SEQUENCE STRATIGRAPHIC INTERPRETATION OF THE FOX HILLS AND HELL CREEK FORMATIONS (MAASTRICHTIAN), EASTERN MONTANA AND ITS RELATIONSHIP TO DINOSAUR PALEONTOLOGY

The Upper Cretaceous Hell Creek Formation in the vicinity of Fort Peck Reservoir in eastern Montana has received significant attention recently due to the large variety and often exceptional preservation of the fossil material that it contains. Workers have mainly focused on taphonomic and paleontologic issues and are now in need of a meaningful framework from which to study evolution, population diversity, and ecology. This study uses sequence stratigraphy to correlate within the Fox Hills and Hell Creek Formations such that the data collected can now be interpreted in relation to a series of linked depositional environments and their relationship through time. Three key surfaces can be traced across the study area. The first is a sequence boundary at the top of the Fox Hills marine shoreface strata. Incision occurs locally at this boundary and the resulting topography is filled by incised valley-fill strata of the Colgate Member of the Fox Hills Formation (lowstand systems tract). The second key surface is a subsequent flooding surface that promotes development of a Glossifungites surface across the interfluve areas where Colgate strata are not present. This flooding surface separates either the white, trough cross-stratified Colgate Sandstone or the marine Fox Hills below from the inclined heterolithic strata that comprise the traditional lower Hell Creek sandstone above. The lower Hell Creek sandstone (from which most dinosaurian species are being excavated) is interpreted as a high-accomodation estuarine complex (transgressive systems tract). The third key surface separates the lower Hell Creek estuarine deposits from the upper Hell Creek fluvial system. These upper Hell Creek deposits consist of mixed sandstone, siltstone and mudstone which are interpreted to have formed on an alluvial plain through processes associated with meandering stream deposition and pedogenic development and reflect a return to low accommodation depositional patterns (highstand systems tract). We suggest that the changes in accommodation through the Maastrichtian, as interpreted from regional stacking patterns, facies changes and facies tract dislocations can be used in conjunction with taphonomic data from paleontologic sites in order to understand and predict the nature of preservation and distribution of fossil material.