Two distinctive, laterally traceable bentonite beds were used to construct three isochronous time slices through the marine sediments of the Upper Cretaceous Greenhorn Cyclothem of the Western Interior Basin. Calcareous nannofossil assemblages from these time slices were analyzed from >40 localities to examine the paleoceanographic conditions in the basin.

Deposition of the lower time slice (X bentonite) approximately corresponded to the time at which free communication between the basin and open oceanic systems first occurred. The most striking trend is the presence of common to abundant nannofossils in the center of the basin and their absence from the eastern and western margins. This indicates that open oceanic conditions conducive to large standing crops of calcareous nannoplankton existed only in a narrow, centrally located channel. Conditions at the basin margins were probably unsuitable for large populations due to salinity constraints. Multivariate analyses indicate two distinct biofacies (northern and southern) which are interpreted to represent distinct surface-water masses. The geometry of the water-mass boundary suggests that the northern surface-water mass actively flowed southward into or over the relatively passive southern water mass.

The upper time slice (HL-3 bentonite) samples the nannofossil distributions at or near the maximum transgression of the Greenhorn Sea. In this time slice, nannofossils occur beyond the limits of the study area except in proximity to the Frontier delta system. Diversities are significantly higher, indicating water-mass conditions more similar to those in open oceanic settings. Multivariate analyses indicate three distinct surface-water masses (northern, southern, and western). The northern and southern water masses exhibit relationships similar to those during the X bentonite time slices. The interaction between the northern and southern water masses promoted vertical stratification of the water column and resultant sediment anoxia. The western biofacies may be indicative of a surface-water mass with significant contributions from nearby fresh-water influx.

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