STRONTIUM ISOTOPIC DATING OF THE WACCAMAW FORMATION AT ACME, NC, AND THE DUPLIN FORMATION AT TAR HEEL, NC: A PLIO-PLEISTOCENE RESEARCH PROGRESS REPORT

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.