Texas Houston is the largest city in Texas and the fourth-largest in the United States, while San Antonio is the second largest in the state and seventh largest in the United States. Dallas–Fort Worth and Greater Houston are the fourth and sixth largest United States metropolitan areas, respectively. Other major cities include El Paso and Austin—the contains a great variety of geologic settings. The state's stratigraphy Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy includes two related subfields: lithologic or lithostratigraphy and biologic stratigraphy or biostratigraphy has been largely influenced by marine transgressive-regressive A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking or the ocean basins filling with water . Transgresssions and regressions may be caused by tectonic events such as orogenies, severe cycles during the Phanerozoic The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 545 million years and goes back to the time when diverse hard-shelled animals first appeared. Its name is derived from the Greek words φαίνω and ζωή, meaning make life appear, since it was once, with a lesser but still significant contribution from late Cenozoic The Cenozoic Era is the most recent of the three classic geological eras and covers the period from 65.5 million years ago to the present. It is marked by the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and the end of the Mesozoic Era. The Cenozoic era is ongoing tectonic activity Plate tectonics is a scientific theory which describes the large scale motions of Earth's lithosphere. The theory builds on the older concepts of continental drift, developed during the first decades of the 20th century by Alfred Wegener, and seafloor spreading, developed in the 1960s, as well as the remnants of a Paleozoic The Paleozoic or Palaeozoic Era (from the Greek palaios , "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic Eon. The Paleozoic spanned from roughly 542 to 251 million years ago (ICS, 2004), and is subdivided into six geologic periods; from oldest to mountain range.

Contents

General geology

Texas Houston is the largest city in Texas and the fourth-largest in the United States, while San Antonio is the second largest in the state and seventh largest in the United States. Dallas–Fort Worth and Greater Houston are the fourth and sixth largest United States metropolitan areas, respectively. Other major cities include El Paso and Austin—the is approximately bisected by a series of faults In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement. Large faults within the Earth's crust result from the action of tectonic forces. Energy release associated with rapid movement on active faults is the cause of most earthquakes that trend southwest to northeast across the state, from the area of Uvalde to Texarkana The Texarkana, TX-Texarkana, AR Metropolitan Statistical Area , as defined by the United States Office of Management and Budget, is a two-county region anchored by the twin cities of Texarkana, Texas and Texarkana, Arkansas, and encompassing the surrounding communities in Bowie County, Texas and Miller County, Arkansas. South and east of these faults, the surface exposures consist mostly of Cenozoic sandstone Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow, red, gray and white. Since sandstone beds often form and shale Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, strata In geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. They may extend over hundreds of thousands of square kilometers of the Earth' that grow progressively younger toward the coast, indicative of a regression that has continued from the late Mesozoic The Mesozoic Era is a period from about 250 million years ago to about 67 million years ago. It is called the Age of Dinosaurs because most dinosaurs developed, and went extinct, during that time. The Chicxulub impact and other events ended the era when a majority of species on earth went extinct to the present. The coastal plain is underlaid by salt domes A salt dome is a type of structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata, forming a diapir that are responsible for many of the oil Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights, and other organic compounds, that are found in geologic formations beneath the earth's surface traps in the region.

North and west of the faults are the Stockton, Edwards The Edwards Plateau is a region of west-central Texas which is bounded by the Balcones Fault to the south and east, the Llano Uplift and the Llano Estacado to the north, and the Pecos River and Chihuahuan Desert to the west. San Angelo, Austin, San Antonio and Del Rio roughly outline the area, and Comanche plateaux; these define a crustal In geology, a crust is the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in incompatible elements than their respective mantles block that was upthrown during the Neogene The Neogene is a geologic period and system starting 23.03 ± 0.05 million years ago and lasting either until today or ending 2.588 million years ago with the beginning of the Quaternary. The Neogene Period follows the Paleogene Period of the Cenozoic Era. Under the current proposal of the International Commission on Stratigraphy , the Neogene. This large region of central Texas, which extends from Brewster County east to Bexar Bexar County is a county located in the U.S. state of Texas. As of July 1, 2008, The U.S. Census Bureau estimated the population at 1,622,884, the 20th most populous county in the nation. Its county seat is San Antonio. In Spanish, "Béxar" is pronounced [ˈbexar], and northeast to the Red River The Red River, or sometimes The Red River of the South, is a major tributary of the Mississippi and Atchafalaya Rivers located in the United States of America. The river gains its name from the red-bed country of its watershed. It is one of several rivers with that name. The Red River is the second largest river basin in the southern Great Plains features extensive Cretaceous The Cretaceous , Latin for "chalky", usually abbreviated K for its German translation Kreide (chalk), is a geologic period and system from circa 145.5 ± 4 to 65.5 ± 0.3 million years (Ma) ago. In the geologic timescale, the Cretaceous follows on the Jurassic Period and is followed by the Paleogene Period of the Cenozoic Era. It is the shale and limestone Limestone is a sedimentary rock composed largely of the mineral calcite . Like most other sedimentary rocks, limestones are composed of grains; however, most grains in limestone grains are skeletal fragments of marine organisms such as coral or foraminifera. Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and outcrops. The limestone in particular is important, both economically for its use in cement In the most general sense of the word, a cement is a binder, a substance which sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term "opus caementicium" to describe masonry which resembled concrete and was made from crushed rock with burnt lime as binder manufacture and as a building material, as well as practically; a porous limestone formation in the Texas Hill Country The Texas Hill Country is a vernacular term applied to a region of Central Texas, that features tall rugged hills that consist of thin layers of soil lying ontop of either limestone or granite. It also includes the Llano Uplift and the second largest granite monadnock in the United States, Enchanted Rock, which is located 18 miles north of is the reservoir of the Edwards Aquifer The Edwards Aquifer is one of the most prolific artesian aquifers in the world. Located on the eastern edge of Edwards Plateau in the U.S. state of Texas, it discharges about 900,000 acre feet of water a year and directly serves about two million people. The Edwards Aquifer is also home to several unique and endangered species, a vital water source to millions.

Almost in the center of these Cretaceous rocks is the Llano Uplift The Llano Uplift is a roughly circular geologic dome of Precambrian rock, primarily granite, in Central Texas in the United States. It is located in the eastern region of the Edwards Plateau, west of the Texas Hill Country. The region is characterized by a central exposure of Precambrian granite , surrounded by a ring of gneiss and schist (Valley, a geologic dome In structural geology, a dome is a deformational feature consisting of symmetrically-dipping anticlines; their general outline on a geologic map is circular or oval. The strata in a dome are upwarped in the center; if the top of a dome is eroded off, the result will be a series of concentric strata that grow progressively older from the outside-in, of Precambrian The Precambrian is an informal name for the span of time before the current Phanerozoic Eon, and is divided into several eons of the geologic time scale. It spans from the formation of Earth around 4500 Ma (million years ago) to the beginning of the Cambrian Period, when macroscopic hard-shelled animals first appeared in abundance about 542 Ma gneiss Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks, schist The schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. By definition, schist contains more than 50% platy, and granite Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granites usually have a medium to coarse grained texture. Occasionally some individual crystals (phenocrysts) are larger than the groundmass in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is sometimes known as a porphyry, surrounded by Paleozoic The Paleozoic or Palaeozoic Era (from the Greek palaios , "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic Eon. The Paleozoic spanned from roughly 542 to 251 million years ago (ICS, 2004), and is subdivided into six geologic periods; from oldest to sedimentary rocks Sedimentary rock is a type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating. The granite here is quarried for construction, but it is perhaps best known to Texans through its manifestation as Enchanted Rock Enchanted Rock is an enormous pink granite pluton rock formation located in the Llano Uplift approximately 15 miles (24 km) north of Fredericksburg, Texas, USA and 15 miles (24 km) south of Llano, Texas. Enchanted Rock State Natural Area, which includes Enchanted Rock and surrounding land, spans the border between Gillespie County and Llano County,.

From San Saba San Saba is a town located in Central Texas. The population was 2,637 at the 2000 census. It is the county seat of San Saba County. The town is known as the birthplace of actor Tommy Lee Jones north to Childress, and from Wichita Falls in the east to Big Spring in the west, the surface consists of late Paleozoic (Pennsylvanian) to early Mesozoic (Triassic The Triassic is a geologic period that extended from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events. The extinction event that closed the Triassic Period has recently been more accurately) marine sediments. These strata grow younger from east to west, until they are overlain unconformably An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe any break in the sedimentary geologic record. The by terrigenous Ogallala The Ogallala Aquifer, also known as the High Plains Aquifer, is a vast yet shallow underground water table aquifer located beneath the Great Plains in the United States. One of the world's largest aquifers, it covers an area of approximately 174,000 mi² in portions of the eight states of South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, sediments of Miocene The Miocene is a geological epoch of the Neogene Period and extends from about 23.03 to 5.33 million years before the present . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words μείων (meiōn, “less”) and καινός (kainos, “new”) and means "less recent" because it has 18% fewer modern sea and Pliocene The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present age. These late Cenozoic deposits dominate the Texas Panhandle The Texas Panhandle is a region of the U.S. state of Texas consisting of the northernmost 26 counties in the state. The panhandle is a rectangular area bordered by New Mexico to the west and Oklahoma to the north and east. The southern border of Swisher County is considered to be the southern boundary of the region, though some consider the region.

The geology of west Texas is arguably the state's most complex, with a mix of exposed Cretaceous and Pennsylvanian strata, overlain by Quaternary The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Tertiary Period, spanning 2.588 ± 0.005 million years ago to the present. The Quaternary includes two geologic epochs: the Pleistocene and the Holocene Epochs conglomerates A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts. Both conglomerates and breccias are characterized by clasts larger than sand (>2. A series of faults trend southeast to northwest across the region, from Big Bend Big Bend National Park is a national park located in the U.S. state of Texas. For more than 1,000 miles , the Rio Grande / Río Bravo forms the international boundary between Mexico and the United States, and Big Bend National Park administers approximately 244 miles (393 km) along that boundary to El Paso El Paso stands on the Rio Grande , across the border from Ciudad Juárez, Chihuahua, Mexico. The image to the right shows Downtown El Paso and Juárez, with the Juárez Mountains in the background. The two cities form a combined international metropolitan area, sometimes called Juarez-El Paso, with Juárez being the significantly larger of the two; there are also extensive volcanic A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, ash and gases to escape from below the surface deposits. The Marathon Mountains northeast of Big Bend National Park Big Bend National Park is a national park located in the U.S. state of Texas. For more than 1,000 miles , the Rio Grande / Río Bravo forms the international boundary between Mexico and the United States, and Big Bend National Park administers approximately 244 miles (393 km) along that boundary have long been of special interest to geologists; they are the folded and eroded remains of an ancient mountain A mountain is a large landform that stretches above the surrounding land in a limited area usually in the form of a peak. A mountain is generally steeper than a hill. The adjective montane is used to describe mountainous areas and things associated with them. The study of mountains is called Orography range, created in the same orogeny Orogeny refers to forces and events leading to a severe structural deformation of the earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts. The word "orogeny" comes from the Greek , and it is the primary that formed the Ouachita and Appalachian Mountains.[1]

Historical geology

Landslide at Palo Duro Canyon (2002)

The Precambrian metamorphic and igneous rocks of the Llano Uplift probably formed during the Mesoproterozoic Grenville orogeny, which was part of the assembly of the supercontinent Rodinia. Over time, the mountains of the Grenville orogeny were eroded flat, and later covered by Paleozoic and Mesozoic sediments that were not uplifted and eroded in their present manner until the late Cenozoic.

Early to middle Paleozoic rocks in Texas are typically carbonate deposited in epeiric seas. Exceptions include a significant area of Cambrian sandstone in west Texas, and some shale strata from the Devonian and Mississippian periods. The Ouachita Mountains were uplifted across the state during the succeeding Pennsylvanian period; this provided a nearby source of sediment for shale and sandstone, along with more marine limestone deposition.[2]

Permian rocks are the best-known of the Texas Paleozoic. They are widespread in north Texas, where their characteristic red beds are spectacularly exposed in Palo Duro Canyon. The strata are also oil-rich where buried in west Texas, such as in the Midland and Odessa region. This crude oil rich area is known as the Permian Basin. Permian Texas was covered by shallow seas to the west, with evaporation flats to the east and north in the Panhandle.[3] Outstanding exposures of Permian strata are located in and around Guadalupe Mountains National Park, the geology of which resulted in the definition of several Permian stratotypes; the region is one of the world's best for studies of the period.[1],[2]

Early and middle Mesozoic strata are, on the whole, poorly represented in Texas. Triassic rocks are limited to sandstone and shale in the Panhandle, while the Jurassic record is almost nonexistent at the surface. This was far from a monotonous time, though, as it featured the creation of the Gulf of Mexico, from a rift southeast of the Ouachita Mountains. Deeply-buried salt deposits and marine limestones under the coastal plain date from the Jurassic, when the first shallow seas formed.[4]

The late Mesozoic record is much richer. Cretaceous rocks--particularly those of the lower Cretaceous--are widespread at the surface, with yet more buried under the coastal plain. The strata consist of massive limestone sequences deposited when the entire region was submerged under the Western Interior Seaway, during the last great marine transgression.[5]

The Western Interior Seaway had withdrawn by the beginning of the Cenozoic, the era that put the finishing touch on Texas's current geology. The modern coastal plain formed during this time; it comprises increasingly thick sediments (perhaps 15 km deep at the coastline) deposited southeastward into the downwarping Gulf of Mexico.[6]

West Texas was rent by volcanism during the Eocene and Oligocene epochs, activity which formed most of the modern topography of the area. Later crustal extension created a series of alternating horsts and grabens similar to those in the Basin and Range province of the western U.S. A late Cenozoic uplift of the Rocky Mountains led to the deposition of a vast fan of eroded sediment to their east, forming the Ogallala Formation that covers much of the Panhandle. Most of the state's current stream valleys and canyons date from the Pleistocene to the present, as the final geologic shaping of the state.[6]

Economic geology

Texas has been one of the leading states in petroleum production since discovery of the Spindletop oil field in 1901. The state also produces uranium. In past years, the state has also produced mercury, silver, and copper.

See also

External links

Footnotes

  1. ^ http://wrgis.wr.usgs.gov/docs/usgsnps/province/inthigh.html USGS
  2. ^ Spearing, Darwin. Roadside Geology of Texas. Missoula: Mountain Press Publishing Company, 1991. ISBN 978-0-87842-265-4 pp. 4–7
  3. ^ Spearing, p. 7
  4. ^ Spearing, p. 8
  5. ^ Spearing, p. 9
  6. ^ a b Spearing, pp. 10–11

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