SEDIMENTOLOGY AND AGES OF CAROLINA BAY SAND RIMS

The eastern and southeastern margins of Carolina bays are commonly bordered by single or multiple, laterally separated shorelines and sand rims, documenting fluctuating lake levels and episodic shoreline stability. These sand rims consist of a combination of eolian and underlying or laterally adjacent shore face sediments. Water-lain pebbles intercalated with eolian sands indicate that these are typical shoreline deposits resulting from fluctuations in water level. Some rims are too subtle to distinguish visually in the field, but can be seen on aerial photography by differences in soil, vegetation, and topography. Optically stimulated luminescence ages of bay rims sampled in Georgia and South Carolina reveal clustering of dates for rim formation and activity during and just prior to stadials over the past 130 ka. At individual bays where concentric rims occur, dating has established that rims are progressively younger toward the center of the bay, reflecting a regressive sequence and confirming that the bays are not single-event features but evolve as a result of processes active episodically over a long period of time. Active shorelines and associated eolian deposition occurred during marine isotope stage (MIS) 2 to late MIS 3 (~12 to 50 ka), MIS 4 to very late MIS 5 (60-80 ka), and late MIS 6 (120 to 140 ka) based on a cumulative total of 45 OSL dates. These age ranges also correspond with the ages of other eolian landforms in the Coastal Plain, including sand sheets and dunefields, and suggest a climatic threshold was crossed during the transition toward glacial stadials, initiating both bay and dune activity. In addition to these age ranges, some OSL dates indicate that bays also were active during the Holocene and Sangamon Interglacials. Sedimentology and archaeology of the upper few meters of bay rims suggest surface reworking by localized colluviation, eolian remobilization, and pedoturbation during the Holocene. Some of the ages represent this type of surface reworking. However, some of the interglacial ages, especially those of the early Holocene, are from sediments that suggest primary littoral or eolian deposition. Therefore certain combinations of bay water level and climatic conditions also allowed bay rim formation during interglacials.