FORAMINIFERAL ASSEMBLAGES AND PALEOCEANOGRAPHY OF THE UPPER CRETACEOUS (CONIACIAN) OF WESTERN KANSAS DURING OCEAN ANOXIC EVENT 3

The late Turonian-early Campanian Niobrara Cycle of the Western Interior Seaway is the longest second-order cycle in the Upper Cretaceous. Samples from the Amoco No. 1 Rebecca Bounds Core in western Kansas were studied spanning the upper Fort Hays Member and the lowest two units of the Smoky Hill Member of the Niobrara Formation. This interval was deposited during the early to middle Coniacian (~88-87 Ma) at a time when organic matter accumulated widely in the western Tethys.

High planktic:benthic ratios (p:b) are present throughout the study interval. Species diversity is low, and the planktic assemblages are dominated by small biserial taxa, primarily Heterohelix globulosa, which are generally more robust than trochospiral taxa. The tiny size and high abundances of this species indicate the stressful nature of the environment including reduced salinities associated with runoff and/or low oxygen conditions associated with influx of Tethyan water masses from the south. The paucity of associated benthics, which were sensitive to oxygen-poor conditions at the seafloor, provide further evidence of the stressful environment in the seaway during Coniacian time.

The strong correlation between the microfossil data and lithology throughout the core interval suggest the presence of cyclicity, perhaps influenced by Milankovitch orbital forcing. Preliminary spectral analyses on foraminiferal abundance and biserial foraminiferal percentage yield a 20 kyr precession signal, which may have influenced precipitation patterns in the Sevier highlands, resulting in freshwater influx to the seaway. In addition, the percentage of planktic foraminifera exhibit a strong obliquity (41 kyr) signal, as well as an eccentricity signal of a lesser magnitude. It is hypothesized that an eccentricity signal is also associated with carbonate content in the study interval. Orbitally influenced precipitation patterns, in addition to clastic dilution and the interplay of warm oxygen depleted Tethyan and cooler Boreal water masses created a truly dynamic environment during the Late Cretaceous. Cyclic changes in precipitation have been observed in other records of organic carbon burial during OAE 3.