NUMERICAL CHRONOLOGY FOR THE PLIO-PLEISTOCENE OF THE NORTH CAROLINA COASTAL PLAIN: SR-ISOTOPE AND AMINO ACID RACEMIZATION RESULTS

Linking the chronology of late Cenozoic units of the North Carolina Coastal Plain to a global biostratigraphic time scale is hampered by the limited presence of planktonic foraminifera. Various studies have assigned Pliocene and Pleistocene ages based on molluscan biostratigraphy, geomorphology, stratigraphic position, and limited foraminiferal occurrences. Numerical dates for units in this age range are restricted to Sr-isotope and amino acid racemization (AAR) results, along with a few U-He results. Paired Sr-isotope and AAR data for the bivalve mollusk Mercenaria from the James City Formation (James City and Kennels Bluff, NC) and the Chowan River Formation (Colerain Bluff and Fountain Quarry, NC), compared with similar paired results from a superposed subsurface ~65 m section (core MLD01, Wehmiller et al., 2012) on the western shore of Pamlico Sound NC, provide some insights into the numerical age(s) of these units. The AAR results are internally consistent for multiple D/L values, and these results provide some qualitative estimates of age differences or similarities for the different units. Nevertheless, with the exception of results for the shallower units (<~35 m) in MLD01, the D/L values are nearly racemic so only minimum age estimates on the order of >1.6 Ma are possible. Sr-isotope results for multiple mollusk specimens from these units demonstrate some scatter, but cluster as follows: James City Fm., 1.0-1.75 Ma (n=6) and Chowan River Fm., 1.7- 3.35 Ma (n=12). Results from ~65 m in MLD01 provide the least ambiguous age estimate for the Plio-Pleistocene boundary in NC: near-equilibrium D/L values have a corresponding Sr-isotope age estimate of 2.65 +/- 0.2 Ma. A pronounced seismic reflection in MLD01 just above this dated position is attributed to the Pliocene-Pleistocene boundary by Mallinson et al. (2010), reinforced with biostratigraphic information from Zarra (1989).