San Antonio 1996 pin

San Antonio 1996 pin

This pin was given to me by Ron Eberhardt, now retired from TxDot. He acquired it at a professional meeting.

San Antonio is a city built on Texas geology. It not only is a center for geologists who make their living in the state looking for geologic resources—oil, gas, water, and minerals, but it sits astride a fundamental juncture in the state’s geology. The Balcones fault system, nearly 30 miles wide, slices northeast-southwest right through the city, separating the high. Upthrown Cretaceous limestone terrain of the Edwards Plateau and Hill Country north of town from the flat, lowland sandstone and mudstone terrain of the Coastal plain south of town.

The Balcones fault “popped up” about ten million years ago (Miocene) elevating the Edwards Plateau nearly 2,000 feet aboue sea level. The fault follows the edge of the old buried Ouchita Range, which means both the fault and the range lie along a deep-seated suture in the Earth’s crust. The fault is not a single break in the rocks around San Antonio, nor a single line of the geologic map but rather a zone of stair-stepping faults.

General Geology of Texas

Balcones Fault system near San Antonio

Balcones Fault system near San Antonio

Texas is approximately bisected by a series of faults that trend southwest to northeast across the state, from the area of Uvalde to Texarkana. South and east of these faults, the surface exposures consist mostly of Cenozoic sandstone and shale strata that grow progressively younger toward the coast, indicative of a regression that has continued from the late Mesozoic to the present. The coastal plain is underlaid by salt domes that are responsible for many of the oil traps in the region.

North and west of the faults are the Stockton, Edwards, and Comanche plateaux; these define a crustal block that was upthrown during the Neogene (29 Ma to present). This large region of central Texas, which extends from Brewster County east to Bexar, and northeast to the Red River features extensive Cretaceous shale and limestone outcrops. The limestone in particular is important, both economically for its use in cement manufacture and as a building material, as well as practically; a porous limestone formation in the Texas Hill Country is the reservoir of the Edwards Aquifer, a vital water source to millions.

Almost in the center of these Cretaceous rocks is the Llano Uplift, a geologic dome of Precambrian gneiss, schist, and granite, surrounded by Paleozoic sedimentary rocks. The granite here is quarried for construction, but it is perhaps best known to Texans through its manifestation as Enchanted Rock.

From San Saba north to Childress, and from Wichita Falls in the east to Big Spring in the west, the surface consists of late Paleozoic (Pennsylvanian—ca. 215 Ma) to early Mesozoic (Triassic—ca 228 Ma) marine sediments. These strata grow younger from east to west, until they are overlain unconformably[1] by terrigenous Ogallala sediments of Miocene and Pliocene age. These late Cenozoic deposits dominate the Texas Panhandle.

The geology of west Texas is arguably the state’s most complex, with a mix of exposed Cretaceous and Pennsylvanian strata, overlain by Quaternary conglomerates. A series of faults trend southeast to northwest across the region, from Big Bend to El Paso; there are also extensive volcanic deposits. The Marathon Mountains northeast of Big Bend National Park have long been of special interest to geologists; they are the folded and eroded remains of an ancient mountain range, created in the same orogeny that formed the Ouachita and Appalachian Mountains.

Location: 07-F6

[1] (Geological—of rock strata) consisting of a series of younger strata that do not succeed the underlying older rocks in age or in parallel position, as a result of a long period of erosion or nondeposition