Bearpaw Formation

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

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 Because the sea did not disappear all at once, but instead slowly withdrew to the south, the Bearpaw Formation is superseded by the terrestrial sediments of the Horseshoe Canyon in Canada, while in Montana the Fox Hills Sandstone represents a near-shore marine environment.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

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

References