CAROLINA BAYS OF THE U.S. ATLANTIC COASTAL PLAIN ARE RELICT THERMOKARST LAKES THAT FORMED EPISODICALLY DURING THE LAST GLACIATION

New studies of the U.S. Atlantic Coastal Plain using LiDAR data and optically stimulated luminescence (OSL) ages have revealed the presence of widespread eolian sands from Delaware to Georgia that were active episodically ~92–5 thousand years ago (ka) but are now stabilized by vegetation. These sands are present in river valleys, on upland terraces, in the Carolina Sandhills, and adjacent to Carolina Bays. Some of these sands form low-relief ridges on the south and east margins of Carolina Bays, which are oriented oval depressions. Some Carolina Bays show cross-cutting relations with other bays. Other Carolina Bays show different stratigraphic relations with respect to eolian dunes in river valleys. Examples from South Carolina include Bear Swamp that is inset into (i.e., younger than) eolian dunes in the valley of the Great Pee Dee River, and Big Bay that is overlain by (i.e., older than) eolian dunes in the valley at the confluence of the Congaree and Wateree Rivers. Cores in Carolina Bays and their associated ridges reveal a few meters of sand and/or muddy sand above an unconformity on various older fine-grained substrates that do not show signs of disturbance. Most published OSL ages from Carolina Bay sand ridges range from ~45–8 ka. Some bays have multiple sand ridges, and ridges closer to individual bays yield younger OSL ages. The stratigraphic relations and OSL ages suggest that Carolina Bays are relict features that did not form during one event of limited duration. Instead, they formed episodically during the same time interval as other eolian sands of the coastal plain (e.g., during the last glaciation when conditions were colder, drier, and windier). The interpretation of Carolina Bays as relict features that formed during the last glaciation suggests that they may be thermokarst lakes such as are present today in high-latitude regions. These lakes develop as a result of thaw and collapse of frozen ground, with subsequent modification by lacustrine and eolian processes. The distribution of Carolina Bays thus may provide information about former distribution of frozen ground. Although the southern limit of continuous permafrost during the last glaciation is thought to have been located in Virginia, deep frozen ground may have extended much farther south into areas where Carolina Bays are present.