Late Cretaceous Sevier Versus Laramide Orogenies in Wyoming-Utah-Colorado, USA: New Insights From Basin Subsidence History

Variability in subsidence rates within Upper Cretaceous strata of the Western Interior Basin offers crucial insights into the response of surface sedimentation styles to Sevier-to-Laramide tectonics and related deep mantle processes. The formation mechanisms of the Late Cretaceous Western Interior Basin in North America have long been a subject of debate. A re-evaluation of the basin's subsidence history reveals rapid subsidence pulses lasting ca. 2 Myr within longer-term (average 5.7 Myr) progradational or aggradational clastic wedges. The timing of these wedges, especially the widespread marine flooding resulting from subsidence, is constrained through the calibration of ammonite zonation with absolute dates. Sevier wedges exhibit a different architecture compared to the Laramide wedges. The former recorded initial rapid and widespread marine transgressions followed by long-term coastal progradation, whereas the latter developed by initial erosional and progradational growth followed by aggradation and long-term coastal transgression. The Sevier clastic wedges, initially accumulated within a N-S elongated, long-wavelength tectonic subsidence zone close to the thrust belt, gradually migrated cratonward. Starting in the early Campanian (ca. 82 Ma), the Laramide Orogeny developed along a NW-SE trend and then migrated northeastward, roughly consistent with coeval long-wavelength frontal basin subsidence. The spatio-temporal variations in long-wavelength tectonic subsidence indicate a shift in the dynamic subsidence's migration direction from eastward to northeastward, driven by changes in Farallon subduction direction and mode. Our work shows how repeated subsidence behavior in the Sevier-to-Laramide transition records evolving architectural responses and the trajectory of coeval dynamic topography.