Organic-walled microplankton paleoecology and biostratigraphy of the Upper Cretaceous Ripley Formation, southwestern Georgia

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Virginia Polytechnic Institute and State University


This study documents the occurrence of dinoflagellate, chlorophyte, and acritarch cysts from the Upper Cretaceous upper Cusseta, Ripley, and lower Providence Formations in the USGS Fort Gaines core, drilled in Clay County, Georgia.

A total of 75 taxa were identified, consisting of 36 genera, 61 species, and 5 subspecies of dinoflagellates, 3 genera and 4 species of chlorophytes, 3 genera and 4 species of acritarchs, and 1 problematical protozoan. 15 of these taxa are unpublished.

The study's cyst assemblage was statistically analyzed to ascertain paleoecologic patterns. The results from cluster analysis and detrended correspondence analysis indicate the presence of four cyst associations. The Deflandrea pannucea association is interpreted as indicative of low salinity related to nearby river discharge. The Deflandrea sp. A association is confined to the inner shelf. The Exochosphaeridium bifidum and Glaphyrocysta reticulosa associations alternate under normal marine conditions. The associations correlate well with observed lithologies and lithologic change.

Comparison of this study's assemblage with other Late Cretaceous assemblages provides limited information, since many biostratigraphically-important species are not present in this study. However, comparisons with Wilson's (1974), Benson's (1976), and Firth's (1984) zonations suggest a Lower Maastrichtian age for all strata in this study.