THE PALEOGEOGRAPHIC EVOLUTION OF THE CAMPANIAN AND MAASTRICHTIAN WESTERN INTERIOR SEAWAY AND ITS RELATIONSHIP TO TECTONICS AND THE OCEANO-CLIMATE SYSTEM

The Early Cretaceous to Paleocene Western Interior Seaway (WIS) was one of the largest post-Paleozoic epeiric seas in geological history. Extensive research on this seaway has substantially expanded our knowledge of its paleogeography, ocean-climate system, and geological setting. Despite this extensive research, no study has quantified how this seaway changed in geographic area over time and how these changes were influenced by tectonics and the Earth’s ocean-climate system. To examine how the WIS changed geographically during the Late Cretaceous, we developed high-resolution paleogeographic reconstructions for each Campanian and Maastrichtian ammonite zone to quantify the area of the Seaway over time. We measured the C-, O-, and Sr-isotope composition of well-preserved ammonite shells (Baculites, Hoploscaphites). We then compared our paleogeographic reconstructions of Seaway area with changes in eustasy, tectonics, paleotemperatures (δ¹⁸O proxy), productivity (δ¹³C proxy), and ⁸⁷Sr/⁸⁶Sr ratios to determine their relationship with tectonics and the ocean-climate system. Results show that the area of the WIS slowly shrank during the Campanian likely due to the onset of Laramide uplift during late Santonian to early Campanian. The area of the Seaway then rapidly contracted during the early to late Maastrichtian in association with an increase in the rate of Laramide uplift across the region. The eustatic, δ¹⁸O, and δ¹³C curves show weak relationships with the areal extent of the WIS, which is likely linked to a stronger regional tectonic control. The Western Interior ⁸⁷Sr/⁸⁶Sr curve generally correlates with the global curve, however, there are anomalies at 81.5 Ma, 75.0 Ma, and 70.5 Ma that do not correlate with the open-ocean. In contrast, these anomalies correlate with decreases in the area of the WIS during tectonically-driven (dynamic subsidence, coupled with Laramide block movements) progradational surges and/or regression. The ⁸⁷Sr/⁸⁶Sr anomalies likely resulted from a combination of factors, including tectonically-driven uplift and erosion of older rocks, as well as changes in submarine groundwater discharge from adjacent landmasses.