The Cretaceous Western Interior Seaway of North America is the focus of many sequence stratigraphic analyses because of extensive outcrops and abundant subsurface data. Although most of these studies are local in scope, some workers have been tempted to correlate stratal stacking patterns and bounding surfaces across large areas based on lithostratigraphic similarities rather than age equivalence. We have integrated biostratigraphic and litho-stratigraphic data to create a chronostratigraphic framework for the Campanian and lower Maastrichtian of the Western Interior Seaway. This chronostratigraphic framework allows comparison of coeval stratal stacking patterns and key surfaces along the western margin of the basin and documents some of the complex sequence stratigraphic relationships that can occur within foreland basins.

Basinwide progradational units occur at the base of the Campanian and the lower Maastrichtian, but wide variability in the stratal stacking patterns within time-equivalent deposits exists along strike for most of the Campanian. Sequence boundaries and maximum flooding surfaces have been documented by workers for local areas within the Western Interior Seaway, but in most cases the regional extent and significance of these surfaces have not been addressed. In addition, recently published eustatic curves do not compare favorably with most of the lithostratigraphic/chronostratigraphic profiles in this study. This suggests that tectonic activity during the Campanian and early Maastrichtian probably played a major role in creating the complex relationships observed in the Western Interior Seaway. Local uplifts and basinal downwarps were a dominant control on relative sea level histories for most of the western margin of the basin. Major sequence-bounding unconformities, with hundreds of meters of erosion, are the direct time equivalents of maximum flooding events in other parts of the basin, reflecting the tectonic complexity typical of foreland basins.