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GEOLOGY. The geologic history of Texas is recorded in rocks found in outcrops throughout the state and in rocks penetrated by boreholes drilled primarily in the search for oil and natural gas. These rocks indicate that Texas has undergone a long and dynamic history of igneous activity, structural deformation, and sedimentary processes. Study of the origin of these rocks by geoscientists has enabled documentation of the state's changing landscape, which was formed by a process that started several billion years ago in the Precambrian Era and continues to the present. Uplift of mountains, inundation by vast inland seas, river transport of large volumes of eroded sediment, volcanic eruption, and earthquake activity are all processes that have been active throughout the geologic development of Texas. Petroleum resources, lignite, metal ores, groundwater, salt, limestone, granite, ceramic clays, and fertile soils are the legacy of the changing face of Texas.
Precambrian rocks more than 570 million years old underlie much of Central and West Texas. These complexly deformed volcanic, intrusive igneous, and metamorphic rocks occur at the surface in the Llano Uplift of Central Texas and in more isolated outcrops in the mountainous Trans-Pecos region. Other Precambrian rocks are found in the subsurface in Trans-Pecos Texas, Central and West Texas, and the Panhandle. Knowledge of Precambrian rocks and their deformation in the state is much less complete than that of the state's younger rocks, partly because of the relative scarcity of exposure. However, as a result of deep borehole sampling, enough has been discovered about the oldest rocks that major tectonic events or cycles (episodes of large-scale deformation of the earth's surface) can be defined. Geologic processes associated with the Sierra Grande-Chaves tectonic cycle (probably a composite cycle) formed the oldest known rocks in Texas, which include extensive granitic, volcanic, and volcano-sedimentary rock suites as much as 1.4 billion years old. These rocks are buried beneath younger strata, mostly in the northwestern part of the state. About 1.2 billion years ago, the metamorphism, igneous activity, and uplift that occurred during the Llano tectonic cycle affected a wide area from the Trans-Pecos to the Llano Uplift. The Llano tectonic cycle corresponds temporally to the Grenville orogeny (episode of mountain building) of eastern North America. Many of the resulting faulted and folded Precambrian rocks contain mineral deposits that formed when the rocks were subjected to extremely high temperature and intense pressure. Geologists continue to search these ancient basement rocks of Texas for commodities of economic importance such as talc, graphite, and granite.
During the Paleozoic Era, which lasted from about 570 million to 245 million years ago, many of the major structures, which are now buried basins and uplifts, and sedimentary accumulations that contain the state's large petroleum reservoirs were formed. During the early phase of the Ouachitan tectonic cycle in the Cambrian Period of the early Paleozoic, rifting of the southern margin of the ancient North American continent led to the development of a marine basin to the southeast, along with several failed rift basins, which are elongated troughs extending inland from the continental margin. After this Cambrian activity ceased, the rifted continental margin subsided and shallow seas advanced landward. During the Cambrian and Ordovician periods, widespread carbonate-platform (limestone and dolomite) sediments were deposited over the Texas Craton, which included Precambrian igneous and metamorphic terranes of West and Northwest Texas, as well as much of North America. This was followed by carbonate and terrigenous-clastic (sandstone and shale) deposition until the Late Devonian Epoch. Lower Paleozoic rocks are now exposed around the Llano Uplift and in far West Texas near Van Horn and El Paso. They also exist in the subsurface throughout most of West and North Texas.
In the late phase of the Ouachitan tectonic cycle, beginning in the Mississippian Period of the late Paleozoic, compressional (collisional) tectonics became dominant. At the time of the Ouachitan tectonic cycle, a long, deep-marine basin called the Ouachita trough bordered the Texas Craton on the east and south. Collision of the North American tectonic plate with the European and African-South American plates uplifted the thick Ouachita trough sediments to form the Ouachita Mountains before the Middle Pennsylvanian. Today, remnants of this mountain chain are exposed only in southeastern Oklahoma, southwestern Arkansas, and the Marathon region of the Big Bend area of Texas. But in most of Texas the ancient Ouachitas are now buried by younger rocks.
During the Pennsylvanian Period, the Ouachita highlands bordered the eastern margin of several inland marine subbasins, including the Midland, Delaware, and Palo Duro basins, which now collectively compose the oil-rich Permian Basin of West Texas. Extensive limestone reefs rimmed the deeper subbasins, and today these reef rocks contain some of the most productive oil reservoirs in the state. Rivers flowing westward from the Ouachitas into the subbasins during the Pennsylvanian fed major delta systems. The resulting fluvial, deltaic, and shallow-marine deposits formed the exposed rocks in north central Texas. By the Permian Period, at the end of the Paleozoic Era, the inland seas were gradually withdrawing from the West Texas subbasins, leaving behind shallow basins rimmed by extensive tidal flats and containing highly evaporative bodies of water in which salt, gypsum, and red muds accumulated in a hot, dry climate. These characteristic Permian "red beds" presently crop out along the eastern edge of the Panhandle as far east as Wichita Falls, as far south as Concho County, and in the Trans-Pecos.
During the early Mesozoic Era (Late Triassic Epoch), about 220 million years ago, the focus of major geologic events in Texas shifted from West and Central Texas to the eastern part of the state. The European and African-South American plates, which had collided with the North American plate to form the Ouachita Mountains, began to separate from North America. This was the initial stage of the Gulfian tectonic cycle, the period during which much of the Coastal Plain of Texas developed. The whole Gulfian cycle consisted of several periods of continental extension (rifting) and compression and is still in progress. In the Triassic, a series of discontinuous rift basins, generally oriented parallel to the edge of the developing ocean basin and extending from Mexico to Nova Scotia, formed as the plates began to separate. As continental separation continued, the rift basins in Texas were eventually buried by thick deposits of Middle Jurassic marine salt in the newly formed East Texas and Gulf Coast basins. Igneous oceanic crust formed in the Gulf Coast Basin during the Late Jurassic; the boundary between oceanic and continental crust probably lies beneath the present-day Texas continental shelf or slope, but its exact location is unknown. Jurassic and Cretaceous deposits formed broad carbonate shelves that were periodically buried in places by deltaic sandstones and shales at the edge of the widening Gulf of Mexico. With increased accumulation of sediments above the Middle Jurassic salt, the salt became unstable and deformed by migrating upward toward depths of lower confining pressure, forming a variety of structures collectively known as salt domes. These structures, which are prominent subsurface features of the Texas Gulf Coast region, formed significant oil and natural gas traps in the tilted and faulted sedimentary rocks that immediately surround them.
By the Early Cretaceous, the shallow Mesozoic seas extended inland, covering much of the state-as far west as the Trans-Pecos region and north almost to the state line. Best exposures of Early Cretaceous rocks are found in the steep canyon walls of the Rio Grande in Big Bend National Park and in the canyons of the Edwards Plateau, as well as in Central Texas from San Antonio to Dallas. During most of the Late Cretaceous, much of Texas lay beneath marine waters that were deeper than the Early Cretaceous seas. The Austin chalk, which crops out from Eagle Pass on the Rio Grande northeastward to the Texarkana area, is a prominent example of the deeper water deposits of the Late Cretaceous. Delta and strandline sandstones of the Woodbine Formation originated in nearshore areas and are the reservoir rocks of the most prolific oilfield in Texas. When discovered in 1930, the East Texas oilfield contained recoverable reserves estimated at 5.6 billion barrels. The Upper Cretaceous rocks dip southeastward beneath the East Texas and Gulf Coast basins. Late Cretaceous volcanoes were widespread in a band roughly parallel to and south and east of the buried Ouachita Mountains. Pilot Knob (Travis County),qv located southeast of Austin, is the eroded remnant of one of these.
The Late Cretaceous was the time of the last major seaway across Texas. At the end of the Mesozoic Era and the beginning of the Cenozoic Era, major uplift and mountain building in the western United States during the Laramide Orogeny affected the southwestern and western border regions of Texas. This regional uplifting formed the Rocky Mountains, and large river systems draining from the young Rockies southeastward across Texas toward the Gulf of Mexico buried the older marine deposits. Major deltas fed by these rivers prograded the early Cenozoic coastline more than 100 miles seaward into the Gulf of Mexico. Among the effects of this major increase in sediment volume moving into the Gulf of Mexico was renewed upward migration of thick Mesozoic marine salt and the formation of salt domes in the coastal plain area near the site of Houston and in South Texas. Additionally, rapid deposition of deltaic sands over older marine muds resulted in a mechanically unstable sediment column, leading to displacement of the sediments by growth faults (large, curved faults that form during sediment accumulation and continue to grow with increasing depth of burial). Linear zones of growth faults of various ages extend from northeastern Mexico into Louisiana and compose traps for large oil and gas fields in offshore Texas. Regional uplift of the western United States beginning in the Miocene Epoch also elevated the Central Texas area. At the boundary between this uplifting plateau and the subsiding Gulf Coast area, a series of normal (extensional) faults formed farther inland than did the extensive growth faults. Examples include the northeast-trending Balcones Fault Zone (forming the Balcones Escarpment near Austin and San Antonio), which separates the Texas Coastal Plain from the Hill Country in Central Texas.
During the middle Tertiary Period of the Cenozoic Era, Trans-Pecos Texas underwent a period of volcanism and regional deformation that produced much of the rugged landscape in this picturesque part of the state. Volcanic activity there, which occurred from the Eocene through the Miocene Epochs some forty-seven to seventeen million years ago, produced about fourteen volcanic centers (calderas) and associated thick lava and ash-flow deposits. Most of the Trans-Pecos volcanism probably occurred during the waning stages of the Laramide Orogeny. The transition from the Laramide compressional regime to a tensional (rift) structural environment, which took place about thirty million years ago, marked the beginning of the period of basin and range deformation in Texas. During this major episode in the geologic development of Trans-Pecos Texas, a series of elongate, north and northwest trending, fault-bounded basins formed along with intervening highlands (ranges), such as the Hueco Basin near El Paso and the Salt and Presidio basins farther east and south. Recent fault activity is evident along the margins of the larger basins. The 1931 Valentine earthquake, the largest of earthquakes in historical times in Texas, was produced by sudden rupture along one of these faults.
During the middle to late Cenozoic Era, streams originating in the recently elevated Rocky Mountains to the west transported large volumes of sand and gravel into the Texas Panhandle, which accumulated in large alluvial fans. Vast amounts of fine-grained, wind-blown sediments were also deposited along with the alluvial material. These fans were deposited on older Paleozoic and Mesozoic rocks and occurred from Texas into what is now Nebraska. Between one and two million years ago, the fans in Texas were cut off from their Rocky Mountain source, and the eastern edge of the alluvial sheet began to retreat westward as rivers draining into the Gulf of Mexico caused headward erosion. The resulting bluff that extends in a north-south line in West Texas and the Panhandle is the Caprock Escarpment. The alluvial and windblown deposits have been designated the Ogallala Formation, which is a major aquifer (water-bearing rock unit) throughout the central and southern High Plains of the United States.
Expansion of the polar ice caps during the latest Cenozoic produced the great Ice Age in North America, during which, for more than two million years, thick sheets of glacial ice successively advanced and retreated. Although the continental glaciers never reached as far south as Texas, the state's climate and sea level underwent major changes during each period of glacial advance and retreat. Sea level during glacial advances was 300 to 450 feet lower than during the warmer interglacial periods because so much sea water was contained in the ice sheets. The climate was both more humid and cooler than that of today, and the largest Texas rivers carried more water and sediment to the Gulf of Mexico than they do today. These deposits underlie the outer fifty miles or more of the Gulf Coastal Plain. Approximately 3,000 years ago, sea level reached its modern position, and the coastal features that are present today, such as the deltas, lagoons, beaches, and barrier islands, have formed since that time.
BIBLIOGRAPHY:Alexander Deussen, Geology of the Coastal Plain of Texas (Washington: U.S. Department of the Interior, 1924). Ross A. Maxwell, Geologic and Historic Guide to the State Parks of Texas (Bureau of Economic Geology, University of Texas at Austin, 1970). Samuel D. Myres, The Permian Basin: Petroleum Empire of the Southwest (2 vols., El Paso: Permian, 1973, 1977). E. H. Sellards, W. S. Adkins, and F. B. Plummer, The Geology of Texas (University of Texas Bulletin 3232, 1932). Elias H. Sellards, W. S. Adkins, and F. B. Plummer, The Geology of Texas: Bibliography and Subject Index of Texas Geology (Austin: Bureau of Economic Geology, University of Texas, 1933). Darwin Spearing, Roadside Geology of Texas (Missoula, Montana: Mountain Press, 1991).
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