QUANTIFYING THE PALEOGEOGRAPHIC EVOLUTION OF THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

The expanded sedimentary record of the Late Cretaceous Western Interior Seaway (WIS) in North America has been well-documented from both natural exposures and subsurface cores across the U.S. Western Interior. This record is correlated with high-resolution bio- and chronostratigraphy, which has resulted in numerous high-resolution paleogeographic maps. Despite this excellent paleogeographic record, relatively little attention has been given to quantifying how the area of the Seaway changed over time and how these changes related to eustasy, climate, and tectonics. To examine these issues, we took high-resolution WIS shoreline maps developed for each Campanian-Maastrichtian ammonite biozone and calculated the area of the Seaway to quantify how it changed over time and in relation to changes in eustasy, δ¹⁸O, δ¹³C, and ⁸⁷Sr/⁸⁶Sr. Results show that the area of the WIS slowly shrank during the Campanian and then rapidly contracted during the early to late Maastrichtian in association with the Laramide Uplift. The area of the WIS shows little relationship with eustasy, which likely reflects a stronger tectonic control on the Western Interior Foreland Basin. The values of δ¹⁸O, δ¹³C, and ⁸⁷Sr/⁸⁶Sr show a weak relationship with the areal extent of the WIS during the Campanian. However, during the Maastrichtian, these proxies show a clearer relationship with the decrease in the area of the Seaway.