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.[1]

Shallow subduction Laramide orogeny
The Laramide orogeny was caused by subduction of a plate at a shallow angle.

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.[2] 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.

Basins and mountains

The Laramide orogeny produced intermontane structural basins and adjacent mountain blocks by means of deformation. This style of deformation is typical of continental plates adjacent to convergent margins of long duration that have not sustained continent/continent collisions. This tectonic setting produces a pattern of compressive uplifts and basins, with most of the deformation confined to block edges. Twelve kilometers of structural relief between basins and adjacent uplifts is not uncommon. The basins contain several thousand meters of Paleozoic and Mesozoic sedimentary rocks that predate the Laramide orogeny. As much as 5,000 meters (16,000 ft) of Cretaceous and Cenozoic sediments filled these orogenically-defined basins. Deformed Paleocene and Eocene deposits record continuing orogenic activity.[3]

During the Laramide orogeny, basin floors and mountain summits were much closer to sea level than today. After the seas retreated from the Rocky Mountain region, floodplains, swamps, and vast lakes developed in the basins. Drainage systems imposed at that time persist today. Since the Oligocene, episodic epeirogenic uplift gradually raised the entire region, including the Great Plains, to present elevations. Most of the modern topography is the result of Pliocene and Pleistocene events, including additional uplift, glaciation of the high country, and denudation and dissection of older Cenozoic surfaces in the basin by fluvial processes.[3]

Wpdms nasa topo bighorn basin
Topographic map of the Bighorn Basin (highlighted in orange), formed by the Laramide Orogeny

In the United States, these distinctive intermontane basins occur principally in the central Rocky Mountains from Colorado and Utah (Uinta Basin) to Montana and are best developed in Wyoming, with the Bighorn, Powder River, and Wind River being the largest. Topographically, the basin floors resemble the surface of the western Great Plains, except for vistas of surrounding mountains.[3]

At most boundaries, Paleozoic through Paleogene units dip steeply into the basins off uplifted blocks cored by Precambrian rocks. The eroded steeply dipping units form hogbacks and flatirons. Many of the boundaries are thrust or reverse faults. Although other boundaries appear to be monoclinal flexures, faulting is suspected at depth. Most bounding faults show evidence of at least two episodes of Laramide (Late Cretaceous and Eocene) movement, suggesting both thrust and strike-slip types of displacement.[3]

Ecological consequences

According to paleontologist Thomas M. Lehman, the Laramide orogeny triggered "the most dramatic event that affected Late Cretaceous dinosaur communities in North America prior to their extinction."[4] This turnover event saw the replacement of specialized and highly ornamented centrosaurine and lambeosaurines by more basal upland dinosaurs in the south, while northern biomes became dominated by Triceratops with a greatly reduced hadrosaur community.[5]

See also

Footnotes

  1. ^ Willis 2000
  2. ^ Dumitru 1991
  3. ^ a b c d  This article incorporates public domain material from the National Aeronautics and Space Administration document "Wyoming Intermontane Basins" by M Hegde.
  4. ^ Lehman 2001, p. 310
  5. ^ Lehman 2001, p. 324

References

  • Dumitru, T.A.; Gans, P.B.; Foster, D.A.; Miller, E.L. (1991). "Refrigeration of the western Cordilleran lithosphere during Laramide shallow-angle subduction". Geology. 19 (11): 1145–1148. Bibcode:1991Geo....19.1145D. doi:10.1130/0091-7613(1991)019<1145:ROTWCL>2.3.CO;2.
  • English, Joseph M.; Johnston, Stephen T. (2004). "The Laramide Orogeny: What Were the Driving Forces?". International Geology Review. 46: 833–838. Bibcode:2004IGRv...46..833E. doi:10.2747/0020-6814.46.9.833.
  • Lehman, T. M. (2001). "Late Cretaceous dinosaur provinciality". In Tanke, D. H.; Carpenter, K. (eds.). Mesozoic Vertebrate Life. Indiana University Press. pp. 310–328.
  • Liu, L.; Gurnis, M.; Seton, M.; Saleeby, J.; Müller, R.D.; Jackson, J.M. (2010). "The role of oceanic plateau subduction in the Laramide orogeny" (PDF). Nature Geoscience. 3 (5): 353–357. Bibcode:2010NatGe...3..353L. doi:10.1038/ngeo829.
  • Livaccari, Richard F.; Burke, Kevin; Sengor, AMC (1981). "Was the Laramide orogeny related to subduction of an oceanic plateau?". Nature. 289 (5795): 276–278. Bibcode:1981Natur.289..276L. doi:10.1038/289276a0.
  • Saleeby, Jason (2003). "Segmentation of the Laramide Slab -- Evidence from the southern Sierra Nevada region" (PDF). Geological Society of America Bulletin. 115: 655–668. doi:10.1130/0016-7606(2003)115<0655:sotlsf>2.0.co;2.
  • Schellart, W.P.; Stegman, D.R.; Farrington, R.J.; Freeman, J.; Moresi, L. (16 July 2010). "Cenozoic Tectonics of Western North America Controlled by Evolving Width of Farallon Slab". Science. 329 (5989): 316–319. Bibcode:2010Sci...329..316S. doi:10.1126/science.1190366. PMID 20647465.
  • Willis, Grant C. (2000). "I thought that was the Laramide orogeny!". Utah's Sevier Thrust System. Utah Geological Survey.

External links

Austin Chalk

The Austin Chalk is an upper Cretaceous geologic formation in the Gulf Coast region of the United States. It is named after type section outcrops near Austin, Texas. The formation is made up of chalk and marl.Dinosaur remains are among the fossils that have been recovered from the formation, although none have yet been referred to a specific genus.The Austin Chalk consists of recrystallized, fossiliferous, interbedded chalks and marls. Exposures of Austin Chalk are mainly seen in quarries, roadcuts, and stream beds where the water eroded the soil. The Austin Chalk outcrops can be seen throughout Dallas, and extend south underneath I-35 down into Austin and San Antonio. Volcanic ash layers are present in the Austin chalk, and were deposited by wind from distant erupting volcanoes around 86 mya. These eruptions occurred along a 250-mile long by 50 mile wide belt of submarine volcanoes, which are located in present-day south-central Texas. This belt of volcanoes coincides with the trend of the Balcones Fault zone and is known as the Balcones volcanic province. Evidence of these ancient volcanoes is only visible in a few places since most were buried by the Austin and Taylor Group, and now are in the subsurface. The presence of this volcanism during deposition of the Austin Chalk is correlated with the Laramide orogeny. Sea level rose for conditions to be right for the deposition of the Austin Chalk, which also coincides with the maximum extent of the Cretaceous Interior Seaway. The depths of the deposition of the Austin Chalk occurred in ~250 m or 820 ft of water. The Austin Chalk is filled with micro-organism fossils known as coccoliths.

Colorado Mineral Belt

The Colorado Mineral Belt (CMB) is an area of ore deposits from the La Plata Mountains in Southwestern Colorado to near the middle of the state at Boulder, Colorado and from which over 25 million troy ounces (778 t) of gold were extracted beginning in 1858. The belt is a "northeast-striking zone defined by: a Proterozoic shear zone system (McCoy, 2001); a suite of Laramide-aged plutons and related ore deposits (Tweto and Sims, 1963); a major gravity low (Isaacson and Smithson, 1976); low-crustal velocities; and high heat flow (Decker et al., 1988)." Mining districts include:[2]

Central City-Idaho Springs district

Leadville mining district, named for Leadville, Colorado

Sneffels-Red Mountain-Telluride districtThe belt lies within a zone that has been geologically active at intervals beginning from near the time of crustal accretion in central Colorado at least 1.6 billion years ago until the present. Parts of the CMB follow shear zones of Precambrian age and the Paleozoic and Mesozoic. Igneous rocks intruded about 60 to 70 million years ago during the Laramide orogeny are associated with the belt and once were thought to be responsible for most of the ore deposits. Now many of the important ore deposits are thought to be genetically related to younger magmatism, some at least as young as about 25 million years.

Dakota Hogback

North terminus: 41.484347°N 105.254917°W / 41.484347; -105.254917

South Terminus: 37.619759°N 104.947553°W / 37.619759; -104.947553The Dakota Hogback is a long hogback ridge at the eastern fringe of the Rocky Mountains that extends north-south from southern Wyoming through Colorado and into northern New Mexico in the United States. The ridge is prominently visible as the first line of foothills along the edge of the Great Plains. It is generally faulted along its western side, and varies in height, with gaps in numerous locations where rivers exit the mountains. The ridge takes its name from the Dakota Formation, a sandstone formation that underlies (the Dakota does not underlie the ridge. The much older Fountain does) the ridge. The hogback was formed during the Laramide orogeny, approximately 50 million years (50 my) ago, when the modern Rockies were created. The general uplift to the west created long faulting in the North American Plate, resulting in the creation of the hogback.While the hogback was created during the Laramide Orogeny, the geologic strata comprising the hogback are much older. For example, fossilized data such as dinosaur footprints have been observed in the exposed strata, created by dinosaurs which lived during the Jurassic Period approximately 150 my ago. Some of these footprints were attributed to the Diplodocus dinosaur and could be seen on the hogback west of Denver, Colorado as recently as the 1980s.The ridge forms a barrier between the high plains and the Rocky Mountain foothills. The ridge is pierced by a few water-cut gaps, which have been used to provide road access between the mountains and the plains. The ridge is paralleled by I-25 from north of Cheyenne, Wyoming, through Colorado, into northern New Mexico. The ridge is to the west. North of Denver its major gaps are; I-80 in southern Wyoming, U.S. Highways 34 at Loveland, U.S. 36 to Rocky Mountain National Park. Interstate 70 passes through a highway cut, revealing the numerous layers making up the ridge. South of Denver, the major gaps are; U.S. Route 24 in Colorado Springs, U.S. Route 50 in Pueblo and finally in Colorado, U.S. Route 160 in Walsenburg.

Dyticonastis

Dyticonastis is an extinct genus of amphisbaenians, or worm lizards, that includes a single species, Dyticonastis rensbergeri, that lived during the late Oligocene and early Miocene in what is now Oregon. Fossils of the species come from the John Day Formation. It belongs to Rhineuridae, a family that includes many other extinct North American amphisbaenians but only one living species, Rhineura floridana, from Florida. Dyticonastis rensbergeri occurs the farthest west of all rhineurid species. Like all rhineurids, Dyticonastis has a shovel-like snout adapted for burrowing underground, but it differs from most other members of the group in having a relatively shallow angle to its snout wedge (about 30 degrees) and in having a widened snout tip. The only other rhineurids that share these features are species of the genus Spathorhynchus, which lived from the Middle Eocene to the Early Oligocene in what is now Wyoming. A 2007 phylogenetic analysis of amphisbaenians found that Dyticonastis and Spathorhynchus are each other's closest relatives, suggesting that both taxa may have evolved through vicariant speciation; the growth of the Rocky Mountains during the earliest stages of the Laramide orogeny in the early Paleogene would have separated North American rhineurids into eastern and western populations, with the western population producing Dyticonastis and Spathorhynchus.

Franklin Mountains (Texas)

The Franklin Mountains of Texas are a small range (23 miles long, 3 miles (4.8 km) wide) that extend from El Paso, Texas north into New Mexico. The Franklins were formed due to crustal extension related to the Cenozoic Rio Grande rift. Although the present topography of the range and adjoining basins is controlled by extension during rifting in the last 10 million years, faults within the range also record deformation during the Laramide orogeny, between 85 and 45 million years ago.

The highest peak is North Franklin Peak at 7,192 feet (2,192 m). Much of the range is part of the Franklin Mountains State Park. The mountains are composed primarily of sedimentary rock with some igneous intrusions. Geologists refer to them as tilted-block fault mountains and in them can be found 1.25 billion-year-old Precambrian rocks, the oldest in Texas.

Geology of Guatemala

The geology of Guatemala encompasses rocks divided into two tectonic blocks. The Maya Block in the north has igneous and metamorphic North American Craton basement rocks, overlain by late Paleozoic metasedimentary rocks, which experienced deformation during the Devonian. Red beds, evaporites and marine limestone from the Mesozoic overlie these rocks. A karst landscape formed in the thick limestone units across the north of the country. During a collisional orogeny, these Paleozoic and Mesozoic rocks were uplifted, thrusted and folded as the Central Guatemalan Cordillera. Paleogene rocks from the early Cenozoic include volcanic and marine clastic rocks, associated with high rates of erosion.

By contrast, to the south of the Motagua Valley, the underlying rocks belong to the Chortis Block—the northern section of the Caribbean Plate. Many geologists have interpreted the Chortis Block as having "translated" eastward to its present position, with Cretaceous brittle deformation and uplift suggesting a connection to the Laramide orogeny building up the Rocky Mountains to the north.

Particularly within the Quaternary, the Cocos Plate which split off the Pacific Plate in the Oligocene has subducted beneath the Caribbean and North American plates, producing a chain of volcanoes along the Pacific Coast of Guatemala. The North American and Caribbean plates are moving with strike-slip displacement along the Motagua-Polochic Fault Zone.Some have looked to the Paleozoic organic shales and the Todos Santos sandstone, with its evaporite "cap" as a potential reservoir for oil and gas.The Sierra de Santa Cruz Massif has an ophiolite zone with serpentized harzburgite around a two kilometer thick gabbro pluton with quartz diorite. The pluton is capped by pillow basalt, diabase, chert and limestone. These in turn are covered over by an additional layer of tuff chert, volcanogenic flysch and breccia with andesite and dacite lava.

Geology of Honduras

The geology of Honduras includes Paleozoic metamorphic rocks, such as the Cacaguapa Schist as its basement rocks. Together with Nicaragua and El Salvador it is underlain by the Chortis Block continental fragment. Currently, the Valle de Catacamas basin extends along the Guayape fault for 290 kilometers. Early tectonic research in 1977 suggested a possible origin for the underlying land in the Pacific Ocean rather than on the Caribbean Plate.Through the Mesozoic, particularly the Cretaceous and into the Paleogene red beds deposited. Throughout the rest of the Paleogene, they became unconformably overlain by andesite lava, sedimentary rocks and rhyolite ignimbrite. In places such as the Plantares geothermal area in the Departamento de Copan, rainwater heats up in complex faults surrounding a graben formed in these rocks. In some cases faulting, like high-angle reverse faults associated with the Montana de Comayuga structural belt are related to the Laramide orogeny that built up the Rocky Mountains far to the north.

An extensive mapping project with keys in English was conducted by the University of Texas-Austin in the early 1970s.

Geology of Montana

The geology of Montana includes thick sequences of Paleozoic, Mesozoic and Cenozoic sedimentary rocks overlying ancient Archean and Proterozoic crystalline basement rock. Eastern Montana has considerable oil and gas resources, while the uplifted Rocky Mountains in the west, which resulted from the Laramide orogeny and other tectonic events have locations with metal ore.

Geology of the Rocky Mountains

The geology of the Rocky Mountains is that of a discontinuous series of mountain ranges with distinct geological origins. Collectively these make up the Rocky Mountains, a mountain system that stretches from Northern British Columbia through central New Mexico and which is part of the great mountain system known as the North American Cordillera.

The rocky cores of the mountain ranges are, in most places, formed of pieces of continental crust that are over one billion years old. In the south, an older mountain range was formed 300 million years ago, then eroded away. The rocks of that older range were reformed into the Rocky Mountains.

The Rocky Mountains took shape during an intense period of plate tectonic activity that resulted in much of the rugged landscape of the western North America. The Laramide orogeny, about 80–55 million years ago, was the last of the three episodes and was responsible for raising the Rocky Mountains. Subsequent erosion by glaciers has created the current form of the mountains.

Hoback Formation

The Hoback Formation is a geologic formation in west-central Wyoming, located within the Hoback Basin (directly north of the Green River Basin). It formed as a result of increased sedimentation rates from the Laramide Orogeny and preserves fossils dating back to the late Paleogene period, through the early Eocene.

The Hoback Formation was likely formed in a forested floodplain environment during a period of humid climate, as indicated by plentiful coal, carbonaceous shale, and fossilized plant remains. Many of the beds observed are dull in color, indicating that they formed in a reducing environment - another sign of a floodplain depositional environment, as standing water and waterlogged soil would be present for a substantial portion of the year. A prominent sandstone facies (with crossbedding, overbank deposits, and large pebbly deposits), thought to represent a large stream, is also present through much of the formation.Fossils found within the Hoback Formation include bone fragments, turtles, larger mammals, molluscs, scales, fish teeth, and a wide variety of fossilized plant material (including fossilized wood). Signs of early Cenozoic crocodiles have also been found.

Laramie Mountains

The Laramie Mountains are a range of moderately high peaks on the eastern edge of the Rocky Mountains in the U.S states of Wyoming and Colorado. The range is the northernmost extension of the line of the ranges along the eastern side of the Rockies, and in particular of the higher peaks of the Front Range directly to the south. North of the range, the gap between the Laramie range and the Bighorn Mountains provided the route for historical trails, such as the Oregon Trail, the Mormon Trail, and the Pony Express.

The Laramie Mountains begin in northern Colorado and extend discontinuously into southeastern Wyoming between Cheyenne and Laramie and northward to Casper. (By some definitions the Laramies are only in Wyoming.) They are named after the Laramie River, which cuts through the range from southwest to northeast and joins the North Platte River east of the range in eastern Wyoming. The mountains in turn give their name to the Laramide orogeny, the uplift of the North American Plate approximately 70 million years ago that created the present Rocky Mountains.

The highest portions of the Laramie mountains are mostly in public ownership, forming part of the Medicine Bow-Routt and Roosevelt national forests.

Little Missouri River (North Dakota)

The Little Missouri River is a tributary of the Missouri River, 560 miles (901 km) long, in the northern Great Plains of the United States. Rising in northeastern Wyoming, in western Crook County about 15 miles (24 km) west of Devils Tower, it flows northeastward, across a corner of southeastern Montana, and into South Dakota. In South Dakota, it flows northward through the Badlands into North Dakota, crossing the Little Missouri National Grassland and both units of Theodore Roosevelt National Park. In the north unit of the park, it turns eastward and flows into the Missouri in Dunn County at Lake Sakakawea, where it forms an arm of the reservoir 30 miles (48 km) long called Little Missouri Bay and joins the main channel of the Missouri about 25 miles (40 km) northeast of Killdeer.The highly seasonal runoff from badlands and other treeless landscapes along the Little Missouri carries heavy loads of eroded sediment downstream. The sedimentary layers, which extend from the headwaters in Wyoming all the way to the mouth in North Dakota, vary in age, but most of the beds along the river belong to the Bullion Creek and Sentinel Butte formations, both deposited during the Paleocene (about 66 to 56 million years ago). The deposits include siltstone, claystone, sandstone, and lignite coal laid down in a coastal plain during the Laramide orogeny.

Mist Mountain

Mist Mountain is a mountain located alongside Highway 40 in the Canadian Rockies of Alberta, Canada.

It reaches an elevation of 3,140 m (10,300 ft) and is visible from Alberta Highway 40 and the Sheep River.

The mountain was named in 1884 by George M. Dawson.Mist Mountain is composed of sedimentary rock that was pushed east and over the top of younger rock during the Laramide orogeny.

Mosquito Range

The Mosquito Range (elevation approximately 14,000 ft) is a high mountain range in the Rocky Mountains of central Colorado in the United States. The peaks of the range form a ridge running north-south for approximately 40 miles (64 km) from southern Summit County on the north end, then along the boundary between Lake and Park counties. The ranges forms a high barrier separating the headwaters of the Arkansas River near Leadville from South Park and the headwaters of the South Platte River near Fairplay. The highest peak in the range is Mount Lincoln at an elevation of 14,286 ft. Other fourteeners in the range are Quandary Peak (14,272 ft), Mount Bross (14,172 ft), Mount Democrat (14,148 ft), and Mount Sherman (14,036 ft).

Rattlesnake Hills greenstone belt

The Rattlesnake Hills greenstone belt represents a fragment of a partially exposed synformal Archean greenstone belt within the Wyoming craton that was intruded by Cenozoic alkalic volcanics. The supracrustal belt has been subjected to multiphase deformation during the Archean and later brittle deformation during the Laramide orogeny. Ductile deformation during the Archean produced foliation, and at least three episodes of folding.

Rock Springs Uplift

The Rock Springs Uplift is an area of uplifted Cretaceous to Eocene rocks in Wyoming surrounded and once covered by sediments of the Green River Formation which were deposited in the Eocene Lake Gosiute. The Rock Springs Uplift formed in the Late Cretaceous through the Eocene and is related to the Laramide orogeny. The structure is a north–south trending anticline with a surface expression of approximately 56 miles (90 km) by 28 mi (45 km). The community of Rock Springs is located on the western margin of the uplift.

A recently discovered lithium deposit is estimated at 228,000 tons. Additional deposits in the same formation were extrapolated to be as much as 18 million tons.

San Rafael Swell

The San Rafael Swell is a large geologic feature located in south-central Utah about 30 miles (48 km) west of Green River, Utah. The San Rafael Swell, measuring approximately 75 by 40 miles (121 by 64 km), consists of a giant dome-shaped anticline of sandstone, shale, and limestone that was pushed up during the Paleocene Laramide Orogeny 60–40 million years ago. Since that time, infrequent but powerful flash floods have eroded the sedimentary rocks into numerous valleys, canyons, gorges, mesas, and buttes. The swell is part of the Colorado Plateau physiographic region.

Sawatch Uplift

The Sawatch Uplift is a prominent geologic anticline in the Rocky Mountains in central Colorado in the United States. It was formed as a bulge in the North American Plate approximately 70–65 MYA during the Laramide orogeny that created the modern Rocky Mountains. The uplift runs north-south for several hundred miles from central Colorado into northern New Mexico. The Sawatch Range and Mosquito Range were both formed as part of the uplift. Approximately 35 MYA, stresses in the continental plate in the center of the uplift caused the center to collapse, forming a trough between the Sawatch and Mosquito Range that is part of the larger Rio Grande Rift. The trough runs southward from Leadville, CO into New Mexico. The northern part of the rift forms the headwater valley of the Arkansas River.

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.

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