Region 3: The Coastal Plain

The sediments that accumulated to form the Coastal Plain of Texas and Louisiana were deposited by numerous rivers that drained the land to the north and northwest. This process began in the late Cretaceous, and the Mississippi River continues to drain the central US today, contributing to the coastline of the Gulf of Mexico. The oldest sediments on the Coastal Plain (from the late Cretaceous) appear in an irregular northeast-southwest-oriented band farthest from the modern coastline (Figure 3.29). Roughly parallel bands of sediments of decreasing age outcrop toward the coast, with the youngest sediments accumulating right at the shore today.

The late Cretaceous sediments of the Coastal Plain are frequently rich in fossils, including shark teeth (Figures 3.30 and 3.31) and ammonites (both the usual coiled forms, and unusual irregular or uncoiled forms, known as heteromorphs) (Figure 3.32). Oysters and other bivalves including Inoceramus (Figure 3.33) are also present, as well as are rare mosasaur and plesiosaur remains. These fossiliferous Cretaceous sediments are found from the base of the Edwards Plateau near the Mexican border to Dallas and into the sliver of Coastal Plain on the eastern half of the Oklahoma-Texas border. The latter area yields rare dinosaur skeletons, most spectacularly (in Atoka County, Oklahoma) the gigantic sauropod Sauroposeidon (Figure 3.34).

The line of late Cretaceous layers continues along the landward edge of the Coastal Plain into Arkansas, bordering the Interior Highlands. This boundary forms a line from the southwest to northeast corners of the state. Cretaceous layers, however, only crop out in the corners of Arkansas, as they are buried by younger sediment in the state’s center. These strata contain large clams and bivalves, ammonites, gastropods, echinoids, shark teeth, and the occasional marine reptile. A few pockets of Cretaceous-aged rock are also found in Louisiana, where salt domes have pushed up through the overlying Cenozoic sediment, occasionally carrying Cretaceous shark teeth to the surface.

Figure 3.29: Generalized geologic map of the Coastal Plain.

The great majority of sediment exposed in the Coastal Plain is Cenozoic in age. Paleogene-aged strata are, for the most part, sandwiched between the western rim of Cretaceous deposits, and the youngest layers of Neogene and Quaternary sediment, which are found nearest the Gulf and along the Mississippi River Valley (see Figure 3.29). Paleogene marine deposits in Texas, southern portions of Arkansas, and northern parts of Louisiana are abundantly fossiliferous, yielding shark teeth, corals, clams and snails (Figure 3.35), and the bones of early whales like Basilosaurus (Figure 3.36).

Ammonoids

Ammonoids are a major group of cephalopods that lived from the Devonian to the end of the Cretaceous. Both nautiloids (the group that today contains the chambered nautilus) and ammonoids have chambered shells subdivided by walls, or septa (plural of septum). These shells are frequently, but not always, coiled. The term “ammonoid” refers to the larger group of these extinct cephalopods, distinguished by complex folded septa. Within ammonoids, “ammonites” is a smaller sub-group, distinguished by the extremely complex form of their septa. Ammonites were restricted to the Jurassic and Cretaceous periods. The form of the septa in nautiloids and ammonoids is not visible in a complete shell; it is most often seen in the trace of the intersection between the septum and the external shell. This trace is called a suture. Sutures are usually visible in fossils when sediment has filled the chambers of a shell, and the external shell has been broken or eroded away.

Ammonite shell break-away cross section; surface plane of a septum and sediment-filled chamber.

Figure 3.30: Cretaceous shark teeth from Kansas. A) B) and C) Scapanorhynchus, up to 1 centimeter (0.5 inches) long. D) Leptostyrax, about 1.4 centimeters (0.6 inches) long. E) and F) Carcharias amonensis, about 0.7 centimeters (0.3 inches) long. G) Cretodus, about 4.5 centimeters (1.8 inches) long.

Basilosaurus was first described by Richard Harlan in 1843, who believed the few bones he found belonged to a marine reptile (hence the name “saurus”). Basilosaurus was a member of an early group of whales known as archaeocetes, which included the ancestors of the two major groups of modern whales―the toothed whales (odontocetes), such as porpoises and sperm whales, and the baleen whales (mysticetes), such as humpback and blue whales. Archaeocetes inhabited the equatorial oceans during the Paleocene and Eocene. The vertebra pictured in Figure 3.36B was found in Arkansas in 1945, and represents the northernmost archaeocete fossil ever found.

Abundant tiny fossils called foraminifera (Figure 3.37) are found throughout the Coastal Plain’s Cretaceous and Cenozoic sediments. Foraminifera, or “forams,” as they are frequently called, are single-celled organisms (protists) with shells made of calcium carbonate. They live in the ocean in huge numbers, both at the bottom and floating in the water column, and are extremely important as index fossils and paleoenvironmental indicators in Cretaceous and Cenozoic sediments.

Terrestrial deposits in eastern Texas, dating from the early Miocene (e.g., Fleming Formation, exposed near Toledo Bend Reservoir), include abundant bones and teeth of amphibians, reptiles, and rodents (Figure 3.38). Larger mammals are also present, including carnivores, tapirs, pigs, and bizarre extinct forms such as the large herbivore Moropus (Figure 3.39). The Coastal Plain’s Miocene sediments also contain petrified wood, including palm (Palmoxylon), the Texas state rock and Louisiana state fossil (Figure 3.40).

Figure 3.38: Miocene rodent teeth from the Texas Coastal Plain. A) 1.5 millimeters (0.25 inches) wide. B) 1 millimeter (0.1 inches) wide.

By the Quaternary period, the glaciation of North America was underway. Many Neogene mammals had gone extinct by this time, yet mastodons, horses, and camels remained common. Fossils from this time are found on the Coastal Plain of Texas and Louisiana, and in the Mississippi River Valley in Louisiana, Arkansas, and the southeasternmost corner of Missouri. The youngest deposits of the Coastal Plain―for example, in the counties around Corpus Christi, Texas―contain the fossils of camels, saber-toothed cats, mammoths, glyptodonts, and dire wolves (Figures 3.41 - 3.43). Because they were the dominant life on land, and because their relatively large, heavy bones preserve more easily, it is easy to focus on Quaternary mammals; however, major groups of flora and fauna shared the landscape with these beasts, and their remains are occasionally found as well. For example, shells of freshwater bivalves are common, as well as those of land and freshwater gastropods, and petrified wood and pollen are also found.

The Waco Mammoth Site, in McLennan County, central Texas, contains the fossil bones of 24 Columbian mammoths (Mammuthus columbi) and other Pleistocene mammals. The site, discovered in 1978, is the largest known concentration of a single herd of mammoths dying from the same event, which is believed to have been a flash flood.