Big Bend Virtual Field Trip -Table of Contents

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Introduction

This is another one of my ongoing projects. 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. Hopefully, we will be able continue gathering more material in the years ahead, allowing me to update and expand this virtual field trip.

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 volcanic 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, as well as in southwestern Arkansas and southeastern Oklahoma. 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 around 150 million years ago in which limestones were deposited. The limestones 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 and deepened during this time. The lime deposition gave way about 80 million years ago to clays, silts, and sands. The environment was gradually transitioning from marine to terrestrial.

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. Mesa de Anguila, Mariscal Mountain, and the Sierra del Carmen all owe their existence to this orogeny. The region including the park continued to rise 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, including the Chisos Mountains, the Rosillos Mountains, the Grapevine Hills, Burro Mesa, and so on. The volcanism is thought to be due to the shallow subduction of the Farallon Plate beneath western North America. Meanwhile, due to the continued elevation of the region, terrestrial sediments, such as alluvial fan deposits, were laid down.

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 may be 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 is probably related to the continued sinking of the subducted Farallon plate, possibly in conjunction with the continued grinding of the Pacific plate against the North American plate to the northwest. The Basin and Range Province structures are roughly parallel to that direction. Crustal extension has led to numerous high angle faults in the park, including the ones that border, on the east and west, the crustal block that contains the Chisos Mountains 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 uplift and/or mountain-building activity, Big Bend will one day be a flat plain, or even a shallow sea, once again.