Hell Creek Formation

Hell Creek Formation
Stratigraphic range: Late Cretaceous to Early Paleogene, Maastrichtian–Danian (Lancian) ~68–66 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Exposure in the badlands near Fort Peck Reservoir
Type	Geological formation
Unit of	Montana Group
Sub-units	Breien, Little Beaver Creek, Middle Sandstone & Pretty Butte Members
Underlies	Fort Union Formation
Overlies	Fox Hills Formation
Thickness	50–100 m (160–330 ft)
Lithology
Primary	Claystone, mudstone
Other	Sandstone, siltstone, conglomerate, amber
Location
Coordinates	46°54′N 101°30′W / 46.9°N 101.5°W / 46.9; -101.5
Approximate paleocoordinates	52°36′N 74°24′W / 52.6°N 74.4°W / 52.6; -74.4
Region	Montana, North Dakota, South Dakota, Wyoming
Country	United States
Extent	Williston Basin
Type section
Named for	Hell Creek, Jordan, Montana
Named by	Barnum Brown
Year defined	1907
Hell Creek Formation (the United States) Show map of the United States Hell Creek Formation (Montana) Show map of Montana

The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions of Montana, North Dakota, South Dakota, and Wyoming.^[1] In Montana, the Hell Creek Formation overlies the Fox Hills Formation. The site of Pompeys Pillar National Monument is a small isolated section of the Hell Creek Formation. In 1966, the Hell Creek Fossil Area was designated as a National Natural Landmark by the National Park Service.

It is a series of fresh and brackish-water clays, mudstones, and sandstones deposited during the Maastrichtian and Danian (respectively, the end of the Cretaceous period and the beginning of the Paleogene) by fluvial activity in fluctuating river channels and deltas and very occasional peaty swamp deposits along the low-lying eastern continental margin fronting the late Cretaceous Western Interior Seaway.^[2] The climate was mild, and the presence of crocodilians along with palm trees suggests a subtropical and temperate climate, with no prolonged annual cold. The famous iridium-enriched Cretaceous–Paleogene boundary, which separates the Cretaceous from the Cenozoic, occurs as a discontinuous but distinct thin marker bedding above and occasionally within the formation, near its boundary with the overlying Fort Union Formation.

The world's largest collection of Hell Creek fossils is housed and exhibited at the Museum of the Rockies, in Bozeman, Montana.^[3] The specimens displayed are the result of the museum's Hell Creek Project, a joint effort between the museum, Montana State University, the University of Washington,^[4] the University of California, Berkeley, the University of North Dakota, and the University of North Carolina which began in 1998.