Badlands National Park
Location: Jackson, Pennington, and Shannon Counties, southwest South Dakota
Time: 37 to 28 million years ago, during the Tertiary period
About this Famous Find: Badlands was designated as a national monument in 1929 to preserve the region's unique geologic setting and associated fossil assemblages. The monument became a national park by an Act of Congress in 1978. The park's rugged deposits are representative of a variety of paleoclimates and span from Eocene to Oligocene in age.
The stratigraphy within the park, from oldest to youngest, is divided into the following geologic formations:
Chadron Formation (37-34 million years ago): The lithology of this formation primarily consists of fluvial mudstone, siltstone, limestone, and fine sand that were deposited under subhumid to humid conditons. Further evidence of a humid tropical paleoclimate exists in the Chadron Formation's faunal assemblage which includes freshwater snails, aquatic turtles, alligators, cursorial and semi-aquatic rhinoceroses, the rhinoceros-like brontotheres, the three-toed horse Mesohippus, saber-toothed cats such as Nimravus, Hyaeonodon, the rabbit Palaeolagus, and the pig-like entelodont Archaeotherium. Many of these taxa, excluding the brontotheres and alligators, also appear in the Brule Formation.
Brule Formation (34-30 million years ago): These sediments are representative of stream and floodplain deposits that accumulated in a savannah-like climate. This formation has been divided into the following members:
Scenic Member (34-32 million years ago): This member is characterized by a series of paleosols composed of gray to brown clays interbedded by siltstone. Many aquatic reptiles disappeared and large terrestrial turtles such as Gopherus became the most abundant reptile. Although the faunal assemblage still contained cursorial rhinoceroses and horses, perissodactyls began to decline and were replaced by artiodactyls like Leptomeryx, a small deer-like animal, and sheep-like oreodonts such as Merycoidodon.
Poleslide Member (32-30 million years ago): Climatic conditions continued to become drier and depositional patterns became reflective of episodic flooding, with rich silts cut by channel sandstones. Oreodonts became more diverse and included the genera Leptauchenia and Agriochoerus. In addition, another small deer-like animal called Protoceroas appeared.
Sharps Formation (30-25 million years ago): Drying climatic conditions continued and frequent volcanic eruptions in the Great Basin episodically covered the Badlands in ash. The distinctive white Rockyford Ash occurs at the base of the Sharps Formation. Sharps lithology is characterized by tuffaceous sandstones, coarse channel sandstones, and floodplain mudstones that were deposited in a steppe environment. Many of the taxa from the Brule Formation carried over into the Sharps Formation, however, Leptauchenia, Palaeolagus, and Nimravus disappeared. New taxa included the lagomorphs Archaeolagus, Desmatolagus, and Gripholagomys and the giant entelodont Daeodon.