WATER MASS EVOLUTION IN WESTERN IOWA, MID-CRETACEOUS

The Bridge Creek Limestone Member of the Greenhorn Formation is of late Cenomanian to early Turonian age and consists of alternating limestone and marl beds in the Western Interior Basin of the U.S.A. The Bridge Creek Limestone represents transgression, with the upper half representing maximum Greenhorn high-stand. Samples cored from near Hawarden and Grant City Iowa provide coverage from closer to the eastern shore. Assemblages of foraminifera indicate that as sea level rose in the latest Cenomanian into the Turonian more diverse planktonic foraminifers spread eastward. High fertility calcareous nannoplankton were pushed east (Grant City) as sea level rose and more diverse, oligotrophic assemblages replaced them to the west (Hawarden). An increase in planktonic foraminiferal porosity located in the upper Neocardioceras juddii ammonite zone has been identified at Hawarden and Grant City. This increase has also been found in Pueblo, Colorado, Deora, Colorado, and Hot Springs, South Dakota. We interpret this increase in Hedbergella delrioensis porosity as a decrease in water density. The decrease in water density was probably caused by an increase in sea level that brought warmer Tethyan water north and eastward.