Two distinctive, laterally traceable bentonite beds were used to construct two isochronous time slices through the marine sediments of the Upper Cretaceous Greenhorn cyclothem of the United States Western Interior basin. Calcareous nannofossil assemblages from these time slices were examined from more than 40 outcrop localities. Nannofossil presence and assemblage diversity and composition were statistically analyzed to examine the paleooceanographic conditions within the basin.

The lower time slice (X bentonite) is at the stratigraphic horizon which approximately corresponds to the time at which free communication between the basin and open oceanic systems first occurred. The most striking trend in the nannofossil distribution is delineated by the presence or absence of nannofossils. The presence of common to abundant nannofossils in the center of the basin (i.e., near the hingeline of the basin) and the absence of nannofossils from the eastern and western basin margins indicate that open marine conditions conducive to significant standing crops of calcareous phytoplankton occurred only in a narrow, centrally located zone. Conditions at the basin margins were probably unsuitable for large populations due to environmental instability. Evidence indicates that atznaueria barnasae was the most ecologically tolerant form.

The upper time slice (HL-3 bentonite) samples the nannofossil distributions at a point at or near the maximum transgression of the Greenhorn Sea. Similar, basinward increases in diversity and abundance are evidence but more subtly expressed. Paleolatitudinal differences in diversity and the relative abundances of some taxa indicate contributions from both tropical and boreal oceanic water masses.

End_of_Article - Last_Page 538------------