FORAMINIFERA OF ANCIENT METHANE COLD-SEEPS AS PALEODEPTH INDICATORS IN THE PIERRE SHALE, SOUTH DAKOTA

Recently, an abundance of ancient methane cold-seeps have been recognized within the Upper Cretaceous Pierre Shale of the Western Interior Basin, U.S.A.. In an effort to understand the depth of formation of these methane seeps we examined foraminifera from four American Museum of Natural History localities (AMNH) in the Baculites compressus Zone (AMNH loc. 3528), Didymoceras nebrascense Zone (AMNH loc. 3535), Baculites cunneatus Zone (AMNH loc. 3545), and the Didymoceras cheyennense Zone (AMNH loc. 3546) in Custer County, South Dakota for changes in foraminifera assemblages. Many studies suggest that the ratio of planktonic to benthic foraminifera (P/B) can be used to establish a water depth. Thus, the foraminifera ratios could be used to establish the depth of methane seep formation. Foraminifera from these three localities were analyzed to identify planktonic versus benthic. Using P/B ratio formula, Depth = e^{(3.58718 = (0.03534%P))}(Van der Zwaan et al., 1990), the approximate depth of formation of the methane cold-seeps is 117m. Further research is necessary to determine the potential effects of anoxic bottom waters on benthic populations of methane cold-seeps or if seeps foraminifera populations reflect opportunistic or stress-loving species different from species found distal to seeps.