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
- The Ouachita-Marathon mountain-building episode, when the South
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
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
collided. This mountain-building event produced large
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
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.
comments and criticisms are welcome - even solicitated
I hope you enjoy the show.
BIG BEND NATIONAL PARK AND SURROUNDING AREA
Virtual Field Trips