Big Bend National Park

Sedimentary Processes

Sedimentary rock is rock that is composed of particles of preexisting rocks that have been transported, deposited, and lithified. There are two important processes that lead to the creation of these rocks. Erosion is the process by which rock is weathered and transported from its source. Deposition is the process by which eroded particles are dropped from water, wind, or solution.

Erosionary Features of Big Bend

Mass wasting is a process by which massive amounts of debris or earth slide or fall all at one time. The picture to the left depicts a rock slide that occurred in 1987 on the west side of the Sierra Del Carmen Mountains in the park. This slide can be found five or six miles south of the north entrance of the park. 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.

Desert Pavement covers the flat plains in Big Bend. When scouring winds blow through the park, the smaller particles of sand and dust are carried away, leaving more massive pebbles and rocks behind.

Santa Elena Canyon is a perfect example of river erosion. Millions of gallons of water flow through this canyon every year. Tons of sand, silt, and gravel are taken with that water and deposited in the Gulf of Mexico. As rivers cut through layer after layer of rock, it leaves a V-shaped canyon. Generally, with a straight section of river, water velocity is greater in the center of the river than the velocity at the river banks. Since the river carries more sediments in the areas of higher water velocity, the center of the river is abraided more. The scale and grandeur of this canyon cannot be shown by a photograph alone. The walls of the canyon are approximately 1500 ft high.

Differential Erosion -- In this picture, Leslie is standing under the Burro Mesa Pouroff. This area is where most if not all of the water drains from the top of Burro Mesa. The dark top layer of rock is a porphoritic trachyandesite and seems to be eroding more slowly than the layer of tuff below it. The term used to describe two beds of rock that erode at different rates is differential erosion.

Unconformity -- The picture to the left depicts erosion that happened at least 30 million years ago. The top layer of dark rock is an Eocene aged lava flow, and the lower layer of rock is a Cretaceous rock formation known as the Javelina formation. What happened to the rock layers that were deposited in the Paleocene? At some point before the lava flow occurred those rocks were washed away by some form of erosion. A gap like this in the rock record is known as an unconformity.

Depositional Features of Big Bend

Marine Deposition -- In a marine environment, different types of sediment are associated with varying depths of ocean water. Close to the beach, sand and course sediments are deposited. Therefore, when these sediments are buried and lithified, a sandstone (A) results. In intermediate depths, finer grained particles settle out and result in the formation of shale (B). In deeper oceanic environments, organic material from shells and inorganic calcite combine to form a finely grained rock known as limestone (C).

Transgresive-Regressive Cycles -- Given the above information, a reasonable assessment of ocean depth over time can be made (provided that no major unconformities are present). For example, given these layers of rock from bottom to top: limestone, shale, sandy shale, and limestone. We can conclude that the earliest layer of rock was deposited in a relatively deep marine environment, then the ocean receded during the time of the next layer. The next layer above the shale indicates that the rock was deposited in a very shallow marine environment. Then the ocean began to rise once again, depositing the sandy shale. Finally, the ocean waters grew deep enough again for limestone to form. Very often, layers of limestone interbedded with shale can be found. Often in a formation, hundreds of these layers can be identified. These indicate cycles of the ocean waters rising (transgressing) and receding (regressing) or commonly known in the geology world as transgressive-regressive cycles

Conglomerate is a type of rock that forms as the result of pebbles or gravel being cemented together. Conglomerate forms in many different environments including river beds, eroding coastlines, and in lake bottoms next to rapidly moving rivers. This conglomerate can be found near the Burro Mesa Pouroff and is formed from massive pebbles composed mainly of rhyolite and quartz. It is cemented with tuffaceous sandstone.

Santa Elena Limestone is the rock that the Rio Grande has cut through to form Santa Elena Canyon and Boquillas Canyon. This limestone was deposited in the early Cretaceous period and contains many small marine fossils. The beds range from 750 to 850 feet thick and stretch for more than a hundred miles. This gives a good indication that the park was submerged in deep ocean water during the early Cretaceous.

Below is a table that gives information about the sedimentary rocks that can be found in Big Bend National Park. From these rocks, see if you can draw any conclusions about the history of the park.

Rock FormationRock Type Rock AgeEnvironment
Glenrose Formation Limestone/Shale Early Cretaceous Deep Ocean
Maxon Formation Sandstone Early Cretaceous Shallow Ocean
Telephone Canyon Formation Marly Limestone Early Cretaceous Deep Ocean
Del Carmen Limestone Cherty Limestone Early Cretaceous Deep Ocean
Sue Peaks Formation Shale Early Cretaceous Intermediate Ocean
Santa Elena Formation Limestone Mid Cretaceous Deep Ocean
Del Rio Clay Shale/Clay Mid Cretaceous Shallow Ocean
Buda Limestone Marly Limestone Mid Cretaceous Deep Ocean
Boquillas Formation Chalk/Limestone Mid Cretaceous Deep Ocean
Pen Formation Marl/Clay Upper Cretaceous Shallow Ocean
Aguja Formation Clay/Coal Upper Cretaceous Swampy
Javelina Formation Clay/Sandstone Late Cretaceous Terrestrial
Black Peaks Formation Sandstone/Conglomerate Paleocene Arid
Hannold Hill Formation Clay Early Eocene Arid
Canoe Formation Lava Flow Early Eocene Arid/Volcanic
Chisos Formation Lava Flow Mid/Late Eocene Arid/Volcanic
South Rim Lava Flow Oligocene Arid/Volcanic
Alluvial Deposits Clays/Conglomerate Pleistocene-Present Arid