Big Bend Virtual Field Trip -Table of Contents

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Introduction to Virtual Geological Field Trips

This is another one of my ongoing projects. It began as a resource for my geology students at Texarkana College. I took them on geology field trips into the Ouachita Mountains area of Arkansas, which was not too far, and there was a lot of geology to see there. However, that area has plenty of vegetation to hide the geology. Places that we went to - road cuts, spillways, hillsides, etc. - became more and more overgrown as the years went by. Hence, I began using pictures from my Big Bend trips in the classroom and eventually putting them online with explanations so that students could look at them at their leisure.

West Texas, and especially the Big Bend area, which includes Big Bend National Park, has long been of great interest to me. Unfortunately, it is a long way from the places I've lived over the past several decades, and I haven't been able to get out there nearly as often as I would like. The current material is from trips my brother, Randy, and I have been taking since May, 2006. We have been driving and hiking through Big Bend, examining the geology on the way. We both take pictures with our digital cameras, so quite a few of the photographs shown here are his, although not always credited. Now that I'm retired and generally useless otherwise, I've continued and hope to continue my trips to Big Bend and expand this work.

The geology of West Texas is fascinating - even unique. And, because of the dryness of the Chihuahua desert, which is nevertheless subject to infrequent episodes of intense erosion, the geology is exposed and readily visible. Erosion is the real star of the show in Big Bend National Park and its environs. Millions of years of weathering and differential removal of the rocks emplaced by igneous and sedimentary processes, has resulted in the creation of the area's incredible topographic features and the revelation of its unique geological history. This virtual field trip is a sort of slide show with notes. I have made considerable effort to be as accurate and up-to-date as possible.

The geologic history of Big Bend can be roughly divided into five episodes.

The Ouachita-Marathon mountain-building episode, when the South American tectonic plate collided with the North American plate. This occurred (roughly speaking) about 300 million years ago. Mountains were raised as the collision pushed sediment on the order of a hundred miles up onto the North American plate. The remnants of this episode can be found in the Marathon uplift and the Solitario in Big Bend Ranch State Park. The "roots" of the old mountains can be found there, as well as in southwestern Arkansas and southeastern Oklahoma – rocks deposited in the early to middle Paleozoic seas. The effects of this mountain building can also be seen in the geologic structures of the Persimmon Gap area in the northern part of the park.

Rifting as South America and North America separated. The mountains were worn flat and Big Bend became a shallow sea in which limestones and other marine sediments were deposited. All this occurred between about 200 million and 85 million years ago. These rocks can be seen in the mountains on the east and north side of the park, Mariscal Mountain, Mesa de Anguila, and even in parts of "The Basin". The Gulf of Mexico formed, deepened, and widened during this time due to seafloor spreading.

The Laramide orogeny, which affected much of western North America between 70 and 50 million years ago as the North American and Farallon plates collided. This mountain-building event produced large folds and faults in Big Bend. Both can be seen on Persimmon Peak at the north entrance to the park. The folds found in Mesa de Anguila, Mariscal Mountain, and the Sierra del Carmen all owe their existence to this orogeny. The marine sedimentation transitioned into continental sedimentation as the region including the park rose in altitude.

Between 46 and 28 million years ago, periods of volcanic activity affected the park and its environs. Igneous rocks due to this activity are ubiquitous in the central and western parts of the park, in the Chisos Mountains, the Rosillos Mountains, the Grapevine Hills, Burro Mesa, the McKinney Hills, among other areas. The volcanism is thought to be due to the shallow subduction of the Farallon Plate beneath western North America.

The park continues to be affected by the crustal extension of the Basin and Range province activity, which began about 25 million years ago (and which is decreasing as little activity has been noted over the past two million years). The cause of this crustal extension is not entirely clear, but it may have to do with the action of the Pacific tectonic plate as it moves northwest and grinds against the North American plate. The Basin and Range Province is elongated roughly in that direction, and the extension is also roughly parallel to that direction. Crustal extension has led to numerous high angle faults in the park, including the ones that bound the crustal block containing the Chisos Mountains on the east and west and the one that resulted in the spectacular cliffs of the Sierra del Carmen in Mexico.

Finally, erosional forces remain at work to continually alter the landscape. The Rio Grande came into being during the last 2 million years, as it established its present course and ate down into the Cretaceous limestone, forming the Santa Elena, Mariscal, Boquillias, and other canyons. Throughout the park, as less resistant rock is worn down, the topographical relief between it and more resistant rock grows. However, even rock highly resistant to erosion is eventually worn down. Without further mountain-building activity, Big Bend will one day be a flat plain once again.