Big Bend Sentinel GeoNotes 

The Trans-Pecos Province of Texas 

The Texas Master Naturalist text (edited by Haggerty and Meuth, 2015, TAMU Press) divides  Texas (267,000 square miles) into 10 natural or ecological regions. These include Blackland  Prairies, Coastal Sand Plains, Edwards Plateau (extends across part of the Trans-Pecos), Gulf  Coast Prairies and Marshes, High Plains, Llano Uplift, Oak Woods and Prairies, Piney Woods of Northeast Texas, Rolling Plains, South Texas Brush Country and the Trans-Pecos (page 130 of text). All of these provinces are controlled by their geology, soils, precipitation and shifting plant communities. Climate change affects all of these provinces and their boundaries which shift with rainfall through time. The Trans-Pecos province (38,000 square miles) is everything west of the Pecos River, a south-southeast trending river course and canyon system, plus an area southeast of the southeast corner of New Mexico, including parts of Andrews, Loving, Winkler, Ector, Ward, Crane, Upton, Crockett and Val Verde counties. 

The Trans-Pecos is by far the most geologically diverse surface area of Texas. It lies within the Cordillera which trends all the way from Alaska to Patagonia. A number of igneous,  depositional and structural settings have contributed to the rock assemblages at the crustal  surface within the Trans-Pecos province. A few of the most important and widespread  geologic areas within the Trans-Pecos are discussed. 

The oldest outcrops are around El Paso and Van Horn, PreCambrian (1.3-1.1 billion years  before the present). Talc has been mined from the metamorphic mountains west of Van Horn for decades. 

South and east of Marathon, and in the Solitario Uplift, outcrops of the Appalachian Mountains  are seen. These Ouachita facies are Cambrian (500 million years) through late Pennsylvanian  (310 million years) in age. They have been structurally deformed by plate tectonics, and form  magnificent thrust folds and faults on many of their outcrops. The Ouachita trend is mostly  buried in the subsurface, and trends from northern Chihuahua to Marathon to Sanderson to the Ouachita Mountains in southeast Oklahoma, then across Arkansas, central Mississippi and Alabama, and connects with the Appalachian trend amongst the gold mines of northwest Georgia. 

World famous Permian-aged rocks (299-252 million years ago) crop out in the Sierra Diablo, Guadalupe, Delaware, Apache and Glass Mountains. Guadalupe Mountains National Park, partly a gift of oilman Wallace Pratt, showcases a portion of the Capitan Reef sequence surrounding the Delaware Basin. 

The southeastern part of the Trans-Pecos province lies within the Cretaceous-aged mesas of the Edwards Plateau. This area is sometimes referred to as the Stockton Plateau west of the Pecos River. The relatively flat-lying limestones, marls and shales are fossil rich and were mostly deposited within and near the Cretaceous Interior Seaway. These age rocks and topography are also present in areas of Big Bend National Park. 

The igneous rocks of the Trans-Pecos, including the Bofecillos, Chinati, Chisos and Davis  mountains, are 77-17 million years before present (late Cretaceous to Tertiary) with most of the volcanic activity from 38-32 million years ago. They represent lava flows, ash-rich explosive eruptions and numerous intrusives like Paisano Peak, Mitre Peak and Elephant Mountain. 

The Trans-Pecos province is currently under crustal extension and is within the Basin and  Range structural province of North America. Erosion is the predominant earth process.  Earthquakes are frequent although usually small. Texas’ largest recorded earthquake during  European times was near Valentine, Texas. It occurred on Aug 16, 1931 (90 years ago) and was estimated at magnitude 6.5 on the Richter scale at a depth of 6 miles below surface.

Sources: Texas Through Time: Lone Star Geology, Landscapes, and Resources (2016): Ewing, T.E. and Christensen, H.: The University of Texas at Austin, Texas Bureau of Economic Geology Udden Series #6, 431p.