A PRELIMINARY LOOK AT THE SEDIMENTS OF THE LOWER HELL CREEK IN NORTHEASTERN MONTANA

The Hell Creek Formation (late Maastrichtian), a predominately fluvial deposit, overlies the Fox Hills Formation, a predominately marine deposit. The contact between the two formations in northeastern Montana is a transitional zone, showing evidence of fluvial channel scours alternating with estuarine tidal flows. As a geologic component of a long term, multi-faceted paleo project operated jointly by the Museum of the Rockies (Montana State University), and the University of California, Berkley, we have been analyzing the sediments in one of many fossiliferous areas (the "Fiske" site) of the Lower Hell Creek . Our long term goal is to reconstruct the paleoenvironment of our immediate area, and to correlate it spatially and temporally with other fossiliferous sites in the larger study area. We began the reconstruction of the paleoenvironment by selecting three large hills in the area that we could correlate visually by tracing key beds. The hills consist of poorly cemented sandstones, friable siltstones and mudstones, plus numerous discontinuous beds of irregular organic deposits that range from peat to low grade coal. Sedimentary structures (graded bedding, cross beds, ripples, hummucky cross stratification) are present but hard to collect because of poor consolidation. Also present are spatially extensive layers of micrite. Some of these are associated with leaf impressions, others are associated with fresh water clams and gastropods, others are barren. Preliminary identification of the invertebrate fauna suggests that we are dealing with low energy lacustrine environments alternating with higher energy fluvial deposits. We have constructed preliminary fence diagrams of the area, and have collected sand, silt and mud samples. We are analyzing the sandstones using sieving techniques (grain size distributions, mean grain size, sorting parameters), compositions, rounding. We are using X-ray diffraction to get a preliminary look at the composition of the mudstones. Ultimately we will be able to combine our geologic assessment of the sedimentary environment with the rest of the project's paleontological data to reconstruct a complete picture of the Late Cretaceous in Northeastern Montana.