Paleontologic and Sr Isotopic Age Analysis of the Waccamaw Formation,

Waccamaw River and Intracoastal Waterway, Horry County, South Carolina

By Kira Badyrka

Whitman College, Walla Walla WA


Paleoecological and geochronological analyses were conducted for the Waccamaw Formation at the type locality on the Waccamaw River near Tilly Lake, SC, and a site on the Intracoastal Waterway near North Myrtle Beach, SC. Five bulk samples from the Waccamaw River and three from the Intracoastal Waterway were collected, wet sieved, and whole specimens and fragments containing bivalve beaks and gastropod apices were picked. More than 4600 specimens from the Waccamaw River site were analyzed, representing 49 bivalve and 50 gastropod genera. These samples represent a high diversity of both bivalves and gastropods, but bivalves are far more abundant than the gastropods, with 3651 bivalves and 1081 gastropods analyzed. The majority of bivalves are suspension feeders and have an infaunal siphonate life mode (72%), and 65% of gastropods are carnivorous predators. Based on preliminary examination of the bulk samples the Intracoastal Waterway has a lower overall diversity of taxa than the Waccamaw River, with a significant decrease in the diversity and abundance of gastropods, suggesting that it represents a post-Gelasian extinction assemblage. Thick, articulated Mercenaria were collected from the Waccamaw River and the Intracoastal Waterway sites for 87Sr/86Sr isotopic analysis. The average of six strontium ratios from samples on the Waccamaw River generated a date of 1.54 Ma and the average of three dates from the lower-most bed for the Intracoastal Waterway site yielded a date of 1.71 Ma. When the errors on these dates are considered, we are unable to distinguish by Sr dating a difference in age of the two locations. These data are consistent with the Pleistocene age of the Waccamaw Formation at sites in North Carolina.