THE VIABILITY OF FORAMINIFERA AS PROXIES FOR OCEANIC ANOXIA IN LATE CRETACEOUS DEPOSITS ON THE EASTERN MARGIN OF THE WESTERN INTERIOR SEAWAY

During the Late Cretaceous Period, the Greenhorn Sea in western Iowa recorded complex mixing between cold boreal waters and warm, tropical Tethyan waters. Section K-18 (Witzke and Ludvigson, 1994 GSA Spec. Pub. 287 43-78) includes the Graneros Shale and Bridge Creek Limestone that are assigned to the Sciponoceras gracile and Neocardioceras juddi ammonite biozones. The calcareous shales and marlstones contain abundant foraminifera that include Heterohelix, Hedbergella, Guembelitria and Whiteinella. Specimens were extracted for geochemical analysis, carefully cleaned with DI water, and a visual clarity index scale of one to five, one being the most clear and pristine, was applied to each specimen. Scanning Electron Microscopy revealed secondary calcite overgrowths and trace elements that include Al, K, Si and Fe, suggestive of secondary clay authigenesis. Relative abundance acmes of Heterohelix are coincident with rich accumulations of crimson cryptocrystalline coprolites with elemental traces of P and Ca, indicative of the mineral Collophane. Our preliminary results indicate that these observed fluctuations in the Heterohelix may reflect changes in the paleoceanographic ventilation during a global oceanic anoxic event (OAE II). Stable isotope analysis and trace element geochemistry provided an independent test to the viability of foraminifera as isotopic proxies for oceanic anoxia during the Late Cretaceous.