Bearpaw Formation

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Bearpaw Formation
Stratigraphic range: Campanian-Maastrichtian, 75–72Ma
Bearpaw-Horseshoe Canyon.jpg
Contact (red arrow) between the underlying marine shales of the Bearpaw Formation and the coastal Horseshoe Canyon Formation.
Type Geological formation
Unit of Montana Group
Underlies Horseshoe Canyon Formation
Overlies Dinosaur Park Formation, Judith River Formation
Thickness 350 meters (1,150 ft)[1]
Lithology
Primary Claystone
Other Siltstone, sandstone, concretionary beds
Location
Coordinates 48°15′0″N 109°30′0″W / 48.25000°N 109.50000°W / 48.25000; -109.50000 (Bearpaw Formation)Coordinates: 48°15′0″N 109°30′0″W / 48.25000°N 109.50000°W / 48.25000; -109.50000 (Bearpaw Formation)
Region Alberta, Saskatchewan, Montana
Country  Canada  United States
Extent Northern Montana to northern Alberta
Type section
Named for Bearpaw Mountains, Montana
Named by Hatcher and Stanton, 1903[2]

The Bearpaw Formation, also called the Bearpaw Shale, is a sedimentary rock formation found in northwestern North America. It is exposed in the U.S. state of Montana, as well as the Canadian provinces of Alberta and Saskatchewan, east of the Rocky Mountains. It overlies the older Two Medicine, Judith River and Dinosaur Park Formations, and is in turn overlain by the Horseshoe Canyon Formation in Canada and the Fox Hills Sandstone in Montana. To the east and south it blends into the Pierre Shale.

Lithology and depositional environment[edit]

A marine formation composed mostly of shale, it represents the last major expansion of the Western Interior Seaway before it completely receded from northwestern North America by the end of the Cretaceous Period.[3] The seaway had previously divided North America in half before orogeny (mountain-building) in the west uplifted the land and forced the seaway to retreat. As the uplift slowed down or ground to a halt in the late Campanian stage, around 74 million years ago, subsidence of the land allowed the seaway to invade once more. This northern expansion is often called the Bearpaw Sea. When the Laramide Orogeny resumed in the early Maastrichtian, the seaway retreated to the south for the final time. Because the sea did not disappear all at once, but instead slowly withdrew to the south, the Bearpaw Formation is superseded by the terrestrial sediments of the Horseshoe Canyon in Canada, while in Montana the Fox Hills Sandstone represents a near-shore marine environment. The Fox Hills too would be replaced by the terrestrial sediments of the Hell Creek Formation in Montana by the late Maastrichtian.

Fauna[edit]

It is famous for its well-preserved ammonite fossils.[4] Other fossils found in this formation include many types of shellfish, bony fish, sharks, rays, birds, and marine reptiles like mosasaurs, plesiosaurs and sea turtles. The occasional dinosaur remains have also been discovered, presumably from carcasses washed out to sea.[5]

Dinosaurs[edit]

Dinosaurs reported from the Bearpaw Formation
Genus Species Location Stratigraphic position Material Description Images

Brachylophosaurus[5]

Indeterminate[5]

Edmontonia[5]

Indeterminate[5]

Kritosaurus

"Nearly complete skull and postcranium."[6]

A hadrosaurid

Pachyrhinosaurus

P. lakustai

A ceratopsid

Prosaurolophus[5]

Indeterminate[5]

Stegoceras[5]

Indeterminate[5]

References[edit]

  1. ^ Lexicon of Canadian Geological Units. "Bearpaw Formation". Retrieved 2009-02-06. 
  2. ^ Hatcher, J.B. and Stanton, T.W., 1903. The stratigraphic position of the Judith River beds and their correlation with the Belly River beds. Science, no. 5, v. 18, p. 211-212.
  3. ^ Latest Cretaceous Western Interior Seaway
  4. ^ Bearpaw fauna in Alberta
  5. ^ a b c d e f g h i Weishampel, David B; et al. (2004). "Dinosaur distribution (Late Cretaceous, North America)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 574-588. ISBN 0-520-24209-2.
  6. ^ "Table 20.1," in Weishampel, et al. (2004). Page 441.