Physical Geology 2002

 

 

 

 

Related Links

Big Bend National Park (NPS Site, this will have the most complete and up-to-date information)

Tarbuck & Lutgens (for basic information on Physical Geology).

Visit Big Bend (travel and adventure information).

http://www.visitbigbend.com/19bbgeo.html

USGS (United States Geological Survey)

Daily Uptadtes on Big Bend (Weather, Rainfall, River Levels and Campground Status).

-See the Sierra del Carmen...updated every 15 minutes.

-infared dopler satelite images...updated every 5 minutes.

Photo Gallery (gorgeous scenes of Big Bend)

mass wasting

mass wasting on the west side of the Sierra Del Carmen Mountains .http://geoweb.tamu.edu/faculty/herbert/bigbend/sed/index.html

Most of the mass wasting that occurs in Big Bend is a result of the rocks expanding in the heat of the day and contracting in the cool of the night. Since the climate of Big Bend is desert, the differential temperatures between day and night can be quite significant.

dike

On the road that leads west from Panther Junction http://geoweb.tamu.edu/faculty/herbert/bigbend/ig/int.html

Dikes are vertical wall-like structures that form as a result of magma being injected into the fractures of rocks. Dikes are discordant features. This means that they cut through existing layers of rocks. When the surrounding rock is eroded, dikes are exposed and often appear as dark walls of rock.

usgs survey map

http://www.maroon.com/bigbend/met/index.html

The pink portion of the map shows an igneous intrusion that happened during the mountain building episodes in the park. The stippled grey portion of the map shows the country rock which was metamorphosed (Big Bend: Lost Mine Trail. Date Unknown).

Santa Elena Canyon

http://www.visitbigbend.com/35aphoto.html

Big Bend National Park

Rio Grande, Big Bend National Park

http://www.bigbendrivertours.com/big_bend_info/bbnpgeology.htm

Introduction

  • According to Native American legend, when the Great Creator completed his great task of creation he was left with a sizable pile of left-overs. Having finally placed the last star in the sky, tree on the earth, and fish in the sea, he threw all the superfluous material into a heap creating what we know as the Big Bend (Maxwell, 1968).
  • In observing the unique and varied geology of Big Bend, the origins of such a myth can be easily understood. The area has a definite sense of randomness to it. Much of the strata is lopsided, or juts awkwardly out among mixed up rocks piles and mountains that seem as if they were collected by some surrealist collage artist and pasted to the scene. It is a rich and complex geomorphologic history splayed out and preserved- an ancient story that can be read by any educated visitor.
  • The name "Big Bend" refers somewhat loosely to the area of Southwestern Texas surrounded on three sides by the Rio Grande. The southernmost portion of this region is designated as Big Bend National Park (Roberts, 1996). Most of Big Bend's geological magnificence is the preserved hydrological carvings of ancient and wetter times. The park is now a dry and fairly harsh desert environment- a place where plants grow thorns or die and all creatures follow the vital dips and curves of the Rio Grande

Geologic Processes

  • Big Bend has gone through many changes over the past 300-500 million years. Some geologists have described the area as a paradise while others often refer to it as a nightmare. The remnants of long ago are exposed due to erosion and lack of vegetation in the area (Visit Big Bend, 200).

Structural Features
Three common structural features are faulting, folding and jointing. All three of these features can be seen in Big Bend National Park. Boquillas Canyon is an example of a normal fault. The Glen Rose, Maravillas, and Tesnus formations are examples of thrust faults. Juniper Canyon is an example of block faulting. The walls of the canyon are called horsts while the floor of the canyon is called a graben (Big Bend: Lost Mine Trail. Date Unknown).

Recycling Rocks (the rock cycle)
Big Bend National Park displays classic and unique examples of sedimentary, igneous, and metamorphic processes.

Sedimentary- There are two important processes that lead to the creation of the dramatic sedimentary cliffs and mesas of Big Bend: Erosion (the process by which rock is weathered and transported from its source) and Deposition (the process by which eroded particles are dropped from water, wind, or solution).
Some of the erosion features of Big Bend include mass wasting, desert pavement, differential erosion and unconformity (Tarbuck, Edward J. and Lutgens, Frederick K, 2002).

Mass wasting is Big Bend's most prominent erosional tendency.

Depositional features can also be found at Big Bend. Marine deposits show that Big Bend has been covered by ocean at different times and various depths. When the area was a beach, deposits of sandstone were left behind. At intermediate depths, shale was deposited. When the area was covered by a deep ocean, limestone was deposited. The Santa Elena limestone can be found at Boquillas Canyon and Santa Elena Canyon. The cycle of the ocean water rising and then receding is called transgressive-regressive cycles. Conglomerate, composed mostly of rhyolite and quartz, has been found near Burro Mesa Pouroff. (Big Bend National Park Geology, 2002).

Picture of conglomerate
http://geoweb.tamu.edu/faculty/herbert/bigbend/sed/index.html

Igneous- Big Bend's geology displys intrusive, extrusive and pyroclastic igneous rocks. Some of the most prevalant igneous features in the park are intrusive. Many times between 70 and 20 million years ago, the Big Bend region experienced great intrusions of magma. Many of the resulting intrusive features can be seen today as a result of erosional processes that have carried and continue to carry away the overlying sedimentary rocks. Laccoliths, dikes, sills, and volcanic necks are many of the intrusive features that can be seen throughout the park.

Pictured to the left is a rhyolite dike that cuts through a bed of shale.

Metamorphic- Big Bend's active igneous history of intrusions and volcanic activity was the source of limited metamorphism, both regional and contact. Today, however, there is only one place in the park which displays exposed metamorphic rock, this is Dominguez Mountain. Most of the other areas where metamorphosis had occurred have long since eroded and have been washed away. (Spearing, 1991, Tarbuck, Edward J. and Lutgens, Frederick K., 2002).

 

  • a portion of the US Geological Survey map of the Big Bend area.

Fossil Remains
The Fossils found in Big Bend tell stories that, although often overwhelming in their variety, are anything but boring. Some of the fossils that have been found in the area include pterosaur, duckbill dinosaurs, ichthyosaur, allosaurs, brontosaurus, oysters, snails, ammonites, giant clams, sea turtles, sharks, and marine lizard. In 1907, J. A. Udden found three different species of dinosaurs, a turtle, and a crocodile. In 1936, Ross Maxwell found the largest amount of fossil remains in the park. In 1941, bones of a duckbill hadrosaur and horned ceratopsian dinosaurs were discovered. Today paleontologists continue to study Big Bend's fossils in an attempt to solve the mysteries of the region's past as well as its present (Maxwell, 1968, Gorp, 1998).

Impacts

"How well the insides of these canyon walls have known each others' shadows" -excerpt from my journal March 22, 2002 at Santa Elena Canyon
  • I returned last night from a week of white water canoeing on the Rio Grande through Big Bend National Park. Spending my days and nights in that desert, I began to feel myself no longer as an objective visitor, but as an animal- just another creature living in the dry light of the sun and the moon as they rose and sank above between the mesas.
  • I felt witness to time itself- to life carving, like the river, always deeper, within the form of materials structured by the past holding it along its coarse. These structures layed out perfectly in sedimentary layers- brandishing the record of metamorphic scars unashamed, proof of persistence- of life sprung up atop igneous intrusions and what must have been massive pyroclastic death (the evidence sewed so neatly in the seams).
  • These stone stories were everywhere- the same on both sides of the river- and perhaps this is what struck me most: That Texas and Mexico were indeed the same land and our hand drawn lines so arbitrary, so shallow. I was, for the faced with the evidence of humanity's microscopic dent in time and filled not with fear but with awe and with the joy of witnessing even a small part of something so vast and eternal.

Literature Cited

Maxwell, Ross A. The Big Bend of the Rio Grande. Austin: The University of Texas at Austin, 1968.

Roberts, David C. A Field Guide to Geology Eastern North America. Boston: Houghton Mifflin Company, 1996.

Spearing, Darwin Roadside Geology of Texas. Missoula: Mountain Press Publishing Company, 1991.

Tarbuck, Edward J. and Lutgens, Frederick K. Earth, an Introduction to Physical Geology, 7th ed. Upper
Saddle River, New Jersey: Prentice Hall Inc., 2002.

Big Bend: Lost Mine Trail. Date Unknown. Big Bend National Park pyroclastic igneous processes. http://geoweb.tamu.edu/faculty/herbert/bigbend/ig/int.html (2002 March 25).

Big Bend National Park Geology. 2002 January 14. The geology of Big Bend National Park. http://www.bigbendrivertours.com/big_bend_info/bbnpgeology.htm (2002 March 25).

Geology of Big Bend National Park. 2002 October 4. Geology fieldnotes Big Bend National Park Texas. http://www.aqd.nps.gov/grd/parks/bibe/(2002 March 25).

GORP. 1998 August 21. Big Bend National Park geology. http://wysiwyg://www.gorp.com/gorp/resource/us_national_park/tx/geo_bigb.htm(2002 March 25).

UTD Science Education. 2000 September 3. What do fossils tell us about the history of Big Bend National Park? 2000. http://www.utdallas.edu/dept/sci_ed/torch/bb00_geo_bd.htm (2002 March 25).

Visit Big Bend. 200 November 11. Big Bend area geological history. http://www.visitbigbend.com/19bbgeo.html (2002 March 25).

Author: Leah Green

[Links to all class members sites will be inserted here.]

Earlham · Geosciences Department · Geociences 211: Physical Geology

This website is part of a Geology 211 class project on Processes in Physical Geology.

Revised 26 March 2002. Send corrections or comments to Leah