WHERE THE BAYS ARE: A TEMPORAL TALE OF CAROLINA BAY GEOMORPHOLOGY AS TOLD IN LIDAR BY THE WANDO AND SOCASTEE TERRACES

80 years after aerial photography revealed thousands of aligned oval depressions on the USA’s Atlantic Coastal Plain, the geomorphology and ages of these “Carolina bays” remain enigmatic. Here, high-resolution LiDAR-derived elevation maps are used to examine Carolina bay landform distribution on terraces in the Cape Fear River Valley of North Carolina, in an attempt to identify and constrain their temporal distribution, which is of import to the gradualist vs. cosmic impact debate. The gradualist theory holds that bays were created episodically by katabatic winds during glacial periods, while an impact event would create all bays simultaneously.

The southeast flowing Cape Fear River has been incising its southwestern channel wall for over 2 million years, migrating up to 35 km laterally as its bed was tilted by the persistent tectonic uplift of the Cape Fear Arch. When combined with general regional uplift, a series of expansive and distinct unpaired fluvial terraces have been preserved, each created in turn during interglacial high stands of the Atlantic. This tableau provides a unique temporal index to the evolution of the coastal plain. Adjacent to the current-era flood plain is the Wando Terrace, considered by others to have formed during marine isotope stage (MIS) 5 interglacial ~80ka–135ka, based on uranium-series and amino acid methods. Continuing towards the Arch, the Socastee Terrace is considered to represent MIS 7 ~186ka–245ka, the interglacial preceding the Illinoian glaciation.

LiDAR elevation maps reveal a Socastee terrace populated with well-defined Carolina bays. Likewise, the successively older Penholoway, Waccamaw and Bear Bluff Terraces are rich in bays. In striking contrast, the LiDAR shows the Wando Terrace to be indisputably devoid of Carolina bay landforms. The absence of bays on the MIS 5 surface suggests a minimum date constraint for bay formation of ~135ka. Younger dates identified in the literature may depict reworking, not initial bay genesis. This finding is problematic for the gradualist theory, as the Wando surface should have offered a viable nursery during the last Glacial Maximum (MIS 2). The temporal data implies that if Carolina bays were created during a cataclysmic event, it must have occurred before MIS 5 and after MIS 7, thus during MIS 6, the Illinoian glaciation ~135ka–186ka.