AGE OF THE DUPLIN AND WACCAMAW FORMATIONS, CAPE FEAR RIVER BASIN, NORTH CAROLINA

The age of Plio-Pleistocene stratigraphic units in southern North Carolina has been greatly debated. Dating these strata is important for correlation to other coastal plain strata. Two formations were studied, the Duplin and Waccamaw. DuBar (1980) dated these units as Pliocene and early to mid Pleistocene, respectively. Cronin et al. (1984) dated the Duplin as late Pliocene and the Waccamaw as latest Pliocene to earliest Pleistocene. Using microfossils, Swain (1968) dated the Waccamaw as late Miocene to early Pliocene. Campbell & Campbell (1992) claimed the youngest Waccamaw sediments are older than 2.0 Ma. Blackwelder (1981) considered all Waccamaw deposits as early Pleistocene. Based on the presence of molluscan taxa also found in the late Pliocene Caloosahatchee Formation of Florida, Lyons (1991) considered at least the lower Waccamaw Formation (?) as late Pliocene.

Strontium isotopic ratios were used to assign dates to the units. Strontium is uniformly distributed in seawater and, having chemical properties similar to calcium, is incorporated in shells of organisms during growth. Although strontium ratios in seawater have fluctuated during the Phanerozoic, specific ratios can suggest time of shell origin.

Macrofauna were hand-picked from bulk samples of the Duplin Formation collected on the Lumber River near Lumberton in Robeson Co. and the Waccamaw Formation at the Register Quarry in Columbus Co., NC. The Waccamaw Formation? was also collected from Walkers Bluff in Bladen Co. and Sykes Landing in Columbus Co. Nine specimens that showed minimal diagenesis and evidence of reworking were analyzed, including barnacles, Ostrea, Plicatula, and Macrocallista. XRD analyses indicate that all Duplin samples were calcite and all Waccamaw samples aragonite. Samples were washed in an ultrasonic bath and 0.1N HCl to remove foreign material and analyzed using a mass spectrometer at UNC Chapel Hill.

Duplin samples were dated at 2.30, 2.40, and 2.80 Ma (mid to late Pliocene). Lower Waccamaw samples from the Register Quarry were dated at 1.55, 1.80, and 2.00 Ma (latest Pliocene to earliest Pleistocene). As the 1.55 Ma date from the Register Quarry is not concordant with the other dates, it may be too young. Upper Waccamaw samples from Walker's Bluff yielded dates of 1.60 Ma (two samples) and one sample from Syke's Landing 1.50 Ma.