During the Campanian and Maastrichtian Stages of the Late Cretaceous, the Western Interior epicontinental sea covered an elongated, asymmetrical trough. The western boundary of this Cretaceous basin extended from western Arizona through western Utah, central Idaho, western Montana and north into the Canadian Arctic.

The eastern edge can be traced to western Minnesota and Iowa. The fine-grained sediment, low rates of deposition, and thin stratigraphic units indicate a limited sediment source on the eastern side of the basin.

The north end of the seaway was open to the boreal regions of northern Canada and Greenland, while the south end of the seaway was connected to the Texas Gulf Coast area. Marine vertebrates and invertebrates migrated into and out of the Western Interior from both of these regions.

THE MANSON IMPACT
  Ever since 1980 when Louis and Walter Alvarez stunned the scientific community with their asteroid impact theory, a search has been underway to locate the crater blasted by the extraterrestrial body hypothesized to have impacted the Earth at the end of the Cretaceous, bringing an end to the reign of the dinosaurs and the ammonites. The two impact features that appear to have been created at or near the end of the Cretaceous include the Chicxulub structure, of the Yucutan Peninsula on the Gulf of Mexico, and the Manson structure in Iowa. The Chicxulub structure is thought to more closely coincide with the Cretaceous-Tertiary (K-T) boundary (64.4 MYA), while the Manson structure is older (73.8 MYA), and occurred during the time of the deposition of the Didymoceras stevensoni Range Zone of the Pierre Shale. Izett, et al. (1993) reported that the base of the Crow Creek Member of the Pierre Shale in eastern South Dakota contains shocked quartz grains, presumably derived from the Manson Impact ejecta, based on radiometric data and proximity.

Possible evidence of the erosive force of a tsunami produced by the Manson Impact is the regional, angular unconformity present at the base of the Crow Creek Member of the Pierre Shale. In Lyman County, South Dakota, the unconformity occurs between the Crow Creek Member and the underlying Gregory Member. Farther east, in Yankton County, South Dakota, erosion has completely removed the Gregory Member and the unconformity lies on top of the even older Sharon Springs Member. It appears that the Pierre Shale, below the unconformity at the base of the Crow Creek Member, has been more deeply eroded in southeastern South Dakota (nearer the Manson site) than in more western areas.

Another possible effect of the impact and the resulting tsunami is tha absence of Didymoceras stevensoni and any fossil molluskan or other distinguishable fossils throughout the entire Didymoceras stevensoni Range Zone in North Dakota, South Dakota or Nebraska. As far away as western South Dakota there is a noticeable absence of fauna between the Didymoceras nebrascense Range Zone and the Extiloceras jennyi Range Zone on the northern, eastern, and southern flanks of the Black Hills. The tsunami could have left the seaway uninhabitable or it may have destroyed all evidence of life in these areas. However, the fauna is remarkably intact and abundant on the western and northwestern flanks of the Black Hills, indicating that there must have been some structure that protected or shielded them from the force of the tsunami.

The above information is from Ammonites and the Other Cephalopods of the Pierre Seaway by Neal Larson, Steven Jorgensen, Robert Farrar, and Peter Larson; Geoscience Press, 1997