Southeastern Section - 60th Annual Meeting (23–25 March 2011)

Paper No. 15
Presentation Time: 5:30 PM-8:00 PM


MCGREGOR, Daren A., Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244, HARRIS, William Burleigh, Geography and Geology, University of North Carolina Wilmington, 601 S. College Road, Wilmington, NC 28403, DIETL, Gregory P., Paleontological Research Institution, 1259 Trumansburg Road, Ithaca, NY 14850 and KELLEY, Patricia H., Department of Geography and Geology, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403-5944,

A series of extinction events affected Western Atlantic mollusks during the Plio-Pleistocene. In southern North Carolina, the Pliocene Duplin Formation captures pre-extinction diversity levels, and the Pleistocene lower and upper Waccamaw Formation follow the first and second phases of extinction, respectively. The paleontology of these formations is well understood, but can benefit from the addition of numerical dates. With strontium isotope dating, the ratio of 87Sr/86Sr in marine molluscan shells can be used to suggest time of deposition.

As part of the third year of a three-year NSF-funded Research Experiences for Undergraduates Program, we obtained strontium dates from two localities: Tar Heel, NC, a locality understood to represent the Duplin Formation, and Acme, NC, a locality with a paleontology consistent with the Waccamaw Formation. Past work in the REU (Graybill et al., 2009) yielded a date range of the Duplin Formation at Lumber River, NC, to be 2.3-2.8 Ma, and the lower Waccamaw Formation at Register, NC, to be 1.55-2.30 Ma. In this study, samples were taken from aragonitic bivalves, with a preference for those with the lowest chance of diagenetic alteration. The samples were cleaned, processed, and then examined for mineralogy using x-ray diffraction analysis before being prepared for dating. At Acme, five samples were taken from Mercenaria (1) and Glycymeris (4). At Tar Heel, five samples were taken from Mercenaria (3), Florimetis (1), and Arcinella (1). Isotopic analysis was conducted at UNC Chapel Hill.

87Sr/86Sr ratios of five macrofossils from Acme, NC, yielded an average date of 1.97 Ma, with a range of 1.80-2.12 Ma. These dates indicate that the section is Gelasian in age, or earliest Pleistocene. Based on the strontium dates and fauna, the section represents the lower Waccamaw Formation. 87Sr/86Sr ratios of five macrofossils from an outcrop near Tar Heel, NC, yielded an average date of 3.34 Ma, with a range of 2.88-3.57 Ma. The wide range in dates reflects a flattening of the marine 87Sr/86Sr curve, a longer period of deposition, or time-averaging. Strontium dates and fauna from this location suggest a late Pliocene Piacenzian age and equivalency to the Duplin Formation. Overall, strontium dates are consistent with past work on other Waccamaw and Duplin Formation sites.