Paleoecology of Mid-Cretaceous Planktonic Foraminifera: A Comparison of Open Ocean and Epicontinental Sea Assemblages

Although our knowledge of living planktonic foraminifera is limited, useful paleoecologic information can be obtained from ancient assemblages. During the mid-Cretaceous, as today, the simple, inflated morphotypes inhabited the nearsurface waters while the flatter, keeled forms probably occupied deeper habitats. Three faunal groups are proposed, an Epicontinental Sea Fauna (ESF) characterized by species of Gubkinella, Guembelitria and Heterohelix, an open marine Shallow Water Fauna (SWF) composed primarily of species of Hedbergella and Globigerinelloides, among others, and finally an open marine Deep Water Fauna (DWF) represented by species of Planomalina, Rotalipora and Praeglobotruncana (keeled taxa). Open ocean assemblages of DSDP Sites 545 and 547 are numerically dominated by species of the SWF in the >63-,um size fraction while the ESF comprises about 5% of the assemblages, and the biostratigraphically important DWF generally comprises less than 2%. These assemblages demonstrate remarkable stratigraphic consistency. In contrast to open ocean assemblages, epicontinental sea assemblages of the Vocontian Basin are characterized by loss of the DWF with decreasing water depth and an increase in the proportion of the ESF. The ESF:SWF ratio also becomes increasingly variable with decreasing water depth. Upwelling also has pronounced effects on the composition of planktonic assemblages as suggested by an increase in the proportion of the ESF, interpreted to include opportunistic species.