LANDSCAPE EVOLUTION OF A CAROLINA BAY

Carolina Bays are elliptical depressions common on the North American Atlantic Coastal Plain. The 300 ha Juniper Bay in Robeson County, North Carolina (N 34° 30’ 00”, W 79° 01’ 18”) is one of these Bays. We collected 29 cores to 7.5 or 15 meters in and outside this bay to understand its geomorphology and stratigraphy. Ground penetrating radar, radiocarbon dates, and shallow cores (<2 m) provide additional insights into bay evolution. Below ~6 m, sediments are part of the Cretaceous Black Creek Group. The top of the Black Creek is irregular in elevation (at one core location reaching the ground surface) and is unconformably overlain by the lithologically variable Pliocene-age Duplin/Yorktown Formation. The Duplin/Yorktown is unconformably overlain by a 2-4 m thick complex surficial unit. This unconformity likely represents the bottom of the original bay basin. In some cases, a buried soil occurs below the unconformity. The surficial unit in and around Juniper Bay shows considerable variation in depositional environments and/or erosional patterns. Some cores show clear evidence of eolian deposition (fine, well-sorted sand). Layers of coarse sand indicate higher energy conditions (reworking along beaches, or lag deposits). Clay varies from 10-60% at similar depths across the bay. The presence/absence of mollusk shells, the presence/absence of buried soils, the depth of erosional lags, and the depth to the uppermost clayey sediments indicate that a variety of depositional and erosional environments occur in and around Juniper Bay. The presence of sand over clayey sediment, where water can perch close to the ground surface, would be a logical stratigraphic setting to enable bay formation. The absence of a clay layer at the southeast end of the bay suggests this clay layer is not a requirement. Radiocarbon dates of fossil wood and in situ roots from the bay indicate that plants were growing on subaerial surfaces during at least four geologic periods: late Holocene, early Holocene, and twice during the late Pleistocene. The presence of paleosols also indicates subaerial exposure. Our stratigraphic and geophysical investigations support the hypothesis that Carolina Bays evolved under the influence of a series of natural events, coalescing from several smaller features, driven by changes in climate and prevailing wind direction.