44-8 Beneath the Ellipse: A Sedimentological and Geophysical Investigation of a Carolina Bay on the Delmarva Peninsula, Eastern Shore, VA

Abstract:

Carolina Bays (CB) are enigmatic geomorphic depressions found along the Atlantic Coastal Plain from NY to northern FL. These circular to elliptical features have raised rims and nearly flat bottoms. Though they have been extensively studied in the Carolinas, their origin remains in dispute and few studies have focused on CBs on the Delmarva Peninsula.  

This study investigates a ~400m wide CB located at an elevation of ~10 m absl in Wattsville, VA. Bucket auger sediment cores and 250 mHz Mala X3M GPR reveal complex subsurface stratigraphy beneath the CB.

Overall, with the exception of the upper ~1m, reflectors in the GPR data do not follow the topography of the CB suggesting that the bay is a shallow feature. The deeper reflectors likely represent pre-existing surfaces, including overwash deposits, dunes, and a clay layer. A horizontal GPR reflector at 4-5 m below the bay floor coincides with a blue-gray clay horizon in our cores. This layer is interbedded with lenses of fine, oxidized sand containing large, >2cm clasts possibly indicating the presence of a paleochannel. The clay is overlain by a thick, sandy unit. Within these sediments, reflectors dip in the opposite direction to the CB topography and include foreset structures consistent with stacked dunes. 1.6-1.8 m below the rim of the CB, we found euryhaline benthic diatoms (Navicula spp. and Paralia sulcata) that are likely indicators of storm overwash. Medium-fine sandy sediments with rounded quartz pebbles further support high energy events. 

The combined sedimentologic and geophysical evidence suggest a multi-stage development history for the CB. Rising post-LGM sea levels elevated groundwater tables and the low-permeability clay and silt layers promoted ponding in upland depressions between dunes. Subsequent shallow-water wave and aeolian processes formed the characteristic CB rim. 

Our initial findings indicate that CBs on the Delmarva Peninsula preserve a more complex stratigraphic history than is evident in surface morphology alone. Future work will include micro and macrofossil identification, XRD analysis, and LOI to further constrain the depositional environments of the bay and underlying sediments. This study emphasizes the importance of subsurface investigations for evaluating models of CB origins and overall coastal evolution in the Mid-Atlantic Region.

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