Nevadan orogeny

The Nevadan orogeny occurred along the western margin of North America during the Middle Jurassic to Early Cretaceous time which is approximately from 155 Ma to 145 Ma.[1] Throughout the duration of this orogeny there were at least two different kinds of orogenic processes occurring. During the early stages of orogenesis an "Andean type" continental magmatic arc developed due to subduction of the Farallon oceanic plate beneath the North American Plate.[2] The latter stages of orogenesis, in contrast, saw multiple oceanic arc terranes accreted onto the western margin of North America in a "Cordilleran type" accretionary orogen.[2] Deformation related to the accretion of these volcanic arc terranes is mostly limited to the western regions of the resulting mountain ranges (Klamath Mountain range and Sierra Nevada) and is absent from the eastern regions.[3] In addition, the deformation experienced in these mountain ranges is mostly due to the Nevadan orogeny and not other external events such as the more recent Sevier and Laramide Orogenies.[4] It is noted that the Klamath Mountains and the Sierra Nevada share similar stratigraphy indicating that they were both formed by the Nevadan orogeny.[5][6] In comparison with other orogenic events, it appears that the Nevadan Orogeny occurred rather quickly taking only about 10 million years as compared to hundreds of millions of years for other orogenies around the World (ex. Trans-Hudson orogeny).[7]

Order of events

Terranedocking
"Cordilleran" style of arc terrane accretion onto a continental land mass. Continued subduction transports the arc terrane to the margin of the continent where it is too buoyant to be subducted so it gets accreted to the continent.

The Nevadan Orogeny began with the formation of a continental volcanic arc due to east dipping subduction of the Farallon Plate beneath the North American Plate.[1] Continued subduction of oceanic crust transported multiple oceanic arc terranes to the western margin of North America where they were accreted onto the edge of the continent.[1] During the accretion of the arc terranes onto North America, the sediment and crustal material between North American and the incoming arc terrane were thrust onto the continent forming ophiolite sequences that are preserved in both the Klamath Mountains and the Sierra Nevada. These mountain ranges are located in northern California-southern Oregon, and central California respectively.[5] The accretion of arc terranes resulted in the generation of three distinct belts in the Sierra Nevada: the Western belt, Central belt, and Eastern Belt.[1] The Klamath Mountains are somewhat more complex in their overall structure than the Sierra Nevada.[5]

Sierra Nevada

Sierra-Nevada-snowpack-comparison-web-quality
Location of the Sierra Nevada, best seen in the left image where the mountain range is covered in snow.

Western belt

The rocks of the Western belt comprise dominantly sedimentary rocks including greywacke and mudstone that have undergone deformation.[1] In the southern part of the Western Belt the rocks have undergone folding as the main type of deformation.[1] The Western Belt is generally separated from the Central Belt by the Melones fault zone which also distinguishes between the metamorphic rocks of the Western and Central Belts of the Sierra Nevada.[1] The Western Belt rocks are interpreted to be a part of the Slate Creek terrane, which was accreted onto the western margin of North America at approximately 150 Ma.[8] The age of these rocks was dated using potassium-argon dating (K-Ar).[8] At the western foothills of the Sierra Nevada there are numerous dikes that have intruded the rocks that range in age from 148-155 Ma.[9] These dikes are proposed to have been formed when the North American plate underwent a change in motion direction so that subduction was no longer occurring in a northeast direction but in the southeast direction.[9] The shear sense along the dikes is a sinistral shear sense which indicates later southeast subduction of the oceanic plate.[9]

Central belt

The Central belt of the Sierra Nevada consists of rocks from the Tuolumne River terrane which were accreted onto the western Margin of North America at an earlier time (>150 Ma) than the rocks of the Slate Creek terrane. In general there are two different zones in the Central belt, which are the Calaveras greenschist complex and the Shoo Fly complex.[2] The Calaveras-greenschist complex is located in the western half of the Central Belt and essentially consists of volcanic arc rocks along with small amounts of chert and argillite.[2] The Shoo Fly complex is to the East of the Calaveras greenschist complex and is dominated by quartz sandstone with small amounts of limestone and phyllite.[2] K-Ar dating of the Tuolumne River terrane indicates it is between 190-170 Ma in age.[8] During this time there would have been significant amounts of folding and thrust faulting near the collision zone for both the Tuolumne River terrane and the existing Northern Sierra Terrane.[1][8] However, most of the deformation that would have been experienced in the collision was restricted to the Tuolumne River Terrane as minimal deformation is seen in the Eastern Belt.[10]

Eastern belt

Continental Arc Sketch
"Andean" style of orogenesis involving the subduction of an oceanic plate beneath a continental plate causing partial melt of the asthenosphere wedge atop the down going plate. The partial melt then rises and causes volcanism to start to build up mountains through lava flows and deposits from eruptions.

The eastern belt of the Sierra Nevada consists of the Northern Sierra Terrane.[8][1] The Northern Sierra Terrane was formed from volcanism at the western edge of North America due to the subduction of an oceanic plate, which eventually resulted in the accretion of the Tuolumne River and Slate Creek terranes to North America.[8] This is analogous to the "Andean" style of orogenesis where subduction of an oceanic plate to approximately 110 km beneath the surface of Earth results in melting of the down-going slab and convecting asthenosphere.[11] This melting may be assisted by the presence of water in what is known as Flux melting.[11] The melt from the slab then rises up through the asthenosphere and through the crust to create large batholiths and volcanism. Although deformation in the western and central regions of the Sierra Nevada is widespread, deformation from the Nevadan Orogeny in the Eastern Belt is somewhat limited.[10] It was determined that the deformation was minimal in the Eastern Belt by looking at dikes that had intruded the rocks which appeared to be mostly undeformed.[10] These mostly undeformed dikes were dated using the K-Ar method and were determined to be between 169 and 209 Ma in age, which implies they were placed well before any deformation related to the Nevadan Orogeny would have occurred.[10] As the age of these dikes are older than the deformation of the Nevadan Orogeny, it is evident that most of the deformation took place towards the western side of the Sierra Nevada, rather than in the eastern regions.[10]

Klamath Mountains

Klamath Mountains location map
Location of the Klamath Mountains highlighted in red. Not highlighted but still present is the Sierra Nevada running along the east margin of the Great Valley in California.

The Klamath Mountains tell a similar story to the Sierra Nevada in that they are the product of multiple different accretionary events of island arc terranes. The current proposed model for the formation of the Klamath mountains involves multiple stages. The first stage of the formation of the Klamath mountains was arc magmatism on the western coast of North America which resulted in the formation of the Western Hayfork Terrane.[12] Once the Western Hayfork Terrane was formed (and had subsequently stopped forming) the region was intruded by mafic dikes attributed to some form of extension at approximately 160 Ma.[12] Once extension ceased in the area, compression began again, resulting in the closure of a very small back arc basin produced by the extension and accreted the ophiolite sequences seen in the Klamath Mountains from the Nevadan Orogeny time (Josephine Ophiolite at 155 Ma).[12] Continued convergence in the Klamath Mountains region would eventually lead to the emplacement of dikes and sills within the Josephine Ophiolite at approximately 153 Ma.[12] The youngest of the accretionary ophiolite sequence in the Klamath Mountains appears to be the Josephine Ophoilite, which is dated to be about 155 to 150 Ma in age using both argon-argon (Ar-Ar) and lead-uranium (Pb-U) methods.[13] Rather than being thrust on top of North America, the Josephine Ophiolite was accreted through a different process that involved being thrust underneath of North America and then eventually being exhumed at the surface.[13] In the Klamath Mountains it has also been observed that there is two other plutons of rock that were accreted during the Nevadan Orogeny, the Abrams and Salmon mica schists of the Stuart Fork Formation.[3] Using the potassium-argon (K-Ar) method of isotopic dating on phyllite, the age of metamorphism in the Stuart Fork Formation was determined to be about 148 Ma.[3] The metamorphism related to the phyllite in the Stuart Fork Formation is from the older Abrams and Salmon mica schists being thrust ontop of the Stuart Fork rocks during the end of the Nevadan Orogeny.[3]

Timing of events in the Sierra Nevada and Klamath Mountains

The Sierra Nevada and the Klamath Mountains were the result of continental magmatic arc and then oceanic arc accretion during the Nevadan Orogeny between 155-145 Ma.[12][2][1] At nearly the same time the Eastern Belt of the Sierra Nevada was forming, the Western Hayfork Terrane of the Klamath Mountains was being constructed.[8][12] As the Nevadan Orogeny progressed, the Tuolumne River Terrane was accreted to the Sierra Nevada at approximately the same time as the formation of the Josephine Ophiolite in the Klamath Mountains (150-155 Ma).[12][2] During the last stages of orogenesis, the sedimentary rocks of the Western Belt were accreted to the Sierra Nevada while the Abrams and Salmon mica schists were thrust ontop of the Stuart Fork Formation in the Klamath Mountains.[8][3]

References

  1. ^ a b c d e f g h i j Schweikert, Richard; Bogan, Nicholas L.; Girty, Gary H.; Hanson, Richard E.; Merguerian, Charles (1984). "Timing and Structural Expression of the Nevadan Orogeny, Sierra Nevada, California". Geological Society of America Bulletin. 95 (8): 967–979. doi:10.1130/0016-7606(1984)95<967:taseot>2.0.co;2.
  2. ^ a b c d e f g Tobisch, Othmar; Paterson, Scott R.; Longiaru, Samuel; Bhattacharyya, Tapas (1987). "Extent of the Nevadan orogeny, central Sierra Nevada, California". Geology. 15 (2): 132–135. doi:10.1130/0091-7613(1987)15<132:eotnoc>2.0.co;2.
  3. ^ a b c d e Lanphere, Marvin; Irwin, William P.; Hotz, Preston E. (1968). "Isotopic Age of the Nevadan Orogeny and Older Plutonic and Metamorphic Events in the Klamath Mountains, California". Geological Society of America Bulletin. 79 (8): 1027–1052. doi:10.1130/0016-7606(1968)79[1027:iaotno]2.0.co;2.
  4. ^ Hacker, Bradley; Donato, Mary M.; Barnes, Calvin G.; McWilliams, M.O.; Ernst, W.G. (1995). "Timescales of orogeny: Jurassic construction of the Klamath Mountains". Tectonics. 14 (3): 677–703. Bibcode:1995Tecto..14..677H. doi:10.1029/94tc02454.
  5. ^ a b c Ingersoll, Raymond; Schweickert, Richard A. (1986). "A Plate-Tectonic Model For Late Jurassic Ophiolite Genesis, Nevadan Orogeny and Forearc Initiation, Northern California". Tectonics. 5 (6): 901–912. Bibcode:1986Tecto...5..901I. doi:10.1029/tc005i006p00901.
  6. ^ Ernst, W.G.; Gottlieb, Eric S.; Barnes, Calvin G; Hourigan, Jeremy K. (2016). "Zircon U-Pb ages and petrologic evolution of the English Peak granitic pluton: Jurassic crustal growth in northwestern California". Geosphere. 12 (5): 1422–1436. Bibcode:2016Geosp..12.1422E. doi:10.1130/ges01340.1.
  7. ^ Hoffman, Paul F. (1988). "United Plates of America, The Birth of a Craton: Early Proterozoic Assembly and Growth of Laurentia". Annual Review of Earth and Planetary Sciences. 16: 543–603. doi:10.1146/annurev.earth.16.1.543.
  8. ^ a b c d e f g h Edelman, Steven H. (1991). "Relationships between kinematics of arc-continent collision and kinematics of thrust faults, folds, shear zones, and foliations in the Nevadan orogeny, northern Sierra Nevada, California". Tectonophysics. 191 (3–4): 223–236. Bibcode:1991Tectp.191..223E. doi:10.1016/0040-1951(91)90058-z.
  9. ^ a b c Wolf, Michael B.; Saleeby, Jason B. (1992). "Jurassic Cordilleran dike swarm-shear zones: Implications for the Nevadan orogeny and North American plate motion". Geology. 20 (8): 745–748. doi:10.1130/0091-7613(1992)020<0745:jcdssz>2.3.co;2.
  10. ^ a b c d e Renne, Paul R.; Turrin, Brent D. (1987). "Constraints on timing of deformation in the Benton Range, southeastern California, and implications to Nevadan Orogenesis". Geology. 15 (11): 1031–1034. doi:10.1130/0091-7613(1987)15<1031:cotodi>2.0.co;2.
  11. ^ a b Rolland, Y; Bosch, D; Guillot, S; De Sigoyer, J; Martinod, J; Agard, P; Yamato, P (2016). "Subduction & orogeny: Introduction to the Special Volume". Journal of Geodynamics. 96: 1–5. Bibcode:2016JGeo...96....1R. doi:10.1016/j.jog.2016.03.006.
  12. ^ a b c d e f g Harper, Gregory D.; Wright, James E. (1984). "Middle to Late Jurassic Tectonic Evolution of the Klamath Mountains, California-Oregon". Tectonics. 3 (7): 759–772. Bibcode:1984Tecto...3..759H. doi:10.1029/tc003i007p00759.
  13. ^ a b Harper, Gregory D.; Saleeby, Jason B.; Heizler, Matthew (1994). "Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California-Oregon: U/Pb zircon and 40Ar/39Ar geochronology" (PDF). Journal of Geophysical Research. 99 (B3): 4293–4221. Bibcode:1994JGR....99.4293H. doi:10.1029/93jb02061.
Barosaurus

Barosaurus ( BARR-o-SAWR-əs) was a giant, long-tailed, long-necked, plant-eating dinosaur closely related to the more familiar Diplodocus. Remains have been found in the Morrison Formation from the Upper Jurassic Period of Utah and South Dakota. It is present in stratigraphic zones 2-5.The composite term Barosaurus comes from the Greek words barys (βαρυς) meaning "heavy" and sauros (σαυρος) meaning "lizard"; thus "heavy lizard".

Coast Range Ophiolite

The Coast Range Ophiolite is an ophiolite of Middle to Late Jurassic age located in the California Coast Ranges. They form the basement of the extreme western margin of central and northern California. Exposures straddle the coast from Santa Barbara County up to San Francisco. The formation then trends inland up to the southern end of the Klamath Mountains.It is arguably the most extensive ophiolite terrane in the United States, and is one of the most studied ophiolites in the North America.

Cretaceous

The Cretaceous ( , krih-TAY-shəs) is a geologic period and system that spans from the end of the Jurassic Period 145 million years ago (mya) to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, and the longest period of the Phanerozoic Eon. The Cretaceous Period is usually abbreviated K, for its German translation Kreide (chalk, creta in Latin).

The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct marine reptiles, ammonites and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.

Geologic time scale

The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth's history. The table of geologic time spans, presented here, agree with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy (ICS).

Geological history of Earth

The geological history of Earth follows the major events in Earth's past based on the geological time scale, a system of chronological measurement based on the study of the planet's rock layers (stratigraphy). Earth formed about 4.54 billion years ago by accretion from the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun, which also created the rest of the Solar System.

Earth was initially molten due to extreme volcanism and frequent collisions with other bodies. Eventually, the outer layer of the planet cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as a result of the impact of a planetoid with the Earth. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered from comets, produced the oceans.

As the surface continually reshaped itself over hundreds of millions of years, continents formed and broke apart. They migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago, the earliest-known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia, 600 to 540 million years ago, then finally Pangaea, which broke apart 200 million years ago.

The present pattern of ice ages began about 40 million years ago, then intensified at the end of the Pliocene. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40,000–100,000 years. The last glacial period of the current ice age ended about 10,000 years ago.

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 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 Saskatchewan

The geology of Saskatchewan can be divided into two main geological regions, the Precambrian Canadian Shield and the Phanerozoic Western Canadian Sedimentary Basin. Within the Precambrian shield exists the Athabasca sedimentary basin. Meteorite impacts have altered the natural geological formation processes. The prairies were most recently affected by glacial events in the Quaternary period.

Geology of the Yosemite area

The exposed geology of the Yosemite area includes primarily granitic rocks with some older metamorphic rock. The first rocks were laid down in Precambrian times, when the area around Yosemite National Park was on the edge of a very young North American continent. The sediment that formed the area first settled in the waters of a shallow sea, and compressive forces from a subduction zone in the mid-Paleozoic fused the seabed rocks and sediments, appending them to the continent. Heat generated from the subduction created island arcs of volcanoes that were also thrust into the area of the park. In time, the igneous and sedimentary rocks of the area were later heavily metamorphosed.

Most of the rock now exposed in the park is granitic, having been formed 210 to 80 million years ago as igneous diapirs 6 miles (10 km) below the surface. Over time, most of the overlying rock was uplifted along with the rest of the Sierra Nevada and was removed from the area by erosion. This exposed the granitic rock to much lower pressure, and it was also subjected to erosion in the forms of exfoliation and mass wasting.

Starting about 3 million years ago a series of glaciations further modified the area by accelerating the erosion. During that time large glaciers periodically filled the valleys and canyons. Landslides and river erosion have been the primary erosive forces since the end of the last glacial period, which ended in this area around 12,000 years BP.

Jurassic

The Jurassic Period ( juu-RASS-ik; from the Jura Mountains) is a geologic period and system that spanned 56 million years from the end of the Triassic Period 201.3 million years ago (Mya) to the beginning of the Cretaceous Period 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles. The start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, and the Tithonian event at the end; neither event ranks among the "Big Five" mass extinctions, however.

The Jurassic period is divided into three epochs: Early, Middle, and Late. Similarly, in stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, and Upper Jurassic series of rock formations.

The Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period were first identified.

By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses: Laurasia to the north, and Gondwana to the south. This created more coastlines and shifted the continental climate from dry to humid, and many of the arid deserts of the Triassic were replaced by lush rainforests.

On land, the fauna transitioned from the Triassic fauna, dominated by both dinosauromorph and crocodylomorph archosaurs, to one dominated by dinosaurs alone. The first birds also appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, and the evolution of therian mammals, including primitive placentals. Crocodilians made the transition from a terrestrial to an aquatic mode of life. The oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates.

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.

Morrison Formation

The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone, and limestone and is light gray, greenish gray, or red. Most of the fossils occur in the green siltstone beds and lower sandstones, relics of the rivers and floodplains of the Jurassic period.

It is centered in Wyoming and Colorado, with outcrops in Montana, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah, and Idaho. Equivalent rocks under different names are found in Canada. It covers an area of 1.5 million square kilometers (600,000 square miles), although only a tiny fraction is exposed and accessible to geologists and paleontologists. Over 75% is still buried under the prairie to the east, and much of its western paleogeographic extent was eroded during exhumation of the Rocky Mountains.

It was named after Morrison, Colorado, where the first fossils in the formation were discovered by Arthur Lakes in 1877. That same year, it became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope. In Colorado, New Mexico, and Utah, the Morrison Formation was a major source of uranium ore.

North American Cordillera

The North American Cordillera is the North American portion of the American Cordillera which is a mountain chain (cordillera) along the western side of the Americas. The North American Cordillera covers an extensive area of mountain ranges, intermontane basins, and plateaus in western North America, including much of the territory west of the Great Plains. It is also sometimes called the Western Cordillera, the Western Cordillera of North America, or the Pacific Cordillera.The precise boundaries of this cordillera and its subregions, as well as the names of its various features, may differ depending on the definitions in each country or jurisdiction, and also depending on the scientific field; this cordillera is a particularly prominent subject in the scientific field of physical geography.

Rocky Mountain Trench

The Rocky Mountain Trench, also known as the Valley of a Thousand Peaks or simply the Trench, is a large valley on the western side of the northern part of North America's Rocky Mountains. The Trench is both visually and cartographically a striking physiographic feature extending approximately 1,600 km (1,000 mi) from Flathead Lake, Montana to the Liard River, just south of the British Columbia–Yukon border near Watson Lake, Yukon. The trench bottom is 3–16 km (1.9–9.9 mi) wide and is 600–900 m (2,000–3,000 ft) above sea level. The general orientation of the Trench is an almost straight 150/330° geographic north vector and has become convenient as a visual guide for aviators heading north or south.

Although some of its topography has been carved into U-shaped glacial valleys, it is primarily a byproduct of geologic faulting. The Trench separates the Rocky Mountains on its east from the Columbia Mountains and the Cassiar Mountains on its west. It also skirts part of the McGregor Plateau area of the Nechako Plateau sub-area of the Interior Plateau of British Columbia. It is up to 25 km (16 mi) wide, if measured peak-to-peak, and varies in valley relief, but is clearly visible by air and satellite/remote sensing and is easily discernible to those ascending any of the mountains or ridges lining it.

The Trench is drained by four major river basins: the Columbia, Fraser, Peace and Liard. Two reservoirs of the Columbia River Treaty fill much of its length today - Lake Koocanusa and Lake Kinbasket. A further British Columbia power initiative created Lake Williston. Rivers that follow the Trench, at least in part, are the Kootenay River, the Columbia River, the Canoe River, the Flathead River, the Fraser River, the Parsnip River, the Finlay River, the Fox River, and the Kechika River. The North Fork of the Flathead River, flowing into Flathead Lake with the other branches of the Flathead River, is part of the Columbia River system. The Kechika is part of the Liard River system, and the Fox, Parsnip and Finlay Rivers are part of the Peace River system. The Canoe River is a short tributary of the Columbia system, draining into Kinbasket Lake, a reservoir on the Columbia River. The Kootenai River, however, does not fully follow the Trench but exits Canada southwest via Lake Koocanusa reservoir to the Libby Dam. The Kootenay River (Canadian spelling) is a tributary of the Columbia, joining the Columbia at Castlegar, BC after a meander through the United States as the Kootenai River (US spelling).

For convenience the Rocky Mountain Trench may be divided into two sections, the Northern Rocky Mountain Trench and Southern Rocky Mountain Trench. The dividing point reflects the separation of north and easterly flows to the Arctic Ocean versus south and westerly flows to the Pacific Ocean. A break in the valley system at ~54°N near Prince George, British Columbia may be used for this purpose. The northern portion of the Trench is dominated by strike-slip faulting, while the southern part of the Trench was created by normal faults. Despite differences in timing and faulting styles of the northern and southern portions, they were aligned with each other because faulting for both was controlled by a pre-existing, west-facing, deep basement ramp with over 10 km (33,000 ft) of vertical offset.

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.

Sierra Nevada (U.S.)

The Sierra Nevada (, Spanish: [ˈsjera neˈβaða], snowy range) is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primarily in Nevada. The Sierra Nevada is part of the American Cordillera, a chain of mountain ranges that consists of an almost continuous sequence of such ranges that form the western "backbone" of North America, Central America, South America and Antarctica.

The Sierra runs 400 miles (640 km) north-to-south, and is approximately 70 miles (110 km) across east-to-west. Notable Sierra features include Lake Tahoe, the largest alpine lake in North America; Mount Whitney at 14,505 ft (4,421 m), the highest point in the contiguous United States; and Yosemite Valley, sculpted by glaciers from one-hundred-million-year-old granite. The Sierra is home to three national parks, twenty wilderness areas, and two national monuments. These areas include Yosemite, Sequoia, and Kings Canyon National Parks; and Devils Postpile National Monument.

The character of the range is shaped by its geology and ecology. More than one hundred million years ago during the Nevadan orogeny, granite formed deep underground. The range started to uplift four million years ago, and erosion by glaciers exposed the granite and formed the light-colored mountains and cliffs that make up the range. The uplift caused a wide range of elevations and climates in the Sierra Nevada, which are reflected by the presence of five life zones (areas with similar plant and animal communities). Uplift continues due to faulting caused by tectonic forces, creating spectacular fault block escarpments along the eastern edge of the southern Sierra.

The Sierra Nevada has a significant history. The California Gold Rush occurred in the western foothills from 1848 through 1855. Due to inaccessibility, the range was not fully explored until 1912.

Sierran Arc

In early Triassic time, an extensive volcanic arc system, called the Sierran Arc began to develop along the western margin of the North American continent. In Southern California, this volcanic arc would develop throughout the Mesozoic Era to become the geologic regions known as the Sierra Nevada Batholith, the Peninsular Ranges Batholith, (in the Peninsular Ranges), and other plutonic and volcanic centers throughout the greater Mojave Desert region.

Smartville Block

The Smartville Block, also called the Smartville Ophiolite, Smartville Complex, or Smartville Intrusive Complex, is a geologic terrane formed in the ocean from a volcanic island arc that was accreted onto the North American Plate during the late Jurassic (~160–150 million years ago). The collision created sufficient crustal heating to drive mineral-laden water up through numerous fissures along the contact zone. When these cooled, among the precipitating minerals was gold. Associated with the Western Metamorphic Belt of the Sierra Nevada foothills it extends from the central Sierra Nevada mountain range, due west, under a section of the Central Valley and California Coast Ranges, in northern California. The ophiolitic sequence found in this terrane is one of several major ophiolites found in California. Ophiolites are crustal rocks from the ocean floor that have been moved on land. Ophiolites have been studied extensively regarding the movement of crustal rocks by plate tectonics.

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