Region 5: The Basin and Range

The portion of the Basin and Range region in far western Texas includes most of “Big Bend Country,” which is a name frequently used for westernmost Texas. The oldest rocks here date to the Permian, when three “fingers” of a shallow sea created tropical marine basins in western Texas and New Mexico. A fossilized reef formed on the Permian shoreline of the Delaware Basin, built largely by algae, sponges, brachiopods, and bryozoans (Figure 3.63). Like modern reefs, this reef complex was built from the shells of organisms, most of which were made of calcium carbonate (CaCO₃), the same composition as limestone. This structure also provided a haven for a great variety of other animals: corals, crinoids, cephalopods, fusilinids (unusually large forams; Figure 3.64), and gastropods thrived there.

Figure 3.63: Permian reefs of Texas. A) Map showing the paleogeographic relationship of the reefs to the topographic lows and highs of the shallow sea that covered the area at the time. The reef occupied the rim of the Delaware Basin. B) Delicate fossil brachiopods and bryozoans, preserved as silica, from the Glass Mountains of Texas.

Figure 3.64. One-celled fusulinid foraminifera from the Permian. A) A cluster of the shells, the size and shape of large rice grains. B) Photograph of a cross-section through a single fusulinid, as seen through a microscope.

After the reef was buried by sediment, much of its calcium carbonate was replaced by other minerals dissolved in groundwater. The most important of these minerals was silica (SiO₂), which has the same composition as glass. The resulting “glass” fossils can be freed from their surrounding limestone by treating the rock with acid, which dissolves the rock, leaving the often-delicate fossils behind (see Figure 3.63B).

Following the Permian, there is a roughly 150-million-year gap in Big Bend Country’s fossil record. The next oldest fossils in the region are from the late Cretaceous and record a very different scene. Around 100 million years ago, the area was covered by the Western Interior Seaway. Fossilized mollusks, corals, fish, shark teeth, and giant marine reptiles, including mosasaurs, are found in the oldest layers. The somewhat younger Aguja Formation represents a swampy coal forest and contains a diversity of fossil plants, ammonites, turtles, 60 species of dinosaurs, and Deinosuchus—a giant crocodilian up to 10.6 meters (35 feet) long (Figure 3.65). The overlying Javelina Formation is thought to record the boundary between the end of the Cretaceous and the Paleogene. Like the Aguja, this formation preserves the remains of a coastal “coal forest,” and it preserves a variety of famous reptiles including Tyrannosaurus rex, Alamosaurus (perhaps the largest dinosaur known from North America; Figure 3.66), and the giant pterosaur Quetzalcoatlus (Figure 3.67).

Some layers of early Paleogene terrestrial sediments (e.g., the Tornillo Formation) found in Big Bend Country preserve a snapshot of life on land around between 65 and 50 million years ago, including fish, reptiles, and mammals (Figure 3.68).