NORTH AND SOUTH CAROLINA PLIO-PLEISTOCENE STRONTIUM ISOTOPE GEOCHRONOLOGY

Although significant extinction events occurred for western Atlantic mollusk faunas during the Plio-Pleistocene, the timing and effect of these events is not well understood in the Carolinas. As a part of a Research Experience for Undergraduates program, several formations thought to encompass these extinction episodes were sampled for strontium isotopic dating in order to better constrain the age of the suspected extinctions. This is a comprehensive report of the dates of samples collected in 2008 and 2009 from stratigraphically correlated sections in North and South Carolina.

Samples were collected from four North Carolina locations: an unnamed unit at the Fountain Quarry, Pitt Co., the Duplin Fm. on the Lumber River, Robeson Co., the lower Waccamaw Fm. at Register Pit, Columbus Co., and the James City Fm. at the Lee Creek Mine, Beaufort Co. The stratotype of the Waccamaw Fm. along the Waccamaw River, and the Waccamaw on the Intracoastal Waterway near Myrtle Beach, Horry Co., SC, were also sampled. Where present, articulated thick-shelled mollusks with pristine physical appearance were selected for 87Sr/86Sr analysis; from one unit, pectinids and barnacles were selected.

The unnamed unit at the Fountain Quarry yielded widely scattered dates between 1.70 and 4.25 Ma, reflecting either diagenetic alteration or shell reworking, though the fauna may support an older age. These dates overlap dates of three shells from the Duplin Fm. on the Lumber River (2.30-2.80 Ma). Waccamaw samples from the Register Pit provided dates from 1.55-2.30 Ma, averaging 1.93 Ma. Waccamaw samples from the stratotype on the Waccamaw River and the site on the Intracoastal Waterway yielded dates from 1.43-1.75 Ma, averaging 1.54 Ma, and from 1.33-1.98 Ma, averaging 1.71 Ma, respectively. These data suggest a lower and upper Waccamaw Fm. James City Fm. samples from the Lee Creek Mine were dated between 1.73 and 2.55 Ma, averaging 2.3 Ma, and are older than the James City stratotype and the Waccamaw Fm. at the Register Pit. These data suggest that the James City Fm. may consist of an older part equivalent to the lower Waccamaw Fm. and the upper part equivalent to the upper Waccamaw Fm.