Deinosuchus quarry in Big Bend

Cretaceous Era Strata in Big Bend National Park

by Wes Fleming

      Big Bend National Park in southwest Texas contains an array of impressive geological formations. These formations show that there has been a large variation in environmental conditions over time. In the early Cretaceous rock of Big Bend, we see thick, dense beds of limestone, interspersed with thin, nodular beds of limestone. This tapers into mid Cretaceous calcareous sandstone layers, chalks, and clays interspersed with cherty limestone beds, then leads to late Cretaceous thick layers of carbonaceous continental clays.

      Through this progression of rock strata, we can infer that this region experienced a general recession of oceanic environment during the Cretaceous era. We also see a series of transgression cycles as the ocean repeatedly receded and moved back in. This oceanic recedance was a result of the interior orogeny experienced in western North America during this time, associated with the Sevier and Laramide orogenesis.

      Following this great change in the environment was a dramatic change in the fauna populating this region. Fossils of ammonites, bivalves and corals have been found in the lower Cretaceous limestone of Big Bend. This indicates that a tropical, fairly shallow-water environment was present before the ocean receded. Most interesting to me, however, are the abundant beds of dinosaur fossils to be found in the upper Cretaceous layers deposited after recedance of the ocean.

Strong banding of limestone with
red sandstone interbeds at Ernst Tinaja

      Beginning in the upper Cretaceous, among fish and shellfish fossils, mosasaur bones have been discovered. These specimens are found in the lowlands of the central part of the park. Later in the Cretaceous, as the sea recessed out of this area, thick beds of carbonaceous clay were deposited. The carbonaceous clay indicates that the area must have been a swampy, bayou-type environment with dense vegetation, similar to that existing to the east of Big Bend today, in Louisiana and east Texas. By the fossil-rich record of Big Bend, this clearly provided a perfect environment for dinosaurs. This is evident in the abundance of skeletal remains found in the park, which include duck-billed dinosaurs, ceratops, flying dinosaurs and many large carnivorous dinosaurs.

      The reason for my particular interest in this area and time stems from an experience which I had over winter term of this year. I went to Big Bend for about a week to do some camping and hiking. I set up camp near the base of a canyon, WAY off of the beaten track, a few miles back the rockiest road you could imagine. The camp site was in a basin inhabited by hills of outwash alluvium, indicating that the canyon was a fairly large one.

Cherty limestone in Ernst Canyon

      The canyon consisted of strongly layered limestones and sandstones. There were some beautiful formations and judging by the tortuous bending of the color bands, it was evident that there had been some strong tectonic activity at work here. Upon comparison of my memories and a geologic map of this region, it is clear that this rock was mid to upper middle Cretaceous limestone and was interbedded with clay, chalk and sandstone. These interbeds are obviously the result of the transgression cycles experienced in this region as sea level receded. Unfortunately, I was almost out of film and took very few photographs of this canyon.

      This is especially unfortunate, as I discovered a HUGE bed of fossils, extending over an area of at least a hundred yards. This bed lay in a thin layer intersecting the canyon. The tilt of this bed was at about fifteen degrees west (getting lower as it progressed westward), indicating the general inclination of the tectonic upthrust which formed the hills housing the canyon. Although I could determine nothing of the nature of the animals, other than their fairly large size (the fossils ranged in size up to about three feet long, averaging a length of about one foot), what I have learned through my research for this project has been very helpful in suggesting ideas for their origin. In accordance with the geologic map, these fossils are most likely to be of the Boquillas or Pen Formations. These are (respectively) limestone interbedded with shales; and clay with some thin sandstone interbeds, both of which are of the lower late Cretaceous.

Geologic Map of Big Bend National Park, Brewster County, Texas

      At first, my research made me think that the fossil bed was a storm-collection of swimming reptile carcasses, such as Ichthyosaur, Mosasaur or Plesiosaur, based on the marine nature and time period of the environment in which the rock formations of the canyon were created. The presence of the shales and sandstones in combination with the limestone indicates to me the possibility that this was more of a near-shore environment. While this supports my first idea, it also leaves room for the possibility that this may have been a shallow, offshore environment at the mouth of a large stream or river. It seems possible that the carcasses of many animals were picked up in a wash created by a large storm and carried downstream and out to sea, into a small basin. If it was possible, a reef growing a short distance out from the stream mouth would easily catch such debris.

      Although these conjectures offer a possible explanation for the locality of large fossils, in all of my research no fossils but small, sea-dwelling creatures have been found in these layers. This anomaly between what I have seen and what the research shows continues to be a great confusion to me.

      All in all, my research indicates that during the Cretaceous, the environment of Big Bend was vastly different than it is today. The area was in fact teeming with life, ranging from ammonites, snails, corals and large marine reptiles of the tropical Mesozoic seaway to great flying reptiles, carnivorous and herbivorous dinosaurs inhabiting the densely vegetated swamps of the late Cretaceous. This abundant wealth of creatures, together with very preservative depositional environments lead to a rich biostratigraphical record of the area.

Geology of Big Bend National Park, Brewster County, Texas, p.6

Related Links


Adkins, W.S. (1928). Handbook of Texas Cretaceous Fossils:
       Univ. Texas, Bur. Econ. Geology: Bulletin No. 2838.

Maxwell, Ross A.; Lonsdale, John T. (1966). Geologic Map of Big Bend National Park, Brewster County, Texas:
       Univ. Texas, Bur. Econ. Geology.

Maxwell, Ross A.; Lonsdale, John T.; Hazzard, Roy T.; Wilson, John A. (1967). Geology of Big Bend National Park, Brewster County, Texas:
       Univ. Texas, Bur. Econ. Geology: Bulletin No. 6711.

Maxwell, Ross A. (1968). The Big Bend of the Rio Grande:
       Univ. Texas, Bur. Econ. Geology: Guidebook 7.

Perkins, Bob F. (1960). Biostratagraphic Studies in the Comanche (Cretaceous) Series of Northern Mexico and Texas.
       Geographical Society of America, New York.

Reynolds, Mitchell W.; Dolly, Edward D. (1983). Mezozoic Paleogeography of the West-Central United States: Rocky Mountain Paleogeography Symposium No. 2.
       Knudsen Printing Company, Denver.

Seeley, H.G. (1901). Dragons of the Air: An Account of Extinct Flying Reptiles.
       D. Appleton & Co., New York.

Udden, J.A.; Baker, C.L.; Böse, Emil (1916). Review of the Geology of Texas:
       Univ. Texas, Bur. Econ. Geology: Bulletin No. 44.

NOTE: The two pictures of Ernst Canyon contained herein are the copyright of Wes Fleming.