FORAMINIFERAL ASSEMBLAGES OF THE CENOMANIAN – TURONIAN BRIDGE CREEK LIMESTONE REVEAL THE COMPLEXITY OF A WARMING STORY OF THE CRETACEOUS WESTERN INTERIOR SEAWAY

The limestone-marl bedding couplets of the Bridge Creek Member of the Greenhorn Formation are characterized by fluctuations in biofacies and organic carbon. Foraminiferal assemblages from easily disaggregated marlstones and calcareous shales of the Bridge Creek Member have been extensively analyzed. In this study, foraminiferal assemblages extracted from limestone beds resulted in a quantitative evaluation of foraminiferal response to cyclically changing conditions that allowed deposition of limestone-marlstone couplets. The results show no systematic response of foraminiferal assemblages extracted from limestone beds. Also, for any given limestone–marl couplets, there are no major changes in foraminiferal population counts between that of the limestone beds and the adjacent marly intervals. However, there are some minor differences such as absence of planktic planispiral morphotypes, slight increase in the proportion of planktic biserial and triserial morphotypes, and slight increase in the proportion of benthics relative to total foraminifera in the limestones. Such conditions suggest that the limestones may have been more productive than the adjacent marlstones. Reduced salinity or greater stratification of the upper water column may have also contributed to the minor differences with assemblages preserved in the marlstones and calcareous shales. Oxygenation of the seafloor and proliferation of benthic foraminiferal assemblages “Benthic Zone” may have been associated with the Plenus Cold Event following an extreme global warmth event due to increased submarine volcanism and pCO2. Limestone-marlstone couplets of the Bridge Creek Limestone are climate cycles but they are complicated and cannot be explained by simple switching modes (wet/dry) in the dilution model or (increase/decrease in calcareous plankton) in the productivity model. Rather, paleoceanography of the WIS during Bridge Creek Limestone time suggests a gradual change from initial control by external processes associated with the paleoceanographic state of the proto-Gulf of Mexico (GoM) including calcareous plankton production rates and evaporation-precipitation gradients between the GoM and WIS, to control mainly by internal processes related to wet/dry cycles and surface runoff from both margins of the seaway.