Latest Cretaceous (Maastrichtian) Paleogeographic Map of Western North America

Modified from a U.S.G.S. paleogeography map

Latest Cretaceous Western Interior Seaway
Scientists call the body of water that lay to the east of the Hell Creek landscape the "Western Interior Seaway", an epieric sea.1 The mud and chalk that were deposited in this seaway now form the bedrock of some states of the Great Plains. Eastern Montana and the Dakotas were flooded by this inland ocean at various times in the Mesozoic. In pre-Hell Creek time, the sea stretched northward all the way to the Arctic Ocean, and was as far west as central Montana. The two main late Cretaceous rock formations of the Montana part of the sea are called the Bearpaw shale (representing off-shore depostion) and the Foxhills Formation (representing near-shore and beach deposition). But during the latest Cretaceous (the time of the deposition of the Hell Creek Formation), the seaway had retreated both southward and eastward, and was then roughly 100 miles to the east (see illustration above). In the Dakotas, these marine rocks are called the Pierre Shale. Creatures such as ammonites, clams, oysters, sharks and mosasaurs lived in this sea. I have not compiled a faunal list for the latest Maastrichtian portion of the Bearpaw shale, but in my opinion, the best web source for information on the creatures of the Cretaceous interior seaway can be found on Mike Everhart's "Oceans of Kansas" site (see link below). His huge website deals with a part of the seaway that is much further to the south, and it is also much older (87-82 million years old) than the age of the Hell Creek Formation (which is ~67-65 million years old), so you definately would not have seen an identical assemblage of sea animals in Hell Creek time. Mike's web pages will still give you a general idea of some of the many kinds of North American sea creatures that lived in the Cretaceous period.

1 An epieric sea is a sea that rests upon continental crust.

Hartman, J.H. and Kirkland, J.I. 2002. Brackish and marine mollusks of the Hell Creek Formation of North Dakota: Evidence for a persisting Cretaceous seaway, in Hartman, J.H., Johnson, K.R., and Nichols, D.J., (eds.) The Hell Creek Formation of the northern Great Plains: An integrated continental record of the end of the Cretaceous, Geological Society of America Special Paper 361, p. 271-296.

Hell Creek Life © 1997-2010 Phillip Bigelow

Oceans of Kansas

1 An epieric sea is a sea that rests upon continental crust.

<Hell Creek Life © 1997-2010 Phillip Bigelow