OCEANIC FRONT IN THE GREENHORN SEA (LATE MIDDLE THROUGH LATE CENOMANIAN)

An abrupt lithofacies change between calcareous shale and noncalcareou s shale occurs in strata deposited in the mid-Cretaceous Greenhorn Sea way in the southeastern corner of Montana. The facies were correlated lithostratigraphically using bentonites and calcarenites. The lithocor relations were then refined using ammonites, foraminifera, and calcare ous nannofossils. Twenty-five time slices were defined within the uppe r middle and lower upper Cenomanian strata. Biofacies analysis indicat e that lithofacies changes record the boundary or oceanic front betwee n two water masses with distinctly different paleoceanographic conditi ons. One water mass entered the seaway from the Arctic and the other f rom the Gulf of Mexico/Tethys. The microfauna and microflora permit in terpretation of the environmental conditions in each water mass. At ti mes when the front was near vertical, the two water masses were of the same density but of different temperatures and salinities.