LATEST CRETACEOUS SEA LEVEL AND THE EVOLUTION OF THE LANCE/HELL CREEK AND MEDICINE BOW/FERRIS DELTAS

A new model based on detailed lithostratigraphic and ichnological analysis of Upper Maastrichtian strata from southern North Dakota to southern Wyoming has emerged to provide a regional perspective on the paleogeographic evolution of the western margin of the Western Interior Sea during the final 5 m.y. of the Cretaceous. The distribution and interrelationships of marginal marine (e.g., estuaries, interdistributary bays, lagoons, and tidally-influenced fluvial channels) and continental (e.g., fluvial, floodplain overbank, and lacustrine) deposits varies within and between the latest Cretaceous Lance (LF), Hell Creek (HCF), Medicine Bow (MBF) and Ferris (FF) fms. In the past, this has led to their characterization as heterogeneous units exhibiting chaotic stratigraphic patterns. Viewed from a regional context, however, the LF and HCF are reinterpreted as part of a single, large, eastward-prograding delta complex. A separate system (Medicine Bow/Ferris Delta) built eastward in southern Wyoming, fed by sediment from the Greater Green River Basin and surrounding Laramide uplifts. A tremendous amount of fluvially-delivered clastic sediment rapidly filled in formerly marine Laramide basins, giving the illusion of widespread sea level fall during the final 2 m.y. of the Cretaceous. In fact, relative sea level was rising during deposition of these units as demonstrated by increased accommodation and sediment sequestration in the basins. Differences in accommodation creation that resulted from variable rates of lithospheric subsidence manifested in dramatically different sedimentary successions and paleoshoreline trajectories. In the HCF of North and South Dakota and Montana, low accommodation/sediment supply (A:S) ratios led to strongly progradational clastic wedges. The MBF and FF of southern Wyoming reflect very high A:S ratios with high aggradation and high-frequency brackish-water and marine incursions. This study underscores the importance of considering depositional elements, as well as facies associations and ichnofacies, within a larger-scale of regional stratigraphy and paleogeography in order to better understand ancient deltas.