Fluvial and Sequence Stratigraphy Analysis of the Hell Creek and Fort Union Formations to Test Models for Sedimentation Across the Cretaceous-Paleogene Boundary in Makoshika State Park, Glendive, Montana

Abstract

The Cretaceous-Paleogene Boundary outcrops extensively throughout Makoshika State Park in Glendive, Montana. A distinct change in sedimentation style occurs across the Cretaceous- Paleogene in the Williston Basin deposits found in eastern Montana and western North Dakota. The well documented Hell Creek Formation of the Upper Cretaceous period consists of fresh and brackish water influenced deposits on the western low-lying coast of the Western Interior Seaway. The overlying Fort Union Formation consists of coals and fluvial sandstone deposits. Geologic explanations for the depositional shift include tectonic forces of the Sevier/Laramide Orogeny to the west, The Cretaceous-Paleogene Extinction Event, and the advancement and retreat of the Cannonball Sea to the east. At several locations throughout Makoshika State Park in Glendive, Montana, the Hell Creek and Fort Union Formations are characterized by the incision of large paleovalleys. The appearance of the incised valleys and their channel fill suggests the change in sedimentation style is due to a change in base level associated with regression and transgression of the Western Interior Seaway. This study uses sequence stratigraphy to test the fundamental forces leading to a shift in the depositional environments of the Cretaceous- Paleogene and to add to a more complete record of the Williston Basin deposits of Eastern Montana and Western North Dakota.