5-8 Beneath the Ellipses: Geomorphologic, GPR, and Core Analysis of a Delmarva Peninsula Carolina Bay

Session: Nearshore and Estuarine Research: Dynamics and Future Resiliency in the Coastal Zone (Posters)

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Abstract:

Carolina Bays (CBs) are shallow depressions with semicircular shapes recognized in LiDAR along the coastal plain from NJ to FL. The formation of CBs is highly debated. Proposed hypotheses include formation by extraterrestrial impacts, aeolian/lacustrine processes, and thermokarst phenomena during the last glacial period. CB ages are not well constrained. OSL dates of rim sediments range from 120-140 ka to as young as 2ka, clustering around 40-11ka. CBs range from a few 10’s of meters to a few kilometers in width with relief ranging from 2-15 meters. Depending on region, CBs have a preferred orientation with southern CBs oriented NW-SE (~N45°W). “Delmarva Bays” (DBs) are oriented NE-SW (~N50°E). Here we use GPR and shallow sediment cores to investigate a prominent DB in Virginia.

A dry, forested DB located near Wattsville, VA is  580m x 3m deep and is cut by Atlantic Road. The eastern portion of the DB is located on government land. Our preliminary research collected four GPR survey lines on Chincoteague Bay Field Station property (using a Mala X3M) across the western portion of the DB. We also extracted two 1.5-3m auger cores from the DB floor and its western rim. Sediments were analyzed for grain size and sediment properties (composition, rounding, frosting), loss on ignition, and microfossil/pollen content. 

Radargram data yielded >20 m of return through Holocene and uppermost Pleistocene Omar Formation. At least four sediment packages are identifiable below the DB. The lower portion of radargrams contain dipping reflectors inverse to the topography of the DB. These deeper reflectors contain discrete hyperbolic anomalies interpreted as either paleosol/buried forest complexes or lagoonal shell bed/reef deposits based on USGS core interpretations. The uppermost reflector, likely the water table, dips south toward the center of the DB. The presence of marine and euryhaline diatoms (@1.6-1.8 m depth) overlain by coarser sandy sediments in our cores suggests overwash events before emplacement of barrier island facies prior to LGM and before DB formation. 

Although data analysis is ongoing, these investigations add to the body of knowledge about the expression/development of DBs, and our GPR and sediment record is revealing new details about late-Pleistocene-Holocene environmental change along the Eastern Shore. 

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