal accommodation. Displacement on Sevier aged thrust faults caused the shaping of the curvature of the Uinta recess prior to uplift of the Uinta/Cottonwood arch.[7]

Related thrust belts

Focusing on the southern portion of the Sevier thrust belt many thrust faults can be found. One thrust system is known as the Garden Valley thrust system in the central Nevada thrust belt. Thrusts within this system include the Pahranagat, Mount Irish, and Golden Gate thrusts. These thrusts were correlated with the southward Gass Peak thrust. The Gass Peak thrust is located in the Las Vegas Range and is a Sevier age structure. This thrust may have been responsible for the largest slip of the major belt along that latitude. These thrusts were located all along the same strike. This region showed small scale extension in the Cenozoic due to reactivation of the thrusts. Such a correlation suggests that the Garden Valley thrust system has a direct link to the Sevier thrust belt. The interpretation of this data led to the central Nevada thrust belt as being an interior section of the Sevier. This correlation provides evidence that the Sevier thrust belt was a result of compression moving eastward through the North American plate.[6]

Cordilleran and Sevier orogenesis relationships

Thinning of the Cordilleran has previously been thought to be evidence and reason for flat subduction in the Sevier and Laramide orogenic events. However, isotopic data suggests that preservation of Cordilleran lithosphere implies Cordilleran thinning is not a sufficient answer for Sevier and Laramide flat subduction. This implies thinning and shearing of the Cordilleran was confined to the fore-arc region.[9] Data suggests throughout the Sevier-Laramide thrusting the crust was also uplifted and extended.[1] The modern Chilean subduction is thought to be a parallel model of the Sevier and Laramide events so there are possibly answers to this question in this modern model. Explanations may include a combination of plate motion rates increasing, the underriding oceanic plate becoming younger as the older portion subducts, and thus the underriding plate being hotter and more buoyant.[9]

Crustal shortening

A study on calcite twinning and carbonate relationships with the Sevier orogenic belt showed that shortening directions were parallel to the thrust faulting, which was an E-W direction. Differential stress magnitudes determined from calcite twinning showed a decreasing trend exponentially toward the craton. Differential stresses causing compressional deformation in the Sevier thrust were greater than 150 MPa. The E-W contraction during the Sevier changed to roughly N-S oblique during the Laramide orogenic event. Sevier shortening has been recorded throughout much of the western United States as far east as Minnesota in the Cretaceous Greenhorn Limestone as preserved by calcite twinning. The distance of stress transfer is roughly equivalent to more than 2000 km. The E-W shortening shown in calcite twinning of the Sevier is parallel to today's principal stresses in the western interior of the North American plate.[6]

See also

References

  1. ^ a b c d Livacarri, R.F., 1991, Role of crustal thickening and extensional collapse in the tectonic evolution of the Sevier-Laramide Orogeny, Western United States, Geology [Boulder], Vol. 19, Issue 11, pp. 1104-1107.
  2. ^ a b c Hintze, L., 2005, Utah’s Spectacular Geology, Department of Geology, Brigham Young University, pp. 57, 60-62, 65.
  3. ^ a b Willis, Grant C. (2000). "I thought that was the Laramide orogeny!". Utah's Sevier Thrust System. Utah Geological Survey.
  4. ^ Burtner, R.G. and Nigrini, A., 1994, Thermochronology of the Idaho-Wyoming thrust belt during the Sevier Orogeny; a new, calibrated, multiprocess thermal model, AAPG Bulletin, Vol. 78, Issue 10, pp. 1586-1612.
  5. ^ a b c d e Paulsen, T. and Marshak, S., 1998, Charleston transverse zone, Wasatch Mountains, Utah; structure of the Provo Salient’s northern margin, Sevier fold-thrust belt, Geological Society of America Bulletin, Vol. 116, Issue 4, pp. 512-522.
  6. ^ a b c d Craddock, J.P. and van der Plujim, B.A., 1999, Sevier-Laramide deformation of the continental interior from calcite twinning analysis, west-central North, Tectonophysics, Vol. 205, Issue 1-3, pp. 275-286.
  7. ^ a b c d Paulsen, T. and Marshak, S., 1999, Origin of the Uinta Recess, Sevier fold-thrust belt, Utah; influence of basin and architecture on fold-thrust belt geometry, Tectonophysics, Vol. 312, Issue 2-4, pp. 203-216.
  8. ^ a b c Taylor, W.J., Bartley, J.M., Martin, M.W., Geissman, J.W., Walker, J.D., Armstrong, P.A., and Fryxell, J.E., 2000, Relations between hinterland and foreland shortening: Sevier orogeny, central North America Cordillera, Tectonophysics, Vol. 19, Issue 6, pp. 1124-1143.
  9. ^ a b c d Livacarri, R.F. and Perry, F.V., 1993, Isotopic evidence for preservation of Cordilleran lithospheric mantle during the Sevier-Laramide Orogeny, Western-United States, Geology [Boulder], Vol. 21, Issue 8, pp. 719-722.
Canadian Rockies

The Canadian Rockies (French: Rocheuses canadiennes) or Canadian Rocky Mountains comprise the Canadian segment of the North American Rocky Mountains. They are the eastern part of the Canadian Cordillera, which is a system of multiple ranges of mountains which runs from the Canadian Prairies to the Pacific Coast. The Canadian Rockies mountain system comprises the southeastern part of this system, lying between the Interior Plains of Alberta and northeastern British Columbia on the east to the Rocky Mountain Trench of BC on the west. The southern end borders Idaho and Montana of the United States. In geographic terms, the boundary is at the Canada–United States border, but in geological terms it might be considered to be at Marias Pass in northern Montana. The northern end is at the Liard River in northern British Columbia.

The Canadian Rockies have numerous high peaks and ranges, such as Mount Robson (3,954 m, 12,972 ft) and Mount Columbia (3,747 m, 12,293 ft). The Canadian Rockies are composed of shale and limestone. Much of the range is protected by national and provincial parks, several of which collectively comprise a World Heritage Site.

Delamar Mountains

The Delamar Mountains are a mountain range in Lincoln County, Nevada, named after Captain Joseph Raphael De Lamar. The range extends for approximately 50 miles (80 km) in a NNE–SSW orientation with a width of about 11 miles (18 km). Surrounding ranges include the Burnt Springs Range and the Chief Range to the north, the Clover Mountains and Meadow Valley Mountains to the east and the Sheep Range and South Pahroc Range on the west. The Delamar Valley lies to the west, the Kane Springs Valley to the east and the Coyote Springs Valley lies to the south of the range.U.S. Route 93 traverses the north end of the range between Crystal Springs and Caliente. The elevation of the route reaches 6243 feet at Oak Springs Summit pass. Nevada State Route 317 follows Rainbow Canyon south along the northeast margin of the range between Caliente and Elgin.The range's crest forms part of the Great Basin Divide between the Meadow Watershed and the Dry Lake Watershed, which includes Delamar Dry Lake and the old mining townsite of Delamar.

The Delamar Mountains Wilderness Area covers the southern portion of the range.

Gates of the Mountains Wilderness

The Gates of the Mountains Wilderness is located in the U.S. state of Montana. Created by an act of Congress in 1964, the wilderness is managed by Helena National Forest. A day use campground near the Gates of the Mountains, Meriwether Picnic site, is named in honor of Meriwether Lewis.

Gates of the Mountains Wilderness (then known as the Gates of the Mountains Wild Area) was the site of the 1949 Mann Gulch fire, which claimed the lives of 13 firefighters and which was the subject of Norman Maclean's book Young Men and Fire.

U.S. Wilderness Areas do not allow motorized or mechanized vehicles, including bicycles. Although camping and fishing are allowed with proper permit, no roads or buildings are constructed and there is also no logging or mining, in compliance with the 1964 Wilderness Act. Wilderness areas within National Forests and Bureau of Land Management areas also allow hunting in season.

Geology of Arizona

The geology of Arizona began to form in the Precambrian. Igneous and metamorphic crystalline basement rock may have been much older, but was overwritten during the Yavapai and Mazatzal orogenies in the Proterozoic. The Grenville orogeny to the east caused Arizona to fill with sediments, shedding into a shallow sea. Limestone formed in the sea was metamorphosed by mafic intrusions. The Great Unconformity is a famous gap in the stratigraphic record, as Arizona experienced 900 million years of terrestrial conditions, except in isolated basins. The region oscillated between terrestrial and shallow ocean conditions during the Paleozoic as multi-cellular life became common and three major orogenies to the east shed sediments before North America became part of the supercontinent Pangaea. The breakup of Pangaea was accompanied by the subduction of the Farallon Plate, which drove volcanism during the Nevadan orogeny and the Sevier orogeny in the Mesozoic, which covered much of Arizona in volcanic debris and sediments. The Mid-Tertiary ignimbrite flare-up created smaller mountain ranges with extensive ash and lava in the Cenozoic, followed by the sinking of the Farallon slab in the mantle throughout the past 14 million years, which has created the Basin and Range Province. Arizona has extensive mineralization in veins, due to hydrothermal fluids and is notable for copper-gold porphyry, lead, zinc, rare minerals formed from copper enrichment and evaporites among other resources.

Geology of Nevada

The geology of Nevada began to form in the Proterozoic at the western margin of North America. Terranes accreted to the continent as a marine environment dominated the area through the Paleozoic and Mesozoic periods. Intense volcanism, the horst and graben landscape of the Basin and Range Province originating from the Farallon Plate, and both glaciers and valley lakes have played important roles in the region throughout the past 66 million years.

Geology of New Mexico

The geology of New Mexico formed beginning over 1.7 billion years ago in the Proterozoic as several poorly understood terranes merged. Five types of igneous and metamorphic crystalline basement rock date to the Precambrian. Throughout the Paleozoic, marine sediments and evaporites formed, followed by a series of major mountain building events and volcanism associated with the subduction of the Farallon Plate. Terrestrial conditions persisted until the late Mesozoic, when a marine transgression flooded the region. Significant volcanic activity including ash falls, lava flows and caldera collapse have defined the Cenozoic in New Mexico, along with the horst and graben rifting of the Basin and Range Province and the formation of the Rio Grande Rift.

Geology of Utah

The geology of Utah includes rocks formed at the edge of the proto-North American continent during the Precambrian. A shallow marine sedimentary environment covered the region for much of the Paleozoic and Mesozoic, followed by dryland conditions, volcanism and the formation of the basin and range terrain in the Cenozoic. Utah is a state in the western United States.

Geology of the Bryce Canyon area

The exposed geology of the Bryce Canyon area in Utah shows a record of deposition that covers the last part of the Cretaceous Period and the first half of the Cenozoic era in that part of North America. The ancient depositional environment of the region around what is now Bryce Canyon National Park varied from the warm shallow sea (called the Cretaceous Seaway) in which the Dakota Sandstone and the Tropic Shale were deposited to the cool streams and lakes that contributed sediment to the colorful Claron Formation that dominates the park's amphitheaters.

Other formations were also formed but were mostly eroded following uplift from the Laramide orogeny which started around 70 million years ago (mya). This event created the Rocky Mountains far to the east and helped to close the sea that covered the area. A large part of western North America started to stretch itself into the nearby Basin and Range topography around 15 mya. While not part of this region, the greater Bryce area was stretched into the High Plateaus by the same forces. Uplift of the Colorado Plateaus and the opening of the Gulf of California by 5 mya changed the drainage of the Colorado River and its tributaries, including the Paria River, which is eroding headward in between two plateaus adjacent to the park. The uplift caused the formation of vertical joints which were later preferentially eroded to form the free-standing pinnacles called hoodoos, badlands, and monoliths we see today.

The formations exposed in the area of the park are part of the Grand Staircase. The oldest members of this supersequence of rock units are exposed in the Grand Canyon, the intermediate ones in Zion National Park, and its youngest parts are laid bare in Bryce Canyon area. A small amount of overlap occurs in and around each park.

Geology of the Zion and Kolob canyons area

The geology of the Zion and Kolob canyons area includes nine known exposed formations, all visible in Zion National Park in the U.S. state of Utah. Together, these formations represent about 150 million years of mostly Mesozoic-aged sedimentation in that part of North America. Part of a super-sequence of rock units called the Grand Staircase, the formations exposed in the Zion and Kolob area were deposited in several different environments that range from the warm shallow seas of the Kaibab and Moenkopi formations, streams and lakes of the Chinle, Moenave, and Kayenta formations to the large deserts of the Navajo and Temple Cap formations and dry near shore environments of the Carmel Formation.

Subsequent uplift of the Colorado Plateau slowly raised these formations much higher than where they were deposited. This steepened the stream gradient of the ancestral rivers and other streams on the plateau. The faster-moving streams took advantage of uplift-created joints in the rocks to remove all Cenozoic-aged formations and cut gorges into the plateaus. Zion Canyon was cut by the North Fork of the Virgin River in this way. Lava flows and cinder cones covered parts of the area during the later part of this process.

Zion National Park includes an elevated plateau that consists of sedimentary formations that dip very gently to the east. This means that the oldest strata are exposed along the Virgin River in the Zion Canyon part of the park, and the youngest are exposed in the Kolob Canyons section. The plateau is bounded on the east by the Sevier Fault Zone, and on the west by the Hurricane Fault Zone. Weathering and erosion along north-trending faults and fractures influence the formation of landscape features, such as canyons, in this region.

Greater Green River Basin

The Greater Green River Basin (GGRB) is a 21,000 square mile basin located in Southwestern Wyoming. The Basin was formed during the Cretaceous period sourced by underlying Permian and Cretaceous deposits. The GGRB is host to many anticlines created during the Laramide Orogeny trapping many of its hydrocarbon resources. It is bounded by the Rawlins Uplift, Uinta Mountains, Sevier overthrust belt, Sieria Madre Mountains, and the Wind River Mountain Range. The Greater Green River Basin is subdivided into four smaller basins the Green River Basin, Great Divide Basin, Washakie Basin, and Sand Wash Basin. Each of which possesses hydrocarbons that have been economically exploited. There are 303 named fields throughout the basin the majority of which produce natural gas, the largest of these gas fields is the Jonah Field.

Green River Formation

The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The sediments are deposited in very fine layers, a dark layer during the growing season and a light-hue inorganic layer in the dry season. Each pair of layers is called a varve and represents one year. The sediments of the Green River Formation present a continuous record of six million years. The mean thickness of a varve here is 0.18 mm, with a minimum thickness of 0.014 mm and maximum of 9.8 mm.The sedimentary layers were formed in a large area named for the Green River, a tributary of the Colorado River. The three separate basins lie around the Uinta Mountains of northeastern Utah:

an area in northwestern Colorado east of the Uintas

a larger area in the southwest corner of Wyoming just north of the Uintas known as Lake Gosiute

the largest area, in northeastern Utah and western Colorado south of the Uintas, known as Lake UintaFossil Butte National Monument in Lincoln County, Wyoming is in a part of the formation known as Fossil Lake because of its abundance of exceptionally well preserved fish fossils.

Greenhorn Limestone

The Greenhorn Limestone or Greenhorn Formation is a geologic formation in the Great Plains Region of the United States. It preserves fossils dating back to the Cenomanian and Turonian of the Late Cretaceous period.

The formation was named for the Greenhorn Station on Greenhorn Creek in Colorado in 1896 by Grove Karl Gilbert; and it is the eponym of the Greenhorn Marine Cycle of the Cretaceous Western Interior Seaway. With the underlying Graneros Shale, it records the progressive stage of Greenhorn Marine Cycle while the overlying Carlile Shale records the regressive stage.

The unit under that name is recognized in the Great Plains Region from Minnesota and Iowa to New Mexico to Montana and the Dakotas.

In much of Alberta and Saskatchewan, the "Second White-Specked Shale" contains limy equivalents of the Greenhorn.In Kansas, the Greenhorn Formation is divided into the (lowest) Lincoln Limestone, Hartland Shale, Jetmore Chalk, and (highest) Pfeifer shale members, each noted by changes in chalkiness and limestone rhythmite patterns. In Colorado and western Kansas Hydrocarbon exploration, the divisions are Lincoln Limestone, Hartford Shale, and Bridge Creek Limestone. In other states, where the formation is less developed, the unit is not subdivided and is named the Greenhorn Limestone, as a formation or as a member of another formation, e.g., Cody Shale.

Laramide orogeny

The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute. The Laramide orogeny occurred in a series of pulses, with quiescent phases intervening. The major feature that was created by this orogeny was deep-seated, thick-skinned deformation, with evidence of this orogeny found from Canada to northern Mexico, with the easternmost extent of the mountain-building represented by the Black Hills of South Dakota. The phenomenon is named for the Laramie Mountains of eastern Wyoming. The Laramide orogeny is sometimes confused with the Sevier orogeny, which partially overlapped in time and space.

The orogeny is commonly attributed to events off the west coast of North America, where the Kula and Farallon Plates were sliding under the North American plate. Most hypotheses propose that oceanic crust was undergoing flat-slab subduction, i.e., with a shallow subduction angle, and as a consequence, no magmatism occurred in the central west of the continent, and the underlying oceanic lithosphere actually caused drag on the root of the overlying continental lithosphere. One cause for shallow subduction may have been an increased rate of plate convergence. Another proposed cause was subduction of thickened oceanic crust.

Magmatism associated with subduction occurred not near the plate edges (as in the volcanic arc of the Andes, for example), but far to the east, called the Coast Range Arc. Geologists call such a lack of volcanic activity near a subduction zone a magmatic gap. This particular gap may have occurred because the subducted slab was in contact with relatively cool continental lithosphere, not hotter asthenosphere. One result of shallow angle of subduction and the drag that it caused was a broad belt of mountains, some of which were the progenitors of the Rocky Mountains. Part of the proto-Rocky Mountains would be later modified by extension to become the Basin and Range Province.

List of orogenies

The following is a list of known orogenies organised by continent, starting with the oldest at the top. The organization of this article is along present-day continents that do not necessarily reflect the geography contemporary to the orogenies. Note that some orogenies encompass more than one continent and might have different names in each continent. Like-wise some very large orogenies include a number of sub-orogenies. As with other geological phenomena orogenies are often subject to different and changing interpretations regarding to their age, type and associated paleogeography.

Lytle Formation

The Lytle Formation/Lytle Sandstone is an Early Cretaceous geologic unit with its northern exposure running north and south within the Front Range foothills and the Dakota Hogback in northern Colorado and southern Wyoming where it is assigned formation rank within the Dakota Group. In south-central Colorado, the Lytle is a member of the Purgatoire Formation.

The Lytle was the last (youngest) non-marine unit to form in the Denver Basin before the region was fully inundated by the Western Interior Seaway. It was formed above sea level from sediments carried by heavily laden rivers flowing from the eroding uplifts of the Sevier orogeny several tens of millions of years before the Rocky Mountains rose. It is particularly noted for abundant brown chert pebbles washed in from the uplifted Permian rock far to the west.Known fossils are fragments of petrified wood eroded from the west and nondescript burrows.

Orogeny

An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere (crust and uppermost mantle) at convergent plate margins. An orogen or orogenic belt develops when a continental plate crumples and is pushed upwards to form one or more mountain ranges; this involves a series of geological processes collectively called orogenesis.Orogeny is the primary mechanism by which mountains are built on continents. The word "orogeny" comes from Ancient Greek (ὄρος, óros, lit. 'mountain' + γένεσις, génesis, lit. 'creation, origin'). Although it was used before him, the term was employed by the American geologist G.K. Gilbert in 1890 to describe the process of mountain building as distinguished from epeirogeny.

Sevier (disambiguation)

Sevier may refer to:

Sevier, Utah

Sevier County, Tennessee

Sevier County, Utah

Sevier County, Arkansas

Sevier River

Sevier orogeny

Thin-skinned deformation

Thin-skinned deformation is a style of deformation in plate tectonics at a convergent boundary which occurs with shallow thrust faults that only involves cover rocks (typically sedimentary rocks), and not deeper basement rocks.The thin-skinned style of deformation is typical of many fold and thrust belts developed in the foreland of a collisional zone or back arc of a continental volcanic arc. This is particularly the case where a good basal decollement exists, usually in a weaker layer like a shale, evaporite, or a zone of high pore fluid pressure. This was first described in Rocky Mountains of the United States, as part of the Sevier Orogeny.

In the rock record, this will increase the influence of more surficial rocks, which usually includes sedimentary rocks. Typically, you will see repeated sections of the same rock over and over as thrust faults, coming up from the decollement, stack the same layer on top of itself. The sediments that are created by this type of deformation are typically lithic sandstones.

Western Interior Seaway anoxia

Three Western Interior Seaway anoxic events occurred during the Cretaceous in the shallow inland seaway that divided North America in two island continents, Appalachia and Laramidia (see map). During these anoxic events much of the water column was depleted in dissolved oxygen. While anoxic events impact the world's oceans, Western Interior Seaway anoxic events exhibit a unique paleoenvironment compared to other basins. The notable Cretaceous anoxic events in the Western Interior Seaway mark the boundaries at the Aptian-Albian, Cenomanian-Turonian, and Coniacian-Santonian stages, and are identified as Oceanic Anoxic Events I, II, and III respectively. The episodes of anoxia came about at times when very high sea levels coincided with the nearby Sevier orogeny that affected Laramidia to the west and Caribbean large igneous province to the south, which delivered nutrients and oxygen-adsorbing compounds into the water column.

Most anoxic events are recognized using the 13C isotope as a proxy to indicate total organic carbon preserved in sedimentary rocks. If there is very little oxygen, then organic material that settles to the bottom of the water column will not be degraded as readily compared to normal oxygen settings and can be incorporated into the rock. 13Corganic is calculated by comparing the amount of 13C to a carbon isotope standard, and using multiple samples can track changes (δ) in organic carbon content through rocks over time, forming a δ13Corganic curve. The δ13Corganic, as a result, serves as a benthic oxygen curve.

The excellent organic carbon preservation brought about by these successive anoxic events makes Western Interior Seaway strata some of the richest source rocks for oil and gas.

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