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.