Mountains, seas, coastal plains, rocky plateaus, high plains, forests — all of this physiographic variety in Texas is controlled by the varied rocks and structures that underlie and crop out across the state. The fascinating geologic history of Texas is recorded in the rocks — both those exposed at the surface and those penetrated by holes drilled in search of oil and natural gas.
The rocks reveal a dynamic, ever-changing earth — ancient mountains, seas, volcanoes, earthquake belts, rivers, hurricanes, and winds. Today, the volcanoes and great earthquake belts are no longer active, but rivers and streams, wind and rain, and the slow, inexorable alterations of rocks at or near the surface continue to change the face of Texas.
The geologic history of Texas, as documented by the rocks, began more than a billion years ago. Its legacy is the mineral wealth and varied land forms of modern Texas.
Geologic Time Travel
The story preserved in rocks requires an understanding of the origin of strata and how they have been deformed. Stratigraphy is the study of the composition, sequence, and origin of rocks: what rocks are made of, how they were formed, and the order in which the layers were formed.
Structural geology reveals the architecture of rocks: the locations of the mountains, volcanoes, sedimentary basins, and earthquake belts.
The map above shows where rocks of various geologic ages are visible on the surface of Texas today. History concerns events through time, but geologic time is such a grandiose concept, most find it difficult to comprehend. So geologists have named the various chapters of earth history.
Precambrian rocks, more than 600 million years old, are exposed at the surface in the Llano Uplift of Central Texas and in scattered outcrops in West Texas, around and north of Van Horn and near El Paso.
These rocks, some more than a billion years old, include complexly deformed rocks that were originally formed by cooling from a liquid state, as well as rocks that were altered from pre-existing rocks.
Precambrian rocks, often called the “basement complex,” are thought to form the foundation of continental masses. They underlie all of Texas. The outcrop in Central Texas is only the exposed part of the Texas Craton, which is primarily buried by younger rocks. (A craton is a stable, almost immovable portion of the earth’s crust that forms the nuclear mass of a continent.)
During the early part of the Paleozoic Era (approximately 600 million to 350 million years ago), broad, relatively shallow seas repeatedly inundated the Texas Craton and much of North and West Texas. The evidence for these events is found exposed around the Llano Uplift and in far West Texas near Van Horn and El Paso, and also in the subsurface throughout most of West and North Texas. The evidence includes early Paleozoic rocks — sandstones, shales, and limestones, similar to sediments that form in seas today — and the fossils of animals, similar to modern crustaceans — the brachiopods, clams, snails, and related organisms that live in modern marine environments.
By late Paleozoic (approximately 350 million to 240 million years ago), the Texas Craton was bordered on the east and south by a long, deep marine basin called the Ouachita Trough. Sediments slowly accumulated in this trough until late in the Paleozoic Era. Plate-tectonic theory postulates that the collision of the North American Plate (upon which the Texas Craton is located) with the European and African–South American plates uplifted the thick sediments that had accumulated in the trough to form the Ouachita Mountains.
At that time, the Ouachitas extended across Texas. Today, the Texas portion of the old mountain range is mostly buried by younger rocks. Ancient remnants can be seen in the Marathon Basin of West Texas due to uplift and erosion of younger sediments. The public can see the remains of this once-majestic Ouachita Mountain range at Post Park, just south of Marathon in Brewster County. Other remnants at the surface are exposed in southeastern Oklahoma and southwestern Arkansas.
During the Pennsylvanian Period, however, the Ouachita Mountains bordered the eastern margin of shallow inland seas that covered most of West Texas. Rivers flowed westward from the mountains to the seas bringing sediment to form deltas along an ever-changing coastline.
The sediments were then reworked by the waves and currents of the inland sea. Today, these fluvial, delta, and shallow marine deposits compose the late Paleozoic rocks that crop out and underlie the surface of North Central Texas.
Broad marine shelves divided the West Texas seas into several sub-basins, or deeper areas, that received more sediments than accumulated on the limestone shelves. Limestone reefs rimmed the deeper basins. Today, these reef limestones are important oil reservoirs in West Texas.
These seas gradually withdrew from Texas, and by the late Permian Period, all that was left in West Texas were shallow basins and wide tidal flats in which salt, gypsum, and red muds accumulated in a hot, arid land. Strata deposited during the Permian Period are exposed today along the edge of the Panhandle, as far east as Wichita Falls and south to Concho County, and in the Trans-Pecos.
Approximately 240 million years ago, the major geologic events in Texas shifted from West Texas to East and Southeast Texas. 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.
A series of faulted basins, or rifts, extending from Mexico to Nova Scotia were formed. These rifted basins received sediments from adjacent uplifts. As Europe and the southern continents continued to drift away from North America, the Texas basins were eventually buried beneath thick deposits of marine salt within the newly formed East Texas and Gulf Coast basins.
Jurassic and Cretaceous rocks in East and Southeast Texas document a sequence of broad limestone shelves at the edge of the developing Gulf of Mexico. From time to time, the shelves were buried beneath deltaic sandstones and shales, which built the northwestern margin of the widening Gulf of Mexico to the south and southeast.
As the underlying salt was buried more deeply by dense sediments, the salt became unstable and moved toward areas of least pressure. As the salt moved, it arched or pierced overlying sediments forming, in some cases, columns known as “salt domes.” In some cases, these salt domes moved to the surface; others remain beneath a sedimentary overburden. This mobile salt formed numerous structures that would later serve to trap oil and natural gas.
By the early Cretaceous (approximately 140 million years ago), the shallow Mesozoic seas covered a large part of Texas, eventually extending west to the Trans-Pecos area and north almost to present-day state boundaries. Today, the limestone deposited in those seas is exposed in the walls of the magnificent canyons of the Rio Grande in the Big Bend National Park area and in the canyons and headwaters of streams that drain the Edwards Plateau, as well as in Central Texas from San Antonio to Dallas.
Animals of many types lived in the shallow Mesozoic seas, tidal pools, and coastal swamps. Today, these lower Cretaceous rocks are some of the most fossiliferous in the state. Tracks of dinosaurs occur in several places, and remains of terrestrial, aquatic, and flying reptiles have been collected from Cretaceous rocks in many areas.
During most of the late Cretaceous, much of Texas lay beneath marine waters that were deeper than those of the early Cretaceous seas, except where rivers, deltas, and shallow marine shelves existed.
River delta and strandline sandstones are the reservoir rocks for the most prolific oil field in Texas. When discovered in 1930, this East Texas oil field contained recoverable reserves estimated at 5.6 billion barrels.
The chalky rock that we now call the “Austin Chalk” was deposited when the Texas seas became deeper. Today, the chalk (and other Upper Cretaceous rocks) crops out in a wide band that extends from near Eagle Pass on the Rio Grande, east to San Antonio, north to Dallas, and east to the Texarkana area. The Austin Chalk and other upper Cretaceous rocks dip southeastward beneath the East Texas and Gulf Coast basins.
The late Cretaceous was the time of the last major seaway across Texas, because mountains were forming in the western United States that influenced areas as far away as Texas.
A chain of volcanoes formed beneath the late Cretaceous seas in an area roughly parallel to and south and east of the old, buried Ouachita Mountains. The eruptions of these volcanoes were primarily on the sea floor and great clouds of steam and ash likely accompanied them.
Between eruptions, invertebrate marine animals built reefs on the shallow volcanic cones. Pilot Knob, located southeast of Austin, is one of these old volcanoes that is now exposed at the surface.
At the dawn of the Cenozoic Era, approximately 65 million years ago, deltas fed by rivers were in the northern and northwestern margins of the East Texas Basin. These streams flowed eastward, draining areas to the north and west. Although there were minor incursions of the seas, the Cenozoic rocks principally document extensive seaward building by broad deltas, marshy lagoons, sandy barrier islands, and embayments.
Thick vegetation covered the levees and areas between the streams. Coastal plains were taking shape under the same processes still at work today.
The Mesozoic marine salt became buried by thick sediments in the coastal plain area. The salt began to form ridges and domes in the Houston and Rio Grande areas. The heavy load of sand, silt, and mud deposited by the deltas eventually caused some areas of the coast to subside and form large fault systems, essentially parallel to the coast.
Many of these coastal faults moved slowly and probably generated little earthquake activity. However, movement along the Balcones and Luling-Mexia-Talco zones, a complex system of faults along the western and northern edge of the basins, likely generated large earthquakes millions of years ago.
Predecessors of modern animals roamed the Texas Cenozoic coastal plains and woodlands. Bones and teeth of horses, camels, sloths, giant armadillos, mammoths, mastodons, bats, rats, large cats, and other modern or extinct mammals have been excavated from coastal plain deposits.
Vegetation in the area included varieties of plants and trees both similar and dissimilar to modern ones. Fossil palmwood, the Texas “state stone,” is found in sediments of early Cenozoic age.
The Cenozoic Era in Trans-Pecos Texas was entirely different. There, extensive volcanic eruptions formed great calderas and produced copious lava flows. These eruptions ejected great clouds of volcanic ash and rock particles into the air — many times the amount of material ejected by the 1980 eruption of Mount St. Helens.
Ash from the eruptions drifted eastward and is found in many of the sand-and-siltstones of the Gulf Coastal Plains. Lava flowed over older Paleozoic and Mesozoic rocks, and igneous intrusions melted their way upward into crustal rocks. These volcanic and intrusive igneous rocks are well exposed in arid areas of the Trans-Pecos today.
In the Texas Panhandle, streams originating in the recently elevated southern Rocky Mountains brought floods of gravel and sand into Texas. As the braided streams crisscrossed the area, they formed great alluvial fans.
These fans, which were deposited on the older Paleozoic and Mesozoic rocks, occur from northwestern Texas into Nebraska. Between 1 million and 2 million years ago, the streams of the Panhandle were isolated from their Rocky Mountain source, and the eastern edge of this sheet of alluvial material began to retreat westward, forming the Caprock of the modern High Plains.
Late in the Cenozoic Era, a great Ice Age descended on the northern North American continent. For more than 2 million years, there were successive advances and retreats of the thick sheets of glacial ice. Four periods of extensive glaciation were separated by warmer interglacial periods. Although the glaciers never reached as far south as Texas, the state’s climate and sea level underwent major changes with each period of glacial advance and retreat.
Sea level during times of glacial advance was 300 to 450 feet lower than during the warmer interglacial periods because so much sea water was captured in the ice sheets. The climate was both more humid and cooler than today, and the major Texas rivers carried more water and more sand and gravel to the sea. These deposits underlie the outer 50 miles or more of the Gulf Coastal Plain.
Approximately 3,000 years ago, sea level reached its modern position. The rivers, deltas, lagoons, beaches, and barrier islands that we know as coastal Texas today have formed since that time.
Source: Bureau of Economic Geology, The University of Texas at Austin; www.beg.utexas.edu/