THE STRATIGRAPHY AND SEDIMENTOLOGY OF AN INCISED VALLEY OF THE FORT UNION FORMATION AND ITS RELATION TO A RESPONSE IN SEA-LEVEL FLUCTUATION FOLLOWING THE CRETACEOUS-TERTIARY BOUNDARY MAKOSHIKA STATE PARK GLENDIVE, MONTANA

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 and the advancement and retreat of the Cannonball Sea to the east.

Several locations throughout Makoshika State Park in Glendive Montana, the Fort Union Formation are characterized by the incision of a large paleovalley. The appearance of the incised valley and its channel fill suggests a change in sea level associated with regression and transgression of the Western Interior Seaway. This study uses sequence stratigraphy to discuss 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.