Western Interior Seaway
The Western Interior Seaway as described by Kauffman (1977) is a complex epicontinental sea extending about 4,800 km from Arctic Canada to the Tethys Seaway and approximately 1,600 km wide from a mobile belt along its western edge in Nevada and Utah to a stable shallow shelf along its east side. Within this basin water depths were variable and are inferred by Kauffman (1977) to have been on the order of 100m, a value accepted by most Cretaceous workers. Other authors cite much deeper conditions (Eicher 1969). The active western margin was the source of coarse clastics deposited in rapidly prograding deltaic complexes into rapidly subsiding basins resulting in a complex series of clastic wedges. Farther to the east the Western Interior Seaway was a stable platform which received the suspended fine clastics from the western part of the basin and surrounding low-lying hills of eastern North America deposited as thick units of mudstone and resulting in classic interfingering patterns with coarser clastics to the west as tectonism pulsed throughout the Cretaceous. Paleocurrent patterns for the Tethys Seaway and the Western Interior Seaway were presented by Kauffman (1977; fig. 10) in assessing distribution of larvae throughout the basin.
The interfingering Cretaceous lithosomes also received air-transported volcanic ash from volcanos in the tectonically active Rocky Mountains to the west, resulting in deposition of numerous layers of bentonite on the Cretaceous sea bottom. These isochronous, pyroclastic events must have dramatically altered bottom conditions, water chemistry, and atmospheric conditions, leading to dramatic changes in Cretaceous ecosystems and biotas. As Laramide tectonism progressed, the Western Interior Basin was inundated by sediment from the west causing progradation of large deltas and restriction of the basin in Maastrichtian time to a small Paleocene remnant sea centered in North Dakota, the Cannonball Sea.
The Cretaceous sediments of the Western Interior carry extensive and beautifully preserved faunas, primarily encased in concretionary limestone nodules. These faunas include rapidly evolving cephalopod molluscs such as Baculites, Didymoceras, Hoploscaphites, and many other taxa which have been developed into a well-documented chronology based on faunal succession, bentonite stratigraphy, and radiometric dating. This chronology allows other physical and biologic events, such as periodic appearances of Decapod-Lagerstätten to be precisely dated.