Bearpaw Formation

Bearpaw Formation
Stratigraphic range: Campanian-Maastrichtian, 75–72 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
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, St. Mary River Formation, Eastend Formation, and others
Overlies	Dinosaur Park Formation, Judith River Formation
Thickness	Up to 350 meters (1,150 ft)[1]
Lithology
Primary	Shale, 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)
Region	Alberta, Saskatchewan, Montana
Country	Canada, United States
Extent	Northern Montana to central Alberta and southern Saskatchewan
Type section
Named for	Bear Paw Mountains, Montana
Named by	Hatcher and Stanton, 1903[2]

The Bearpaw Formation, also called the Bearpaw Shale, is a geologic formation of Late Cretaceous (Campanian) age. It outcrops in the U.S. state of Montana, as well as the Canadian provinces of Alberta and Saskatchewan, and was named for the Bear Paw Mountains in Montana.^[2] It includes a wide range of marine fossils, as well as the remains of a few dinosaurs. It is known for its fossil ammonites, some of which are mined in Alberta to produce the organic gemstone ammolite.^[3]

Lithology and depositional environment

The formation was deposited in the Bearpaw Sea, which was part of the Western Interior Seaway that advanced and then retreated across the region during Campanian time.^[4] It is composed primarily of dark grey shales, claystones, silty claystones and siltstones, with subordinate silty sandstones. It also includes bedded and nodular concretions (both calcareous and ironstone concretions) and thin beds of bentonite. As the seaway retreated toward the southwest, the marine sediments of the Bearpaw became covered by the deltaic and coastal plain sediments of the overlying formations.^[1][5][6]

Relationship to other units

The Bearpaw Formation conformably overlies the Dinosaur Park Formation of the Belly River Group in central Alberta, and the Judith River Formation in the plains to the east and Montana. It is overlain by the Horseshoe Canyon Formation in central Alberta; by the Blood Reserve Formation and the St. Mary River Formation in southern Alberta; by the Eastend Formation in southern Saskatchewan; and by the Fox Hills Formation in Montana. To the east, it merges into the Pierre Shale.^[1]

Fauna

The Bearpaw Formation is famous for its well-preserved ammonite fossils. These include Placenticeras meeki, Placenticeras intercalare, Hoploscaphites, and Sphenodiscus, the baculite Baculites compressus and the bivalve Inoceramus, some of which are mined south-central Alberta to produce the organic gemstone ammolite.^[3]

Other fossils found in this formation include many types of shellfish, bony fish, sharks, rays, birds, and marine reptiles like mosasaurs such as Prognathodon overtoni and Plioplatecarpus peckensis, plesiosaurs such as Dolichorhynchops herschelensis, Albertonectes and Nakonanectes, and sea turtles. Dinosaur remains have occasionally been discovered, presumably from carcasses that washed out to sea.^[7][8]

Dinosaurs

Dinosaurs from the Bearpaw Formation
Genus	Species	Location	Member	Material	Notes	Images
Brachylophosaurus[7]	Indeterminate[7]
Daspletosaurus
Edmontonia[7]	Indeterminate[7]
cf. Kritosaurus				"Nearly complete skull and postcranium."[9]	A hadrosaurid
Prosaurolophus[7][10]	P. maximus[10]			Three juvenile specimens[10]	A Saurolophinae hadrosaurid, also known from the Dinosaur Park and Two Medicine Formations
Stegoceras[7]	Indeterminate[7]

Plesiosaurs

Plesiosaurs from the Bearpaw Formation
Genus	Species	Location	Member	Material	Notes	Images
Albertonectes[11]	A. vanderveldei	Alberta		A complete, well-preserved postcranial specimen, missing only the skull.	An elasmosaurid plesiosaur. Albertonectes has the longest neck of any known plesiosaur.
Nakonanectes[12]	N. bradti	Montana		A nearly complete skeleton including the skull.	A small elasmosaurid plesiosaur with an unusually short neck.
Terminonatator[13]	T. ponteixensis	Saskatchewan		A partially articulated incomplete skeleton, including a skull.	An elasmosaurid plesiosaur.
Dolichorhynchops[14]	D. herschelensis	Saskatchewan		An incomplete skeleton	One of the latest known polycotylids.

Mosasaurs

Mosasaurs from the Bearpaw Formation
Genus	Species	Location	Member	Material	Notes	Images
Mosasaurus[15]	M. missouriensis	Alberta and Montana		Several specimens, including a near complete skeleton with stomach contents	A large mosasaurine mosasaur.
	M. conodon	Saskatchewan			A large mosasaurine mosasaur.
Prognathodon[16]	P. overtoni	Alberta		Several exceptionally preserved specimens	A large mosasaurine mosasaur.
Plioplatecarpus[17]	P. primaevus	Saskatchewan			A widespread genus of plioplatecarpine mosasaur.
	P. peckensis	Montana
Tylosaurus[18]	T. saskatchewanensis	Saskatchewan		A single semi-complete skeleton	A large tylosaurine mosasaur.

Turtles

Turtles from the Bearpaw Formation
Genus	Species	Location	Member	Material	Notes	Images
Nichollsemys	N. baieri	Alberta		Known from skulls	A basal chelonioid sea turtle.

References

This page was last edited on 10 February 2024, at 17:51