PALEOGEOGRAPHIC EVOLUTION OF THE WESTERN INTERIOR SEAWAY OF NORTH AMERICA: QUANTIFYING GEOGRAPHIC CHANGES FROM THE CONIACIAN TO MAASTRICHTIAN

The Late Cretaceous Western Interior Seaway (WIS) was the largest post-Paleozoic epeiric sea in North America’s geological history. While the paleogeography of the WIS has been extensively reconstructed by various studies, less attention has been given to quantifying how it changed in area over time and how this relates to eustasy, climate, and tectonics. To quantify changes in seaway area, we created high-resolution WIS facies maps for each Western Interior ammonite zone from the Coniacian to Maastrichtian. We then examined how these changes correlated with shifts in eustasy, δ¹⁸O, δ¹³C, and ⁸⁷Sr/⁸⁶Sr. Our analysis shows that the WIS reached its greatest extent during the Coniacian to early Campanian, then gradually shrank through the middle and late Campanian as the region transitioned tectonically from fold and thrust belt- to Laramide-style tectonics. The Seaway contracted more rapidly during the Maastrichtian as Laramide uplift intensified. The area of the WIS shows weak correlations with eustasy and climate proxies (δ¹⁸O, δ¹³C), suggesting minimal climatic influence on seaway area. The ⁸⁷Sr/⁸⁶Sr curve closely matches the open-ocean curve, indicating strong connectivity to the open ocean into the late Maastrichtian. However, ⁸⁷Sr/⁸⁶Sr anomalies at 81.5 Ma, 75 Ma, and 70.5 Ma correspond to reductions in WIS area, linked to progradational surges driven by Laramide tectonics.