The Trans-Pecos region is located in the western part of the state of Texas and its environment and topography differ from the other parts of the state (Figure 1). The Trans-Pecos is scattered with mountain chains unlike the piney woods in east Texas, hill country in central Texas, and plains in northern Texas. It is defined with its eastern boarder of the Pecos River, the Rio Grande as the southern and western border, and the thirty-second parallel to the north which serves as the border between Texas and New Mexico. The goal of this website is to serve as an overview of the tectonic events that have shaped the Trans-Pecos. Each event discussed is a brief snapshot of a much more complex event and warrants further investigation of interested parties. A map of common place names (Figure 2) is included for geographic reference.
Figure 1. Map of Trans-Pecos from Texas Parks and Wildlife
Figure 2. Names of Places in the Trans-Pecos from Sims, San Antonio Audubon Society
Although the remnants of the Marathon-Ouachita orogeny (Figure 3) have been eroded and buried beneath sediments in the Trans-Pecos region, it had an impact on the southern Trans-Pecos landscape during the late Paleozoic. This orogeny was the result of the Laurentia plate subducting beneath the Gondwanan continental-margin arc. The impact of this episode created a 3000-km-long belt from Mississippi to Sonora, passing through the southern part of the Trans-Pecos.
Figure 3. Marathon-Ouachita Orogeny from Encyclopaedia Britannica 2007
Jurassic and Cretaceous rifting affected the western edge of the Trans-Pecos forming the Chihuahua trough. The Chihuahua trough was created by the divergence of the North American and South American continents. This can further be defined by the counterclockwise rotation of the North American plate. The eastern edge of the trough runs along the Rio Grande from El Paso to Big Bend National Park. This area in the Trans-Pecos experienced little subsidence compared to west of the Rio Grande. The rifting allowed the trough to connect with seawater depositing marine sediments. This coincided with the beginning of the formation of the Western Interior Cretaceous Seaway that deposited many of the marine sedimentary rocks exposed in the Trans-Pecos.
The Laramide orogeny affected the western part of the North American continent reaching from Canada in the north to Mexico in the south. The Cretaceous to Paleogene folding and thrust faulting was similar to the folding and thrusting of the Marathon-Ouachita orogeny. The tectonic episode is the result of the subduction of the Farallon plate beneath the North American plate 90-50 Ma ago (Figure 4). The Laramide folding in the Trans-Pecos occurred in two distinct belts. The western belt extended from El Paso through Presidio, reaching Sierra Rica south of Big Bend National Park. It predominantly contains high ridges of folded Cretaceous strata. The eastern belt runs along the Sierra del Carmen in Big Bend National Park extending into Mexico and has not been studied in as much detail as the western belt. Between these two belts formed the Terlingua Uplift and the Fresno-Terlingua Monocline that has helped shape the region.
Figure 4. Formation of Laramide Orogeny from USGS
Mountain building continued after the Laramide orogeny from 47-18 Ma. The Trans-Pecos volcanic field occured during this time (Figure 5). The initial volcanic activity was the result of the continued subduction of the Farallon plate beneath the North American plate. This created a volcanic arc ranging from southern Mexico to Canada. This first pulse of volcanic activity occurred 47 Ma ago. A period of little volcanic activity occurred as the plate movement ceased. After the subduction a second pulse of volcanic activity started during a period of extension and rifting known as the Rio Grande Rift. This rifting allowed magma chambers to rise in the thinning plate. A series of Paleogene volcanic eruptions took place in the Trans-Pecos from El Paso to Big Bend National Park. The Chisos, Davis, and Chinati mountain ranges are the result of this volcanism and are eroded remnants of the volcanoes. This second pulse created laccoliths, dikes, and sills that intruded Cretaceous limestone (Figure 6). The resistant igneous bodies remain as the sedimentary rocks eroded. Further erosion from streams and rivers have transected some of these laccoliths exposing cross-sectional views of the magma chambers. One of many laccolith exposed is the Wax Factory Laccolith in Big Bend Ranch State Park (Figure 7).
Figure 5. Volcanic Activity in the Trans-Pecos from ReasearchGate
Figure 6. Formation of Volcanic Features from USGS
Figure 7. Wax Factory Laccolith Picture by Kristopher B Farmer
The Trans-Pecos has undergone a tremendous amount of tectonic activity on a geologic scale of time. Mountain chains have been built, then eroded only to be replaced with new mountain chains. Numerous volcanoes have erupted over the landscape and tectonic stress has uplifted and subsided the land. Many geologist find themselves drawn to the Trans-Pecos due to the geologically diverse landscape and lack of vegetation. The exposure of so many rock formations can be a paradise to many and a nightmare to some. Even the seasoned field geologist must stop and fully take in all of the surroundings to understand all of the factors at play. Our understanding of this area is fluid with new interpretations and explanations ever present.
Encyclopaedia Britannica, Permian Basin, Accessed online Encyclopaedia Britannica April 2016
Henry, C.D. 1998. Geology of Big Bend Ranch State Park. Bureau of Economic Geology, The University of Texas at Austin. Guidebook 27. 72 p.
Maxwell, R.A. 1968. The Big Bend of the Rio Grande: A Guide to the Rocks, Geologic History, and Settlers of the Area of the Big Bend National Park. Bureau of Economic Geology, The University of Texas at Austin. Guidebook 7. 138 p.
ReasearchGate, Petrogenesis and Tectonic Setting of the Peralkaline Pine Canyon Caldera, Trans-Pecos Texas, USA. Accessed online ResearchGate April 2016
Sims,R., The Trans-Pecos. San Antonio Audubon Society, Accessed online San Antonio Audubon Society April 2016
Texas Parks and Wildlife. Texas Ecoregions, Accessed online TPWD April 2016
USGS, Geologic History of Lake Mead National Recreation Area, Accessed online USGS April 2016
USGS, Geologic Provinces of the United States, Accessed online USGS April 2016
Yates, R.G. and Thompson, G.A. 1959. Geology and Quicksilver Deposits of the Terlingua District Texas,U.S. Department of the Interior, Geological Survey Professional Paper 312. 114p.
Web Page Created by Kristopher B. Farmer April 16, 2016