The geologic history of Texas is recorded in the rock strata that fill the many subsurface sedimentary basins and crop out across the state. The origin of these strata documents a changing geography that began several billion years ago in the Precambrian Era. Mountains, seas, rivers, volcanoes, and earthquakes are part of the geologic story of Texas, and the resources produced by geologic phenomena (petroleum, coal, lignite, metals, ground water, salt, limestone, ceramic clays, and various soils) are the legacy of the state's changing face.
Texas is underlain by Precambrian rocks more than 600 million years old. The deformed ancient volcanic and intrusive igneous rocks and sedimentary rocks were formed early in the Earth's history. They are now exposed in the Llano Uplift and in a few small areas in Trans-Pecos Texas.
During the early Paleozoic, broad inland seas inundated the stable West Texas region (Texas Craton), depositing widespread limestones and shales. Lower Paleozoic rocks are now exposed around the Llano Uplift and in the mountains of Trans-Pecos Texas. The Texas Craton was bordered on the east and south by the Ouachita Trough, a deep-marine basin extending along the Paleozoic continental margin from Arkansas and Oklahoma to Mexico. Sediments accumulated in the Ouachita Trough until late in the Paleozoic Era when the European and African continental plates collided with the North American plate. Convergence of the North and South American plates in this area produced fault-bounded mountainous uplifts (Ouachita Mountains) and small basins filled by shallow inland seas that constituted the West Texas Basin.
Broad limestone shelves and barrier reefs surrounded the deeper parts of the marine subbasins. Rivers flowed to the landward edges of the basins, forming deltas, and coastlines shifted repeatedly as nearshore sediments were deposited and then eroded by marine processes. Pennsylvanian strata that are products of these processes are exposed today in North-Central Texas. Near the end of the Paleozoic Era, the inland seas retreated southwestward, and West Texas became the site of broad evaporite basins where salt, gypsum, and red muds were deposited in a hot, arid climate. The strata originally deposited in the Permian Basin are exposed in the Rolling Plains of West and Northwest Texas and in Trans-Pecos Texas.
The Mesozoic Era in Texas began about 245 million years ago when the European and African plates began to break away from the North American plate, producing a belt of elongate rift (fault-bounded) basins that extended from Mexico to Nova Scotia. Sediment from adjacent uplifts was deposited in these basins by streams. While Europe and Africa drifted farther away, the basins were buried beneath marine salt as the East Texas and Gulf Coast Basins were created. During the rest of the Mesozoic Era, broad limestone shelves were periodically buried by coastal plains and deltaic deposits as the Texas continental margin gradually shifted southeastward into the Gulf of Mexico. In the East Texas Basin, deeply buried salt deposits moved upward forming salt ridges and domes, providing a variety of folded structures and traps for oil and gas.
In West Texas, during the early Mesozoic Era, a large shallow lake occupied the abandoned site of the Permian Basin, but eventually waters from the Gulf of Mexico encroached and flooded West Texas beneath a shallow sea. Dinosaurs roamed the land and shallow waters, and marine reptiles dominated the Mesozoic seas until the waters withdrew from West Texas, near the end of the era. Mesozoic strata are exposed along the western and northern margin of the Gulf Coast and East Texas Basins and extensively across West Texas.
When the Cenozoic Era dawned in Texas, about 66 million years ago, the East Texas Basin was filling with lignite-bearing deposits of river and delta origin. The early Cenozoic Mississippi River flowed across East Texas, and a large delta occupied the region north of Houston. Smaller deltas and barrier islands extended southwestward into Mexico, very much like the present Texas coast. Delta and river sands were transported southeastward into progressively deeper waters of the Gulf of Mexico. In the Gulf Coast Basin, deeply buried lower Mesozoic salt moved upward to form domes and anticlinal structures. Now, Cenozoic strata are exposed throughout East Texas and in broad belts in the coastal plain that become younger toward the Gulf of Mexico.
In Trans-Pecos Texas, extensive Cenozoic volcanoes erupted, thick lava flows were deposited over older Mesozoic and Paleozoic strata, and rift basins were formed. Cenozoic volcanic rocks are now well exposed in the arid region of Trans-Pecos Texas.
In northwestern Texas, late Cenozoic streams deposited gravel and sand transported from the Rocky Mountains of southern Colorado and northern New Mexico. During the Ice Age (Pleistocene Epoch, beginning about 2 million years ago) the Pecos River eroded northward into eastern New Mexico and isolated the alluvial eolian deposits of the Texas High Plains from their Rocky Mountain source. The isolated High Plains were eroded by several Texas rivers during and since the Ice Age, causing the eastern margin (caprock) to retreat westward to its present position.
While the northern part of the continent was covered by thick Pleistocene ice caps, streams meandered southeastward across a cool, humid Texas carrying great volumes of water to the Gulf of Mexico. Those rivers, the Colorado, Brazos, Red, and Canadian, slowly entrenched their meanders as gradual uplift occurred across Texas during the last 1 million years. Sea-level changes during the Ice Age alternately exposed and inundated the continental shelf. River, delta, and coastal sediments deposited during interglacial (high-sea-level) stages are exposed along the outer 80 kilometers of the coastal plain. Since sea level reached its approximate present position about 3,000 years ago, thin coastal-barrier, lagoon, and delta sediments have been deposited along the Gulf Coast.
Texas is a composite of nature's processes. Texas today is but one frame in a dynamic geological kaleidoscope of changing rivers, subsiding basins, shifting beaches, uplifting mountains, and eroding plateaus. The face of modern Texas is the link that connects its geologic past to its inevitable future.
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Proper appreciation of regional and global deformation comes only from seeing our planet as a fluid overlain by a thin, brittle crust. Although Earth's rocky face seems solid and stationary, it is made of plates that move over millennia because of flow of rocks far below Earth's surface. Tectonics is the study of regional and global deformation history and the plate tectonic processes that control such movement. Our understanding of past movements is summarized on tectonic maps, such as this Tectonic Map of Texas.
The plates that compose Earth's surface move horizontally and vertically relative to each other, at a rate of as much as a few centimeters per year. Motion of Earth's surface can be abrupt, as those who live in earthquake-prone areas can testify, but it is usually gradual and imperceptible. Yet over long periods of time -- millions of years -- parts of Earth's surface can move tens of thousands of miles relative to other parts. Texans who have handled rock samples recovered from the state's deep petroleum and geothermal wells know that such rocks can be nearly too hot to touch. At greater depth, temperatures are higher still. Because rock strength decreases exponentially with increasing temperature, Earth's interior is weak and able to flow plastically. It is this deep seated flow that causes mountain ranges to be thrust up and plateaus torn apart to form ocean basins.
Tectonic maps document movement history by highlighting structural relationships among segments of Earth's crust that may extend across hundreds of miles. Such maps show crustal architectural patterns that indicate the sequence of tectonic events. To better understand the uses of these maps, compare this Tectonic Map of Texas with the Bureau of Economic Geology's page-size Geology of Texas map.
Geologic maps show where rock strata (layers) occur at Earth's surface or under a thin veneer of soil and vegetation. These maps have elaborate color patterns because their purpose is to depict many distinctive rock formations.
In contrast to the complicated color pattern of the geologic map, the Tectonic Map of Texas has a simple color pattern that depicts basic map elements, called tectonostratigraphic units. These units are sequences of sedimentary rock strata or groups of metamorphic and igneous rocks that share a common history of deformation. Combining individual geologic formations removes distracting detail that obscures the shared deformation histories of large blocks of crust. On the Tectonic Map of Texas, for example, the various Paleozoic formations between Midland, Dallas, and Amarillo have been combined. Not shown is the thin veneer of younger Cretaceous, Tertiary, and Quaternary deposits that lie at the surface over much of the area.
Structural information taken from records of deep wells is illustrated on the tectonic map by color coding that shows depth to a particular formation chosen as a reference horizon. Different rock formations have been used in various parts of Texas as reference horizons; these once were nearly horizontal layers at Earth's surface. As a result of deformation, parts of these formations have been raised or lowered, and color coding on the map shows how deeply buried these horizons are now. For example, in West Texas darker shades of blue mark the deep West Texas and Anadarko Basins. The reference horizon used there is the Paleozoic Ellenburger Formation, a petroleum reservoir rock widely penetrated by oil and gas drilling.
Tectonic maps show major structural features, including tectonic fronts that mark edges of major basins and former mountain ranges (orogenic belts). Crosscutting relations show the relative ages of features. For example, the blue patterns of Paleozoic basins and uplifts in West Texas are crosscut by the light green of the younger Gulf Coast Basin east of San Antonio, Austin, and Dallas.
Several tectonic cycles have affected Texas. These are informally listed on this map as "tectonic episodes," but they are actually local subdivisions of global plate movements that did not begin or end everywhere at the same time, and which -- to a certain extent -- are arbitrary milestones in a continuous history of movement. Each cycle produced tectonostratigraphic units that record initially the generation of rifts and divergent continental margins, followed by destruction of an ocean basin and mountain building (orogeny). The Tectonic Map of Texas distinguishes three principal tectonic cycles:
(1) Precambrian cycles are recorded in the ancient rocks of the Llano region and near Van Horn and El Paso. Of these the best known is the Llano cycle of between 1,200 and 1,080 million years ago (mya). At the close of this cycle, parts of present-day Texas were attached to rocks that now are located in Antarctica and southwest Australia.
(2) The Paleozoic Ouachitan cycle began with continental rifting about 550 mya, followed by inundation of much of Texas by shallow seas. This cycle closed with the collision of South and North America which caused the Ouachita mountain-building event, ending about 245 mya. At this time much of Texas was in the shadow of vast mountain ranges that crossed the southern and east-central part of Texas.
On the tectonic map, two major features record this Ouachitan history. The most prominent is the foreland area of West Texas, shown in shades of blue. Here the legacy of ocean opening and the rise and fall of sea level created the stratigraphic and structural features that would later trap vast quantities of oil and gas. The term foreland signifies that the paleophysiography and structure of this area were shaped by a nearby mountain belt. The ancient and almost entirely eroded mountain belt is the other Ouachitan feature shown on the map. The Ouachitan mountain belt lay south and east of the Ouachita tectonic front. Its mostly buried roots extend from the Marathon area of West Texas, where deeply eroded relics of the mountain belt are exposed, through a great northward-curving arc to near Dallas, thence into Oklahoma. This zone of profound crustal contraction continues in the Appalachian Mountains of eastern North America and beyond.
(3) The current tectonic cycle in Texas is the Gulf Coast, which began in Texas with continental rifting in the Late Triassic about 220 mya and eventually led to creation of oceanic crust in the Gulf of Mexico. Well after this ocean began to open in South and East Texas, between 85 and 50 mya (Late Cretaceous to Paleocene), a mountain-building event called the Laramide Orogeny occurred in West Texas. This event is part of widespread deformation in the western United States, Canada and Mexico that created the Rocky Mountains.
The Tectonic Map indicates where the Gulf Coast and Laramide events had their strongest impact on Texas geology. Rocks shown in green and brown (Gulf Coast Cretaceous and Tertiary strata), mainly east of Dallas, Austin, and San Antonio, were deposited during the creation of the Gulf of Mexico and Atlantic Ocean. Byproducts of basin formation depicted on the map include normal faults and intrusions of mobile salt (salt diapirs). Between Del Rio and Dallas, the edge of the Gulf Coast Basin follows the older Ouachitan tectonic front, testifying to the tendency for deformation to be localized through time along preexisting fault zones. Green and tan patterns and fault traces extending southeast of El Paso mark the edge of the Laramide orogenic belt and the frontal edge of the Rocky Mountains.
Formation of both the Gulf of Mexico and the Rocky Mountains is part of continuing global deformation. The Atlantic Ocean is widening as Europe and North America separate, while the Pacific Ocean basin is closing as the North American plate and Asia converge. The earliest phases of this modern pattern of movement can be read in the Gulf Coast and Laramide tectonic history of Texas.
Tectonic maps help illustrate Earth's restless history by highlighting the major episodes of plate tectonic motion, including mountain building and ocean formation. With this map as a guide, map users can retrace the experience of Texas in Earth's history of regional and global movement.
-Stephen E. Laubach
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The fundamental physical and biological properties of Texas lands that are represented on this map collectively define basic land-resource categories: regions of ground-water recharge, lands containing economic mineral resources, areas containing land-surface materials having economically important physical properties, regions exhibiting distinctive land forms, areas influenced by dynamic physical processes, and areas dominated by biological factors. One or more of these categories define a land-resource unit, differentiated on this map by color and labeled by alphanumeric abbreviations. Each of the generalized map units commonly represents more than one land-resource unit.
Ground-Water Recharge Units
Surficial recharge sands and gravels (map units Rs1 through Rs4) are among the most economically important of the Texas land resources. Aquifers supply nearly 60 percent of the total fresh-water demand of the state, and they are the sole source of water in numerous areas of Texas. Recharge areas are generally underlain by uncemented or loosely cemented sand or mixed sand and gravel. Surface waters can, however, recharge aquifers by passing through virtually any type of bedrock or surficial sediments that have permeability (for example, map units G, L7, and others not specifically designated as recharge sands on this map) sufficient to enable water to flow into aquifers.
Ground-water movement and storage in the recharge sediments and aquifers occur within open spaces (porosity) between the sediment grains and can compose as much as 25 percent of the sediment volume. The degree to which these open spaces are interconnected (to allow subsurface water flow) determines the permeability of the recharge and aquifer material. The map differentiates between recharge units on the basis of sediment grain size (gravel, sand, and clay), permeability, and topography (for example, rolling hills, barrier islands, and low relief terrains).
Because Texas produces a large, diverse array of mineral resources, particularly nonmetallic minerals, it has historically been ranked among the top five states in total annual yield of mineral commodities. Mineral-resource units depicted on the map (map units SI through S5, G, C I and C2, Rb I, and LI through L7) include regions where known significant resources or potential deposits exist. For example, major quarries in hard limestone (map unit LI), sandstone (map unit S4), and granite (map unit G) in Central and East Texas provide building, dimension, and facing stone for commercial and residential structures. Crushed limestone, sandstone, and other rock and sediment types furnish hard-rock aggregates in road bases. Recharge sands also host large deposits of uranium in the Karnes City area of South Texas. Caliche (map unit L6) and greensand-ironstone (map unit SI) are locally common road bases. Iron ore has also been n-tined from greensand-ironstone in northeast Texas. Cement plants on and near chalk bedrock (map unit L5) extend from San Antonio to Dallas, and areas depicted by map units C2 and Rb I have yielded bituminous coal, ceramic clay, and gypsum in North-Central Texas.
Physical-property units determine the suitability of an area's physical characteristics for various uses by humans. The physical characteristics of substrate material or soil are the most important, and land properties that impose engineering limits on construction are among the most significant of the map units. These limits include slope stability, foundation strength, excavation potential, compressibility, plasticity, corrosion potential, and infiltration capacity, among others.
The recharge sand units (map units Rs1 through Rs4) exhibit excellent engineering properties for building. They include high foundation strength, low corrosion potential, low compressibility, low expansion (shrink-swell) potential, moderate slope stability, and ease of excavation. Limestone, sandstone, and granite (map units L1 through L5, S1, S5, and G) share many of these desirable characteristics. In contrast, land-resource units that have a high clay content are typically unsuitable as a construction base. Clays and shales (map units C1, C2, S2, S3, and Rb1) erode easily, forming lowlands; compose weak foundation and construction materials; and have expansive and corrosive properties that damage roads and foundations. Clay-dominated units, however, commonly well suited for solid-waste disposal, are sources of industrial clays and constitute prime agricultural lands.
For certain land-resource areas in Texas, topographic relief and land-surface configuration control land use. Unlike other land-resource units, however, few generalizations can be made about physical properties of the land-form units because of their statewide diversity. Substrate materials range from very hard to soft, topography ranges from flat prairie and coastal lands to rugged mountain terrain, and agricultural suitability ranges from poor range land with sparse vegetation to highly productive farmland. Mountain, canyon, and desert vistas (map units Dm1, Dm2, and Af) in Trans-Pecos Texas, badland red-bed terrains (map units Rb1 and Rb2) in North-Central Texas and the Panhandle, the limestone-supported Hill Country of Central Texas (map units L1and L3), and dune fields and barrier islands of coastal Texas are but a few examples of land-form units that have created prime recreational attractions in the state.
Land-resource units in which dynamic physical processes (for example, flooding and wind erosion) are paramount greatly affect the natural suitability of many areas of the state for human activities. In some areas, these processes are continuous; in other regions, the processes occur periodically, rapidly, and sometimes with intensity. The periodicity and intensity of these processes and resultant land-surface changes strongly affect human ability to use land and water resources in the affected areas.
The land-resource units that are grouped under dynamic processes are stream, coastal, and eolian (wind) deposits: flood-prone valleys and terraces (map unit A), areas susceptible to hurricane-surge flooding (map unit Bi and other coastal units), sand dunes and blowouts (map unit W1), and windblown sands (map unit W2). Other units that are less influenced by dynamic processes include limestone terrains susceptible to sinkhole development (map unit L7) and mountainous terrains where rock and mud slides may occur (map units Dm1 and Dm2). Primary dynamic-process units are restricted to the Texas Coastal Zone, West Texas, and the Panhandle.
The nutrient-rich soils in stream and river valleys that are subject to periodic flooding are best used for agriculture in rural areas and for greenbelts in urban settings. River terraces require thoughtful commercial development because they remain possible flood zones during infrequent, but major, flooding events. Topographically low areas along the coastal zone, such as barrier islands, are susceptible to storm-surge flooding and shoreline erosion. Mobile sand dunes and blowouts, involving generally continuous dynamic processes, are components of lands that are commonly left undeveloped as scenic park lands. Mountainous areas subject to rock and mud slides are remotely inhabited, and they are also prime scenic lands.
Only one biological-resource unit is represented on this map-wetlands (map unit M). This land-resource unit includes fresh-, brackish-, and saltwater marshes in coastal and deltaic settings that can be mapped as separate units on a larger scale map. Swamps and riparian lands, also generalized, are included in the wetlands unit. Numerous other coastal marshes and swamps cannot be shown at this scale. In the coastal bay areas, many other biological units can be identified on the basis of benthic populations. Sea grasses and oyster reefs are examples. For more details about the information summarized on this map, please consult the Bureau's 1:500,000-scale map and the text that accompanies the publication Land Resources of Texas.
-E G. Wermund
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This vegetation/ cover-type map of Texas is a result of efforts by the Texas Parks and Wildlife Department to delineate and categorize existing vegetation and land-cover types statewide using Landsat data (1972 to 1980), computer classification analysis, and systematic accuracy verification procedures. It represents information summarized from a mosaic of larger scale vegetation maps published by the Texas Parks and Wildlife Department. These maps (1:300,000) portray detailed vegetation and land-cover information at 80-meter resolution.
Texas comprises 10 broad natural regions differentiated on the basis of physiography, rainfall, and other climatic factors, as well as vegetation and fauna. Herein, the vegetation of Texas has been divided into a total of 53 cover types, including 47 plant associations of 2 or 3 characteristic dominant or codominant species. Plant association names are followed by a term that describes the structure of the dominant species, such as forest or grassland. Units shown on the map include naturally occurring vegetation as well as cover types, which are the result of land use.
Texas is located at the crossroads of four major natural subdivisions of North America-Gulf Coastal Forests and Prairies, Great Western Lower Plains, Great Western High Plains, and the Rocky Mountain Region. Topographically, excluding the mountainous region west of the Pecos River, Texas is a series of plains and prairies that descend in elevation from northwest to southeast. There are three prominent topographic features: the Basin and Range physiographic province in the Trans-Pecos, the north-south Caprock Escarpment of the High Plains in the Panhandle, and the arc-shaped Balcones Escarpment in the central area of the state. The 367 miles of coastline are extensive and biologically diverse.
Before Texas was settled by European immigrants, changes in vegetation boundaries occurred slowly in response to graduallong-term changes in climate. Major localized changes to established vegetation also occurred as a result of natural wildfires and storms, but such changes did not affect the overall boundaries of the major vegetation communities. Prior to settlement and profound land-use conversion in the 1880's, the central 80 percent of Texas was short to tall grassland, the 10 percent of Texas that lies west of the Pecos River was desert grassland, and the eastern 10 percent of the state was forest land.
Much of the Texas landscape has been extensively modified by land use. Among these changes are rapid urbanization and continued exploitation of natural resources that include water resource development, conversion of wildlands to agricultural and forestry practices, mineral and energy production, urban and industrial expansion, recreational or leisure developments, and transportation infrastructure. Unfortunately, these land-use conversions have extensively modified the natural vegetation of entire regions. For example, of the original Blackland Prairie, only about 0.04 percent of the native grasslands remains in small, scattered tracts, making such remnants unique and valuable. Similarly, 95 percent of the native brush in the lower Rio Grande Delta has been lost to agricultural and urban development, increasing the value of the remaining brush as a habitat for wildlife, including many endangered species. Although most of the eastern forested region is still timbered, about 66 percent of the bottomland hardwood forest has been cleared or replaced by reservoirs, tame pasture, and crops; much of the upland hardwood forest has been converted to pine plantations for commercial timber products.
Not only have native plants been displaced, but many have been replaced by alien species. Consequently, boundaries of many of the vegetation types are now changing rapidly-not as a result of natural changes in climate but because of human induced landscape-scale modification.
The great plant diversity and complex patterns of plant distribution in Texas developed in response to a matrix of complex environmental factors including geology, topography, climatic zones, rainfall belts, and soil types. There are more than 5,000 species of vascular plants (trees, shrubs, vines, wildflowers, grasses, and grasslike plants such as sedges and rushes). Of this number, about 400 are endemic. Nearly half (523) of the grass species indigenous to the United States occur in Texas. More than 500 species of vascular plants are introduced. Unfortunately, many of these alien species have degraded or destroyed habitat for native plant species.
The greatest number of plant species occurs in the Trans-Pecos and eastern forest regions. The areas of least diversity are the Texas Panhandle, the Southern High Plains, and the Rolling Plains. The largest number of endemic species occurs in Trans-Pecos Texas and on the Edwards Plateau and Rio Grande Plains. This pattern of floral diversity directly affects wildlife diversity and abundance. The occurrence and distribution of plants in Texas supports more than 1,200 native vertebrate species and countless invertebrates that are dependent upon vegetation for food and cover.
This vegetation and cover-type map represents a snapshot at an instant in time of the dynamic landscape at a regional level. Increased application of cover-type analysis for land-use decisions could improve the probability of achieving a long-term balance between socioeconomic considerations and natural resource needs.
Additional information concerning this map, such as detailed descriptions of each of the mapped plant associations and photographs of each association, is available on the Nature Section, Plant Life Unit of the Texas Parks and Wildlife Department website.
-Roy G. Frye, Kirby L. Brown, and Craig A. McMahan
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Geologists study the natural scenery of Texas and sort its variations into distinctive physiographic provinces. Each province or landscape reflects a unified geological history of depositional and erosional processes. Each physiographic province is distinguished by characteristic geologic structure, rock and soil types, vegetation, and climate. The elevations and shapes of its landforms contrast significantly with those of landforms in adjacent regions. The Physiographic Map of Texas displays seven physiographic provinces and their principal subdivisions; the accompanying table describes their major physical differences. The following descriptions selectively emphasize those characteristics that distinguish provinces and their subdivisions.
Gulf Coastal Plains
The Gulf Coastal Plains include three subprovinces named the Coastal Prairies, the Interior Coastal Plains, and the Blackland Prairies. The Coastal Prairies begin at the Gulf of Mexico shoreline. Young deltaic sands, silts, and clays erode to nearly flat grasslands that form almost imperceptible slopes to the southeast. Trees are uncommon except locally along streams and in Oak mottes, growing on coarser underlying sediments of ancient streams. Minor steeper slopes, from 1 foot to as much as 9 feet high, result from subsidence of deltaic sediments along faults. Between Corpus Christi and Brownsville, broad sand sheets pocked by low dunes and blowouts forming ponds dominate the landscape.
The Interior Coastal Plains comprise alternating belts of resistant uncemented sands among weaker shales that erode into long, sandy ridges. At least two major down-to-the coast fault systems trend nearly parallel to the coastline. Clusters of faults also concentrate over salt domes in East Texas. That region is characterized by pine and hardwood forests and numerous permanent streams. West and south, tree density continuously declines, pines disappear in Central Texas, and chaparral brush and sparse grasses dominate between San Antonio and Laredo.
On the Blackland Prairies of the innermost Gulf Coastal Plains, chalks and marls weather to deep, black, fertile clay soils, in contrast with the thin red and tan sandy and clay soils of the Interior Gulf Coastal Plains. The blacklands have a gentle undulating surface, cleared of most natural vegetation and cultivated for crops.
From sea level at the Gulf of Mexico, the elevation of the Gulf Coastal Plains increases northward and westward. In the Austin San Antonio area, the average elevation is about 800 feet. South of Del Rio, the western end of the Gulf Coastal Plains has an elevation of about 1,000 feet.
The eastern Grand Prairie developed on limestones; weathering and erosion have left thin rocky soils. North and west of Fort Worth, the plateaulike surface is well exposed, and numerous streams dissect land that is mostly flat or that gently slopes southeastward. There, silver bluestem-Texas wintergrass grassland is the flora. Primarily sandstones underlie the western margin of the Grand Prairie, where post oak woods form the Western Cross Timbers.
The Balcones Escarpment, superposed on a curved band of major normal faults, bounds the eastern and southern Edwards Plateau. Its principal area includes the Hill Country and a broad plateau. Stream erosion of the fault escarpment sculpts the Hill Country from Waco to Del Rio. The Edwards Plateau is capped by hard Cretaceous limestones. Local streams entrench the plateau as much as 1,800 feet in 15 miles. The upper drainages of streams are waterless draws that open into box canyons where springs provide permanently flowing water. Sinkholes commonly dot the limestone terrane and connect with a network of caverns. Alternating hard and soft marly limestones form a stairstep topography in the central interior of the province.
The Edwards Plateau includes the Stockton Plateau, mesalike land that is the highest part of this subdivision. With westward decreasing rainfall, the vegetation grades from mesquite juniper brush westward into creosote bush tarbush shrubs.
The Pecos River erodes a canyon as deep as 1,000 feet between the Edwards and Stockton Plateaus. Its side streams become draws forming narrow blind canyons with nearly vertical walls. The Pecos Canyons include the major river and its side streams. Vegetation is sparse, even near springs and streams.
Central Texas Uplift
The most characteristic feature of this province is a central basin having a rolling floor studded with rounded granite hills 400 to 600 feet high. Enchanted Rock State Park is typical of this terrain. Rocks forming both basin floor and hills are among the oldest in Texas. A rim of resistant lower Paleozoic formations (see the Geology of Texas map) surrounds the basin. Beyond the Paleozoic rim is a second ridge formed of limestones like those of the Edwards Plateau. Central live oak mesquite parks are surrounded by live oak ashe juniper parks.
An erosional surface that developed on upper Paleozoic formations forms the North-Central Plains. Where shale bedrock prevails, meandering rivers traverse stretches of local prairie. In areas of harder bedrock, hills and rolling plains dominate. Local areas of hard sandstones and limestones cap steep slopes severely dissected near rivers. Lengthy dip slopes of strongly fractured limestones display extensive rectangular patterns. Western rocks and soils are oxidized red or gray where gypsum dominates, whereas eastern rocks and soils weather tan to buff. Live oak ashe juniper parks grade westward into mesquite lotebush brush.
The High Plains of Texas form a nearly flat plateau with an average elevation approximating 3,000 feet. Extensive stream-laid sand and gravel deposits, which contain the Ogallala aquifer, underlie the plains. Windblown sands and silts form thick, rich soils and caliche locally. Havard shin oak mesquite brush dominates the silty soils, whereas sandsage Havard shin oak brush occupies the sand sheets. Numerous playa lakes scatter randomly over the treeless plains. The eastern boundary is a westward-retreating escarpment capped by a hard caliche. Headwaters of major rivers deeply notch the caprock, as exemplified by Palo Duro Canyon and Caprock Canyons State Parks.
On the High Plains, widespread small, intermittent streams dominate the drainage. The Canadian River cuts across the province, creating the Canadian Breaks and separating the Central High Plains from the Southern High Plains. Pecos River drainage erodes the west-facing escarpment of the Southern High Plains, which terminates against the Edwards Plateau on the south.
Basin and Range
The Basin and Range province contains eight mountain peaks that are higher than 8,000 feet. At 8,749 feet, Guadalupe Peak is the highest point in Texas. Mountain ranges generally trend nearly north-south and rise abruptly from barren rocky plains.
Plateaus in which the rocks are nearly horizontal and less deformed commonly flank the mountains. Cores of strongly folded and faulted sedimentary and volcanic rocks or of granite rocks compose the interiors of mountain ranges. Volcanic rocks form many peaks. Large flows of volcanic ash and thick deposits of volcanic debris flank the slopes of most former volcanoes. Ancient volcanic activity of the Texas Basin and Range province was mostly explosive in nature, like Mount Saint Helens. Volcanoes that poured successive lava flows are uncommon. Eroded craters, where the cores of volcanoes collapsed and subsided, are abundant.
Gray oak pinyon pine alligator juniper parks drape the highest elevations. Creosote bush and lechuguilla shrubs sparsely populate plateaus and intermediate elevations. Tobosa black grama grassland occupies the low basins.
The Physiographic Map of Texasis a useful guide to appreciate statewide travel. Texas abounds with vistas of mountains, plateaus, plains, hills, and valleys in which many rock types and geologic structures are exposed. A variety of vegetation grows, depending on local climate.
-E. G. Wermund
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In river basins of Texas, streams transport a precious resource--surface waters. They contain the essential nutrients for maintaining life in plants, animals, and people. They are the feedstock of nature-forests, prairies, meadows, swamps, and marshes, as well as pastures, and croplands. All the products that build our communities-concrete, bitumen, bricks, wood, paper, and plastics-require their waters.
A river basin is the entire area drained by a stream and its tributaries. The highest land surrounds the river basin, forming its outside boundary as well as dividing it from adjacent basins. From this boundary, or divide, all water falling into the basin flows downhill to a pour point. At the pour point, small stream basins spill into larger stream or river basins.
The 13 major river basins of Texas vary greatly in size, shape, and stream patterns. Although the river basins share many common features, each is unique. River basins reflect the climate, geology, topography, and vegetation of an area.
Five Texas river basins originate outside Texas. Two begin in Colorado: the Rio Grande in the San Juan Mountains at about 14,000 feet and the Canadian River in Raton Pass at nearly 8,500 feet. Three rivers commence in New Mexico: the Red River as Blanca Creek at 4,640 feet, the Brazos River in Running Water Draw at about the same elevation, and the Colorado River in Sulfur Draw at about 4,000 feet. The eight other Texas river basins originate within Texas.
The Canadian and the Red Rivers have their outlets, or pour points, beyond Texas. Our remaining 11 river basins spill into the Gulf of Mexico. Only the Rio Grande and the Brazos River discharge directly into the Gulf of Mexico, where they built substantial deltas in the recent past. On the River Basin Map of Texas, the remains of their deltas make the shoreline bulge into the Gulf.
The remaining river basins spill into estuaries and bays along the coast. The Sabine and Nueces Rivers flow into Sabine Lake and were formerly joined as one river basin. Similarly, the Trinity and San Jacinto Rivers drain into the connected Galveston and Trinity Bays. The Colorado River, which formerly combined with the Brazos River in delta building, now flows into Matagorda Bay. The Lavaca River empties into Lavaca Bay, the Guadalupe and San Antonio together build a delta into San Antonio Bay, and the Nueces River discharges into Corpus Christi Bay. Marshes, seagrasses, shrimp, fish, and bottom fauna such as oysters require the nourishment provided by nutrients contained in the inflow of fresh water and sediments from these rivers.
As shown on the table below the map, Texas river basins vary greatly in area and length. The largest, the Rio Grande, contrasts markedly with the smallest, the San Jacinto River, in both size and length. The Red, Colorado, and Brazos Rivers have similar areas, but the Brazos River is 25 percent longer than the other two. Most of the river basins have elongated shapes. Only the Nueces, Lavaca, and San Jacinto Rivers round into teardrop shapes.
Poorly drained coastal basins are situated between many of the large river basins. Coastal basins along the Gulf of Mexico contain only small streams, commonly named "bayous," east of the Brazos River. Their small discharges into the bays are estimated, and these basins lack instruments to measure stream flow.
Climate, especially rainfall and evaporation, strongly controls the flows of rivers and streams in Texas. In the Sabine River basin, mean annual rainfall is nearly 60 inches and annual evaporation is less than 70 inches, whereas in the Rio Grande basin, mean annual rainfall ranges from 8 to 20 inches and annual evaporation is as much as 105 inches. Therefore, East Texas streams flow year round, but most West Texas streams flow only part of the year or intermittently.
East of the Trinity and south of the Red Rivers, river basins generally contain dark and murky streams because of high organic content. West of Austin, principal streams in river basins run clear when not in flood. In East Texas, the flow in basins, which is the volume of water flowing past a point per unit of time, remains relatively constant. In west and north Texas, flow changes rapidly from none or dry to flood stage following storms; consequently these intermittent streams are called "flashy."
We take advantage of the climatic effects on basin streams in selecting where we build reservoirs, as shown in the table on the River Basin Map of Texas. The largest reservoir is Toledo Bend in the Sabine River Basin, which has a storage capacity of 4,472,900 acre-feet. One acre-foot is the amount of water it takes to cover 1 acre of land 1 foot deep. The smallest reservoir, which is far to the west, is the Mackenzie Reservoir of the Red River, holding 46,250 acre-feet.
River basins strongly reflect their geology. For example, in wetter East Texas where uncemented sands and muds dominate the terrain, river valleys are wider and contain broadly meandering streams carrying abundant suspended brown mud. Meanders are the large snakelike bends that migrate back and forth across the floodplain over time. Eastern stream bars are composed of fine sands, silts, and clays. East Texas rivers have gentle mean gradients of 1.4 to 2.3 feet per mile; the gradient is the slope of the stream bed from its high point of origin to its final pour point.
In contrast, far West Texas streams cut deep gorges with nearly vertical canyon walls into bard limestones and sandstones. There stream courses are incised and normally flow quite clear. Western stream bars have larger proportions of gravel and coarse sand components. Western rivers have steeper mean gradients of 7.3 to 12.5 feet per mile.
Bedrocks, the sources of sediments transported by streams, contribute to the stream character. The Red River gets its name from the red sediment it erodes from red Permian strata. These same Permian strata contain thick salt beds that dissolve into waters of the Canadian and upper Brazos Rivers, making them too salty to drink. Black rocks that come from volcanic lavas are especially common in the Rio Grande gravels. These gravels also contain many rock types transported from New Mexico, like granites.
In terrains where river basins cross adjacent hard and soft rocks, stream patterns and gradients reflect the differences in erosion resistance. Waterfalls commonly increase flow rates, dropping over hard resistant rocks onto soft rocks. Pedernales Falls State Park contains a good example. In gently sloping hard and soft rock formations, streams flow in valleys preferentially eroded into soft rocks; they rarely erode through the harder rocks.
Geologic structures commonly control the directions, patterns, and local gradients of streams. Streams typically occupy areas of easily eroded, highly fractured bedrocks. Fault- and fracture-controlled drainage is common throughout the state. River basins commonly align parallel to faults and fractures and then abruptly bend at high angles, forming kinks controlled by intersections of structures. In East Texas, salt domes near the land surface cause streams to curve around them.
No Texas river basin is in a natural state along its entire length. All are somewhere impacted by humans and have dams, levees or engineered channels, and wastewater treatment plants. Texas rivers quench our thirst, carve a magnificent scenery, provide recreation, create habitats for wildlife, and supply water for agriculture and industry. As the population of Texas increases, we must continue to guard the availability and quality of these assets and wisely allocate our water resources.
-E. G. Wermund
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Aquifers are a critical part of the water cycle. Rainwater that falls on land can (1) evaporate, (2) be taken up by plants, (3) run off into streams, or (4) seep underground through soil, sediment, and rocks. The fraction of the water that moves into the groundwater is calledrecharge. Rock or sediment that stores and transports water underground in amounts and quality sufficient to be useful to humans is called an aquifer.
Water is stored in spaces within the sediment or rock that are called pores. Pores range from microscopic spaces between mineral grains, to fractures, to caves. Below the water table, most pores in a rock are filled with water, so this rock is described as saturated. The aquifer property that lets water move through connected pores is known as permeability. Gravel, sand, and sandstone are sediment and rock types that are commonly very permeable. Clay and shale are examples of sediments and rocks having small or poorly connected pores through which water does not move easily. The permeability of limestone and igneous rocks depends on the geologic history of the rock.
Water in an aquifer is not stagnant but instead moves under pressure. It enters the aquifer at the surface in the recharge zone. Water leaves the aquifer at discharge areas. These can be springs or wells or areas along the bed of a river or along the coast. The flow path from the recharge area to the discharge area can be short and simple, or it can be hundreds of miles long and take fresh water to depths of several thousand feet before bringing it back to the surface. Some parts of the flow path have a low-permeability layer above the aquifer, which is called the confined part of the aquifer. When a well is drilled into a confined aquifer, pressure in the aquifer will make water rise in the well. If pressure in the confined aquifer makes the water in the well rise above the land surface, the well is a flowing artesian well. Water is pumped from wells for irrigation, city and home water supplies, manufacturing, and mining activities.
Groundwater dissolves or chemically interacts with minerals in the rock. What we call fresh water contains less than 1,000 milligrams of dissolved material in each liter of water. In a limestone aquifer, water dissolves some of the mineral calcite, which gives water the taste and poor lathering properties of hard water. At greater depths and in less-permeable rocks, water becomes increasingly salty. It also becomes saline where it comes in contact with salt deposits and where, in arid areas, evaporation of groundwater concentrates dissolved minerals.
People can alter the natural system. If water removed from the aquifer exceeds the water replaced by recharge, springs may dry up, salt water may move into an area that previously had fresh water, or the ground surface may subside, increasing the risk of flooding. Spilling or misuse of materials such as salt water, oil, chemicals, pesticide, or fertilizer can contaminate groundwater.
Texas gets more than half its water supply, more than 2.5 trillion gallons, from aquifers. Because each aquifer system in Texas is unique, this map captures only the general distribution of major and some minor aquifers. The nine major aquifers in Texas supply more than 90 percent of the state's groundwater.
Some of the most productive aquifers in the state are in relatively young (to 1.6 million years [m.y.]old) sediments deposited by streams and are known as alluvial aquifers. The Brazos River alluvium aquifer, for example, was formed when sand, gravel, silt, and clay were deposited in the floodplain and channel of the ancient Brazos River. Alluvial aquifers along many Texas rivers are used for domestic water supply and for irrigation. Because they are shallow and composed of permeable sediments, alluvial aquifers are susceptible to contaminaÂtion. Natural and human sources of salinity have reduced the water quality in some alluvial aquifers, such as the Lipan aquifer.
Another group of aquifers are formed in deposits associated with rivers that flowed during the Cenozoic (as much as 65 mya). The Seymour aquifer is hosted by sand and gravel deposits of ancient east-flowing rivers. Its age is indicated by the valleys that modern rivers have cut into these deposits. Sand dunes as well as river sediment contributed to the Pecos River alluvium, and older rocks (Rustler) beneath the Pecos alluvium exchange water with it. The most prolific aquifer in Texas, the Ogallala aquifer of the Southern High Plains, is Cenozoic sands and gravels that were also deposited in a river-system complex. This aquifer is recharged from small playa lakes on the High Plains surface and is used heavily for irrigation. Water can flow between the Ogallala aquifer and underlying aquifers. Thousands of feet of sand, silt, and gravel deposits fill intermontane basins of West Texas known as bolsons. Each bolson has a distinct sequence of sedimentary fill and its own water balance. Because recharge and basin size are limited, water conservation is critical in these desert areas.
Also during the Cenozoic, rivers carried gravel, sand, and clay toward the Gulf of Mexico and deposited them in coastal-plain, beach, and marine environments. The Gulf Coast aquifer is a composite of several geologic formations composed of layers of sand and clay. In some areas, the water in the Gulf Coast aquifer can move easily from sand unit to sand unit; in others, the clay layers confine the water, and each sand unit acts as a separate aquifer. Names given to some of these partly separated aquifers are Catahoula, Jasper, Evangeline, and Chicot. In some areas heavy pumpage of the aquifer has had harmful effects, such as subsidence of the land surface and intrusion of saline water into the aquifer. Deep wells may tap several aquifers.
On the upper coastal plain an older Cenozoic sequence of coastal-plain sands, gravels, and clay hosts the Carrizo-Wilcox, Queen City, Sparta, and Yegua-Jackson aquifers. In most areas these aquifers are separated by clay-rich confining layers. Water enters these aquifers where sandstones crop out at the surface and moves down the aquifer into the subsurface, where it is pumped from wells. In some areas, water has reacted with low-grade coals (called lignite) or with iron-rich sediments and is of poor quality. The Carrizo-Wilcox aquifer is used heavily for irrigation in the southwest Texas Winter Garden area.
A band of rocks of Cretaceous age (140 to 65 m.y. old) hosts aquifers across the middle of the state. The fossiliferous Edwards limestone is one of the most dynamic aquifers in the country. Where Hill Country creeks and rivers cross the outcrops of the Edwards limestone, water is captured by caves and fractures, flows to depths as great as 3,000 feet through Edwards limestone layers, and discharges at some of the famous springs of Central Texas. But conflict between agricultural and urban pumpers and ecosystems dependent on springflow arises during drought. Cretaceous sandstone aquifers include the Trinity, Woodbine, Nacatoch, and Blossom. On the Edwards Plateau, limestones and sandstones are considered as one aquifer, the Edwards-Trinity, which is in hydrologic connection with over- and underlying Ogallala, Pecos River alluvium, and Dockum aquifers.
Small aquifers are important where water resources are scarce. The Capitan Reef Complex and Bone Spring-Victorio Peak of West Texas, for example, are limestone aquifers that are recharged in mountain areas. The Blaine aquifer, hosted by interbedded limestone, sandstone, and gypsum beds, is locally saline. The Hickory, Marble Falls, and Ellenburger-San Saba are minor aquifers around the Llano Uplift. These very old rocks (430 to 550 m.y.) lie on top of even older igneous and metamorphic rocks that are generally not porous or permeable enough to serve as aquifers. Fractured igneous rocks (for example, basalts and tuffs) and associated alluvial deposits are used for water resources in sparsely populated and arid areas of Trans-Pecos Texas. Similarly, fractured, folded, and slightly metamorphosed rocks and overlying alluvium are minor aquifers in the Marathon area.
Ashworth, J. B., and Hopkins, Janie, 1995, Aquifers of Texas: Texas Water Development Board, Report 345, 69 p.
Brune, Gunnar, 1975, Major and historical springs of Texas: Texas Department of Water Resources, Report 189, 94 p.
Ryder, P. D., 1996, Segment 4, Oklahoma, Texas, in Groundwater atlas of the United States: Reston, VA, U.S. Geological Survey, 30 p.
Texas has produced more oil and natural gas than any other state and to date remains the largest daily producer. Oil and natural gas are found in most parts of the state. No state or any other region worldwide has been as heavily explored or drilled for oil and natural gas as Texas. Currently (August 2003), 151,605 active oil wells and 66,951 active gas wells produce oil and natural gas in the state.
Texas Oil Production
Although Texas wasn't the first state to produce oil, Texans weren't far behind. Drilling for oil in Texas occurred at Oil Springs, near Nacogdoches in East Texas, in 1866, less than a decade after Colonel Edwin Drake's 1859 Titusville, Pennsylvania, well brought the U.S. into the age of oil. Oil had been found before in Texas, but it had been either through natural surface seeps or drilling for water. Then, in 1894, the Texas age of oil began with the first major discovery, Corsicana field, in East Texas. The first boom came in 1901 with Spindletop field in the Gulf Coast Basin. Thousands of other discoveries have followed. East Texas oil field, the largest oil field in Texas or in any of the U.S. Lower 48 states, was discovered in 1930. Annua l Texas oil production peaked in 1972 at 1,263 MMbbl (million barrels), and thereafter production rapidly dwindled. Although oil production in Texas is in decline , significant opportunities for incremental recovery exist in advanced exploration and production techno logies. On average, only 35 percent of original oil in place in Texas reservoirs has been recovered. Technology plays a pivotal role in increasing recovery rate, improving economics, and assisting in exploration of complex oil reservoirs. If technology can be applied to an increasing ly complex and mature resource base, oil production decline in Texas can be slowed.
Texas Natural Gas Production
Historically , natural gas in Texas was discovered as a byproduct of oil. This form of natural gas, which is in contact with crude oil in the reservoir, is termed associated gas, and in earlier years it was wastefully flared and vented off without being produced. With increased oil exploration and discoveries in Texas, annual natural gas production steadily rose and peaked also in 1972 at 9.6 Tcf (trillion cubic feet). However, unlike oil production, since the early 1980's, Texas gas production has maintained a steady production level. This was achieved through several large field discoveries, such as Newark, East, field in North-Central Texas, as well as a multitude of smaller sized fields that required application of advanced exploration and development technologies. Texas natural gas production levels were maintained by increasing numbers of producing wells, which are now at an all-time high. Today many of the new exploration and production activities involve natural gas rather than oil.
U.S. and World Ranking
Through the application of advanced technologies, incremental oil recovery from mature oil fields continues to make Texas the state that leads in oil production. In terms of year 2002 oil and natura l gas production. Texas produced 17 percent (366 MMbbl) and 30 percent (5.7 Tcf) respectively, of the U.S. total. Indeed, if Texas were a nation, it would rank as one of the top 10 producers in the world. In terms of proved oil and natural gas reserves, Texas has 22 percent (5,015 MMbbl) and 23 percent (44.3 Tcf), respectively, of the U.S. total. Reserves are the estimated quantities that analysis of geological and engineering data demonstrates with reasonable certainty in future years to be recoverable from known reservoirs, under existing economic and operating conditions.
Major Producing Regions
Oil and natural gas production in Texas can be divided into seven major producing basins. The Permian Basin dominates oil production in the state, and the Gulf Coast Basin dominates natural gas production. Major oil fields in Texas include Wasson, Yates, and Spraberry in West Texas, as well as the largest Texas oil field, East Texas field in the East Texas Basin. The Permian Basin has been the most prolific oil-producing province in U.S. history. East Texas field has produced more oil than any other field in the lower 48 states. Major natural gas fields in Texas, in terms of today's production rate. include Newark, East, field in the Fort Worth Basin; Carthage field in East Texas; Panhandle, West, field in the Anadarko Basin; and Giddings field in the Gulf Coast Basin. Excluding Panhand le, West, field, all major natura l gas fields in Texas are a product of application of advanced technologies, such as hydraulic fracturing and horizontal drilling, which have resulted in increased production from these low-permeability and complex fields.
Although oil and natural gas production in Texas has declined from its peak, advanced exploration and development technologies will enable Texas to remain the major oil and natural gas producer in the U.S. Because easy-to-find oil and natural gas resources have been fully exploited in Texas, the future mix of oil and gas resources will be increasingly complex and technologically challenging.
Oil and natural gas production in Texas, although not as great as in the past, remains an important source of economic benefit, in terms of value, jobs created, and taxes. According to the Texas Comptroller's input-output model of Texas' economy, the total economic value of oil and gas is 2.91 times the value of production. Additionally, 19.1 jobs are created per million dollars of oil and gas production. Assuming oil and natural gas prices of $25/bbl and $5/Mcf, and year 2002 annual production of 366 MMbbl and 5.7 Tcf, wellhead value exceeds $37 billion. Annual natural gas value is currently 3.1 times that of the oil wellhead value to Texas. In terms of economic value trick led down through the Texas economy and jobs created , this figure equates to nearly $1 10 billion and 719,115 jobs. Severance , ad valorem , and indirect taxes provide additional economic benefits of more than $6 billion to Texas. The leasing of mineral rights to State- and University-owned lands statewide, moreover, provides royalty and leasing revenue that replenishes the Permanent University and School Funds, important sources of revenue for pub lic education in Texas.
Railroad Commission of Texas
The Railroad Commission of Texas, established in 1891, is the oldest regulatory agency in the state and one of the oldest of its kind in the nation. The Railroad Commission has regulatory divisions that oversee Texas' oil and natural gas industry, gas utilities, pipeline and rail safety, safety in the liquefied petroleum gas industry, and surface mining of coal and uranium. As the regulatory agency for the oil and gas industry, it provides extensive drilling and production statistics. The Railroad Commission continues to serve Texas in its stewardship of natural resources and the environment, its concern for the individual and communa l safety of citizens , and its support of enhancing development and economic vitality for the betterment of Texas as a whole.
-Eugene M. Kim and Stephen C. Ruppel
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Soil, a natural body composed of minerals, organic matter, liquids, and gases, occurs on Earth’s surface and supports plant growth. Soils form in environments ranging from desert landscapes to coastal grassflats permanently covered by water up to 2.5 m deep. Soil formation is related to five factors: parent material, climate, topography, living organisms, and time. The soil under your feet determines land use, kinds of crops grown, need for fertilizers, and erosion potential. The state of Texas is divided into 15 major land resource areas, each of which is a grouping of similar soils, vegetation, climate, and topography.
Southern Desertic Basins, Plains, and Mountains
These soils formed in an area of linear mountain ranges and broad desert basins bordered by sloping alluvial fans and piedmont slopes known as the Basin and Range. Shallow soils, including Brewster, Lajitas, and Mainstay soils, formed on mountainous terrain in igneous bedrock. Soils that are shallow to a root-restrictive layer of cemented caliche (CaCO3) occur in gravelly sediments weathered from igneous sources, such as Delnorte and Boracho soils, and from limestone sources, such as Philder soils. Very deep soils formed in basin sediments from limestone, such as Armesa and Reyab soils, and from mixed sources, such as Reakor soils. Liv soils, moderately deep to igneous bedrock, formed in gravelly igneous sediments. Very deep, loamy Musquiz soils occur on broad plains.
Southern High Plains
These soils formed on a nearly level plain on an elevated plateau, commonly bordered by moderately steep escarpments on west and east margins. Numerous playa basins dot the plains. The area is characterized by deep, well-developed soils, with clay increasing in subsoil horizons and accumulations of calcium carbonate. Sherm, Darrouzett, Pullman, Lofton, and Randall soils have clayey subsoil horizons and shrink-swell properties. Acuff, Olton, and Gruver are loamy soils having dark surface horizons (higher organic matter), whereas Amarillo, Dallam, Rickmore, and Vingo are loamy soils having less organic matter. Patricia, Brownfield, Jalmar, and Triomas soils have sandy surface horizons. Nutivoli and Penwell are sandy, less-developed soils. Conlen, Sunray, Spurlock, and Veal soils are calcareous throughout, and Mobeetie and Berda soils are loamy and occur along flanking escarpments.
Central Rolling Red Plains
These soils formed on an erosional surface characterized by rolling plains having ancient stream terraces associated with stream dissection. Soils (mostly red) formed in gently dipping Triassic and Permian sedimentary deposits and alluvium weathered from outcropping bedrock. Miles, Delwin, and Springer are well-developed soils having sandy surface horizons. Woodward and Vernon soils are moderately deep to sandstone and mudstone bedrock, respectively. Loamy Tillman and Hollister soils are very deep with shrink-swell properties.
Texas North Central Prairies
These soils formed on a dissected plateau with narrow, steep-sided valleys carved by generally southeastward flowing streams. Soil parent materials are primarily sedimentary rocks of Pennsylvanian age. Bonti, Bluegrove, Callahan, Stoneburg, and Throck soils, moderately deep to sandstone, siltstone, or claystone, occur on gently sloping to steep, broad ridges and plains. Deep Truce soils and very deep Anocon soils formed on similar landscapes. Very deep Kirkland soils formed in clayey alluvium over siltstone or claystone.
These soils formed on mesas and plateaus of erosion-resistant limestone containing deeply incised canyons, limestone ridges and hills, and gently sloping valley floors. Tarrant, Lozier, Ector, Langry, Brackett, Eckrant, and Real soils are shallow to limestone and differ in texture, mineraology, or organic matter content. Conger, Kavett, Oplin, and Zorra soils have a root-restrictive layer of cemented caliche (CaCO3) over limestone bedrock. Very deep soils occurring on broad plateaus and in alluvial-fan and valley-fill sediments include loamy, calcareous Reagan soils. Clayey Tobosa soils occur on alluvial plains, broad uplands, and depressions.
Texas Central Basin
These soils formed on an erosional surface of outcropping Precambrian igneous and metamorphic rocks and sedimentary rocks of Cambrian and Cretaceous age. The landscape is dominated by hills of granite, gneiss, and schist that are incised by southeastward-flowing rivers. Shallow Keese soils formed over granite and gneiss on gently sloping to steep hillslopes, Moderately deep Ligon soils formed in schist and gneiss on gently sloping, broad, convex ridges.
Rio Grande Plain
These soils formed on a broad coastal plain consisting of sediments of Tertiary and Quaternary age. The southern extent of this nearly level plain is within the ancestral valley cut by the Rio Grande. The coastal-plain landscape is dissected by generally southeastward flowing streams. Weesatche, Duval, Sarnosa, Hidalgo, Brennan, pernitas, Uvalde, Pryor, Elindio, and McAllen soils are deep and very deep, well-developed, loamy soils that occur on nearly level to moderately sloping plains and broad ridges. Olmos, Delmita, and Randado soils, shallow to a root-restrictive layer of cemented caliche (CaCO3) formed in gravelly Pleistocene sediments. Langtry soils are shallow, Montell and Catarina soils are clayey sodium-affected soils, and Maverick soils are clayey and moderately deep to weathered shale bedrock. Falfurrias, Sarita, and Nueces soils are very deep, sandy soils on the sand-sheet prairie that covers the southeast parts of the South Texas Coastal Plain.
These soils formed on a rolling landscape with low to moderate relief dissected by numerous narrow streams. Outcropping sandstones, shales, and limestones of Cretaceous age cover the landscape, and unconsolidated sands and gravels fill the rivers and streams. Duffau, Gasil, and Windthorst soils are deep, highly weathered soils that formed in interbedded sandstone and shale. These soils formed on convex uplands and are very susceptible to erosion. Chaney, Crosstell, and Callisburg soils have clayey subsoils and are deep to claystone or shale.
These soils formed on gently rolling to hilly, dissected limestone plateaus and in adjacent, gently sloping valleys. Steep slopes border valleys along major streams, and most soils formed in flat-lying limestones and calcareous shales of Cretaceous age. Shallow soils—including Aledo, Brackett, Purves, and Real—occur on hills and ridges and differ in texture, mineralogy, and organic matter content. Moderately deep Bolar soils occur on similar landscapes. Clayey Sanger soils, which formed in shale parent materials, have shrink-swell properties.
Texas Blackland Prairie
These Soils formed on a nearly level to gently rolling plain, dissected by generally southeastward flowing streams—a landscape that developed on outcrops of calcareous shales of Cretaceous age. The Austin Chalk (Balcones Escarpment) borders the Blackland Prairie to the west. The shale parent materials have produced a significant extent of clayey soils having high shrink-swell properties, including Houston Black, Heiden, Frelsburg, Bleiblerville, and Latium soils. Loamy soils on similar landscape positions, which formed in interbedded sandstone and shale, include Hallettsville, Crockett, Wilson, and Carbengle.
Texas Claypan Area
These soils formed on nearly level to sloping plains dissected by perennial streams and their tributaries. Large floodplains and stream terraces are associated with meandering river systems. Over most of the area, soils have well-developed, clayey, subsoil horizons with sandy and loamy surface textures. Woodtell, Edge, Crockett, and Straber soils occur on interstream divides and ridges, and Tabor soils are on stream terraces. Padina and Silstid soils have sandy surface layers more than 20 inches thick.
Western Coastal Plain and Flatwoods
These soils formed on nearly level to steep, coastal-plain uplands that are intricately dissected by streams. Parent materials are alluvial and marine sediments of Tertiary age. Pineywoods soils are mostly highly weathered, acidic soils that support pine-hardwood vegetation. Cuthbert, Bowie, Kirvin, Eastwood, Scottsville, Woodtell, and Pinetucky are deep soils that occur on interstream divides and low ridges. Trawick soils formed in glauconitic sediments. Conroe, Pickton, Lovelady, and Wolfpen soils have sandy surface layers more than 2- inches thick, and Fuller and Keltys soils are loamy and deep to mudstone. Flatwood soils are highly weathered and acidic and support pine=hardwood vegetation characterized by loblolly pine. The very deep Otanya, Kirbyville, and Evadale soils occur on low-relief uplands and flat plains.
These soils formed in alluvium on flood plains, the nearly level plains that border a stream and that are subject to inundation under river flood-stage conditions. Tinn, Trinity, Kaufman, Pledger, and Brazoria soils have clayey textures and high shrink-swell properties. Loamy Norwood soils have an irregular distribution of organic matter with soil depth.
Gulf Coast Prairie
These soils formed in alluvial and marine sediments of (primarily) Quaternary age that were deposited under fluctuating sea-level conditions. The area is characterized by low local relief and dissection by rivers that flow to the Gulf of Mexico. Victoria, Laewest, Edroy, Beaumont, League and Lake Charles soils are well-developed, clayey soils with high shrink-swell properties. Orelia, Dacosta, Edna, Labelle, Gessner, Bernard, Katy, Telferner, Wockley, Cieno, and Nada soils have loamy surface textures and loamy and clayey subsoil horizons, and they differ primarily on drainage class and mineralogy.
Gulf Coast Saline Prairies
These soils formed in Quaternary sediments on nearly level coastal lowland plains, including coastal marshes, tidal flats, and barrier islands. Clayey, saline soils include Barrada, harris, Surfside, and Francitas. Sandy Mustang and Daggerhill soils occur on dune landforms on barrier-island landscapes.
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The varied Earth materials that define the diverse surface formations of Texas provide many valuable industrial rocks and minerals used by modern society. Use of Earth materials by Texas residents started in prehistory with clay for pottery, flint for projectile points and tools, grinding stones for food production, and many other uses of local rocks.
Today Texas is an important producer of the industrial mineral resources that are used widely by the state’s ever-growing population, typically ranking among the top five states in annual production value of nonfuel minerals. These varied resources are used extensively in the construction and chemical industries, and their production typically is a direct reflection of the state’s economic vitality. More than 90% of current Texas industrial mineral value comes from production of cement, crushed stone, and construction sand and gravel.
Industrial mineral consumption typically tracks regional population and is reflected in the doubling of Texas’ population since 1970 to its current 23 million residents. For the purposes here, this group of minerals also includes some energy mineral resources—lignite and uranium—that are important producers of electricity for residential, commercial, and industrial consumers. The soils and water essential to Texas’ agricultural and forest industries can also be considered industrial materials.
Texas Energy Minerals
Although coals of various geological ages and ranks are present in Texas, current production is dominated by extensive “brown coal” lignite deposits of Tertiary age in the Gulf of Mexico Coastal Plain. Local Tertiary bituminous coals occur in the Rio Grande Valley, as well as in Cretaceous and Pennsylvanian strata in other regions. Extensive mining of Gulf Coast lignites began in the 1970’s for fuel at “mouth-of-mine” power plants to supply growing regional electricity needs. Texas’ electricity demand increased almost 400% from 1970 through 2005 and shows no signs of slowing. Lignite production has totaled more than 1.3 billion tons, peaking at 56.5 million tons in 1993 and continuing at an annual rate of nearly 47 million tons (2005). The lignite beds extend into the subsurface toward the Texas coast, but current economics only allow surface extraction near the outcrop belt. Tertiary strata of the Coastal Plain host uranium deposits in south Texas. Uranium production started in the early 1960’s and reached a peak of 3,900 tons of U3O8 in 1980. Although south Texas uranium production declined steadily through the 1990’s, recent price increases associated with a rise in demand have resulted in minor current production.
Texas Construction Materials
Crushed stone, gravel, and sand that are consumed in large quantities as aggregate by the construction industry dominate annual state production in terms of tonnage. Texas is the largest crushed-stone-producing state, with more than 200 quarries; an equal number of operations produce sand and gravel from unconsolidated surface deposits. Cement, another vital construction material manufactured principally from limestone and clay, is by far the most valuable industrial mineral product in Texas, with 2005 production estimated at more than $1 billion (40+% of Texas’ annual industrial mineral value) from 12 cement plants.
Gypsum is used in plaster and wallboard, as well as cement manufacture. Clays of various types largely in the Coastal Plain are used in many products, with common clay being consumed in large quantities in the manufacture of bricks and tile. Dimension stone, used mostly for monuments and building exteriors, but with growing high-end residential use, is supplied by Texas’ granites, limestones, and sandstones.
Texas Chemical Minerals
Many industrial minerals are used in the chemical industries—from primary industrial applications to secondary applications in which they serve as sources of valuable elements. For example, salt (sodium chloride) has diverse uses, but most is produced as a chlorine source for the manufacture of hydrochloric acid, a widely used industrial chemical. Lime (calcium oxide produced by calcining limestone) has many uses as well, including water purification, paper manufacture, and sugar refining. Zeolites are valued for their ion-exchange capacity and are used in water- and other purification processes. Bentonitic clays have diverse applications in industrial processes, including drilling-fluid production and vegetable-oil refining. Ball and kaolin clays are used in ceramics and as fillers and coating agents in the rubber and paper industries. Limestone also has many chemical uses, including flue-gas desulfurization of SO2produced by coal-fired electricity-generation plants; this process produces synthetic gypsum, which is becoming an alternative to natural gypsum in wallboard and cement manufacture.
Sulfur, produced by more than 60 refineries of “sour” crude oil and natural gas from Texas and imported sources, is another widely used element, principally in the manufacture of sulfuric acid. The principal domestic source of helium is from natural gas in the Texas Panhandle fields. Sodium sulfate is produced from brines underlying alkaline lakes in west Texas. Talc deposits in west Texas are mined for fillers in ceramic, paper, plastic, and rubber products.
Geology of Major Producing Regions
Most industrial minerals are relatively common Earth materials that can only be produced commercially by relatively low-cost surface extraction techniques. Thus, industrial-mineral production typically occurs in areas where favorable geologic units occur at the surface relatively near the population centers that will consume the products.
These essential mineral resources are products of past geologic events that have affected this part of the Earth’s crust. Ancient plate tectonic processes created a vast mountain range, the roots of which are represented by the Precambrian metamorphic rocks and granites exposed in the Llano region of central Texas and smaller exposures in west Texas. Texas was covered by shallow seas during the early Paleozoic (Cambrian-Ordovician), late Paleozoic (Permian), and late Mesozoic (Cretaceous). These environments produced the extensive carbonate strata that form the Edwards Plateau and other surface belts of limestone that are essential to Texas’ crushed stone, cement, and lime production. Evaporation of these shallow seas in the Permian and Cretaceous also produced local gypsum deposits. Even more extensive early Mesozoic evaporates, present under the Texas Coastal Plain, have been deformed into local salt domes that supply salt via underground mines and brine operations. Sulfur has been produced in large quantities from microbial alteration zones in Coastal Plain salt dome cap rocks and in the Permian evaporates in west Texas.
Surface deposits of Cenozoic age blanketing older deposits in much of Texas provide many valuable industrial mineral resources. Cenozoic strata were deposited by river and coastal processes that distributed the gravels, sands, and muds eroded from the Rocky Mountains and the continental interior. Deposition of these thick sedimentary layers built the Coastal Plain and extended the Texas shoreline to its current position (and farther during the glacial period that resulted in lower sea level 18,000 years ago). Swamps related to deltaic environments provided environments for extensive plant growth and are preserved as Tertiary lignite deposits. Coastal Plain sediments are also the source of clay for bricks and ceramics. Ash from volcanoes in Trans-Pecos Texas and elsewhere in southwestern North America provided unusual Coastal Plain sediments that were altered to valuable industrial zeolites, bentonites, and other clay deposits. Tertiary volcanic ash also was the source of uranium that was concentrated by groundwater to form Texas’ uranium deposits. Most industrial sand and construction sand and gravel are produced from the unconsolidated alluvial deposits of Texas’ major river systems.
The total value of Texas’ industrial mineral production for 2005 was more than $2.4 billion, with additional value supplied by lignite and uranium production. Further, industrial rocks and minerals are produced in virtually every Texas county, principally related to local construction and industrial activities. Industrial-mineral production provides local employment, and unusual mineral concentrations provide specialty products for regional distribution. As Texas’ population continues to grow, production of energy and industrial minerals will continue to satisfy the demands of residential, commercial, and industrial customers.
Ecoregions denote areas of general similarity in ecosystems and in type, quality, and quantity of environmental resources. They are designed to be a spatial framework for research, assessment, management, and monitoring of ecosystems and ecosystem components. Ecoregions stratify the environment by its probable response to disturbance (Bryce and others, 1999). These general-purpose regions are critical to the structuring and implementation of ecosystem management strategies across Federal agencies, State agencies, and non-governmental organizations responsible for different types of resources within the same geographical areas (Omernik and others, 2000).
Ecological and biological diversity of Texas is enormous. The state encompasses barrier islands and coastal lowlands, large river floodplain forests, rolling plains and plateaus, forested hills, deserts, and a variety of aquatic habitats. There are 12 level III ecoregions and 56 level IV ecoregions in Texas, and most continue into ecologically similar parts of adjacent states in the U.S. or Mexico.
This map is based on the premise that ecological regions are hierarchical and can be identified through analysis of spatial patterns and the composition of biotic and abiotic phenomena that affect or reflect differences in ecosystem quality and integrity (Wiken, 1986; Omernik 1987, 1995). These phenomena include geology, physiography, vegetation, climate, soils, land use, wildlife, and hydrology. The relative importance of each characteristic varies from one ecological region to another.
A hierarchical scheme indicates different levels of ecological regions. Level I divides North America into 15 ecological regions. Level II divides the continent into 52 regions (Commission for Environmental Cooperation Working Group, 1997). At level III, the continental United States contains 104 ecoregions, and the conterminous United States has 84 ecoregions (U.S. Environmental Protection Agency, 2003). Level IV, depicted here for Texas, is a further refinement of level III ecoregions. Explanations of the methods used to define the U.S. Environmental Protection Agency's (EPA) ecoregions are given in Omernik (1995), Omernik and others (2000), and Gallant and others (1989).
This map is modified from a collaborative project between EPA Region VI, EPA National Health and Environmental Effects Research Laboratory (Corvallis, Oregon), the Texas Commission on Environmental Quality (TCEQ), and the U.S. Department of Agriculture- Natural Resources Conservation Service (NRCS). Collaboration and consultation also occurred with the U.S. Geological Survey (USGS)-Earth Resources Observation Systems Data Center.
Bryce, S. A., Omernik, J. M., and Larsen, D. P., 1999, Ecoregions- a geographic framework to guide risk characterization and ecosystem management: Environmental Practice, v. 1, no. 3, p. 141-155.
Commission for Environmental Cooperation Working Group, 1997, Ecological regions of North America- toward a common perspective: Montreal, Quebec, Commission for Environmental Cooperation, 71 p.
Gallant, A. L., Whittier, T. R., Larsen, D. P., Omernik, J. M., and Hughes, R. M., 1989, Regionalization as a tool for managing environmental resources: Corvallis, Oregon, U.S. Environmental Protection Agency, EPA/600/3-89/060, 152 p.
Omernik, J. M., 1987, Ecoregions of the conterminous United States (map supplement): Annals of the Association of American Geographers, v. 77, no. 1, p. 118-125, scale 1:7,500,000.
______ 1995, Ecoregions- a spatial framework for environmental management, in Davis, W. S., and Simon, T. P., eds., Biological assessment and criteria- tools for water resource planning and decision making: Boca Raton, Florida, Lewis Publishers, p. 49-62.
Omernik, J. M., Chapman, S. S., Lillie, R. A., and Dumke, R. T., 2000, Ecoregions of Wisconsin: Transactions of the Wisconsin Academy of Sciences, Arts and Letters, v. 88, no. 2000, p. 77-103.
U.S. Environmental Protection Agency, 2003, Level 111 ecoregions of the continental United States (revision of Omernik, 1987): Corvallis, Oregon, U.S. Environmental Protection Agency- National Health and Environmental Effects Research Laboratory, Map M-l, various scales.
Wiken, E., 1986, Terrestrial ecozones of Canada: Ottawa, Environment Canada, Ecological Land Classification Series No. 19, 26 p.
The BEG acknowledges James M. Omernik, Principal Investigator, EPA, and Anne Rogers, Texas Commission on Environmental Quality (TCEQ), for assistance and permission to reproduce this map.
Managing Editor: Peter Eichhubl
Media manager: Cathy J. Brown
Graphics: John T. Ames and Jamie H. Coggin
Authors: Glenn E. Griffith (Dynamac Corporation), Sandra A. Bryce (Dynamac Corporation), James M. Omernik (USGS), Jeffrey A. Comstock (Indus Corporation), Anne C. Rogers (TCEQ), Bill Harrison (TCEQ), Stephen L. Hatch (Texas A&M University), and David Bezanson (Natural Area Preservation Association)