NOVEL INSIGHTS ON THE PALEOCEANOGRAPHY OF THE EASTERN SIDE OF THE CENOMANIAN WESTERN INTERIOR SEAWAY FROM INTEGRATED GEOCHEMICAL AND MICROPALEONTOLOGICAL ANALYSIS

While there are ample studies of the paleoceanography of the western and central regions of the North American Western Interior Seaway, there has been little focus on eastern part of the seaway. There is a need to better understand this area of the seaway to get a more comprehensive view of shallow marine environments in greenhouse climates. An integrated investigation of elemental geochemistry and calcareous nannofossil micropaleontology was carried out on outcrops of the Cenomanian Graneros Shale from Kansas and Iowa. Multivariate statistical analysis determined that detrital sediment influence and changes between wet and arid climates were two important factors that controlled the paleoceanography of the eastern seaway. The analysis also indicated the variable importance of a detrital sediment rich “eastern” water mass and a disoxic “southern” water mass sourced from the Gulf of Mexico. After the initial transgression of the Greenhorn Sea, the climate became wetter and terrigenous influence from the North American craton in the east, which is often overlooked, was high. The peak of terrigenous influence corresponds with a lack of microfauna, which may indicate a similar oceanographic setting to the modern Gulf of Mexico nearshore dead zone. As sea level continued to rise above this point, there was a shift to the influence of the disoxic southern waters. Further transgression led to deposition of the offshore chalky Greenhorn Formation. Nannofossil and geochemical evidence indicates that the change from mudstone and marl deposition to chalk was caused by an increase upwelling-driven productivity and a shift to more arid climate. The use of an integrated micropaleontological and geochemical analysis has shed light on the paleoceanographic and paleoecological dynamics of shallow marine environments of the Cretaceous and could have important implications for similar modern shallow marine settings.