PRELIMINARY RESULTS: LANDSCAPE EVOLUTION AND ENVIRONMENTAL PERTURBATIONS AT THE CRETACEOUS-TERTIARY BOUNDARY, HELL CREEK AND FORT UNION FORMATIONS, EASTERN MONTANA

In the terrestrial realm, events at the Cretaceous-Tertiary (K-T) boundary are identified primarily from strata in the Hell Creek (Upper Cretaceous) and Fort Union (Lower Paleocene) formations exposed in eastern Montana and western North Dakota. Although extensive research has been conducted on the biotic extinction and subsequent recovery across the boundary, little attention has been given to physical environmental changes that occurred at that time. Documentation of the extinction event alone does not provide a complete picture of the environmental upheavals that characterized the transition from the Mesozoic into the Cenozoic. This study investigates the nature, timing, rates, and possible causes of environmental changes and landscape evolution that occurred during deposition of the upper Hell Creek and lower Fort Union Formations in Makoshika State Park, Montana.

Hell Creek strata contain single sandstone bodies, up to 15m thick, isolated within thick shale successions. Characteristics within these sandstones suggest deposition in both meandering and braided streams. An abrupt change in lithofacies defines the base of the Fort Union Formation. These strata contain abundant coal; laminated and variegated siltstone; shale and fine-grained sandstone; and medium-grained single and multistoried sandstone bodies. The sandstone bodies contain a higher stacking density than those in the Hell Creek Formation. Lithofacies in the Fort Union Formation suggest deposition in peat mires, lakes, and meandering and braided streams.

In this area, the abrupt changes in lithofacies across the formational boundary indicate that sudden environmental changes and resulting landscape evolution occurred at the K-T boundary. Here, we propose three potential forcing mechanisms that drove the abrupt environmental change at the: 1) loss of plants and increased sediment flux following the end-Cretaceous bolide impact, 2) changing sedimentation and accommodation patterns within the depositional basin during transgression of Cannonball Seaway, and 3) changing sediment flux and accommodation patterns during the Laramide Orogeny. This research is ongoing and will test these hypotheses in an attempt to find the causes of environmental perturbations during the Late Cretaceous and Early Paleocene.