GEOSPATIALLY REFERENCED AND GROUND-TRUTHED HOLOCENE ESTUARINE SEISMIC STRATIGRAPHY OF THE WESTMORELAND PARK REGION, CENTRAL POTOMAC ESTUARY, VA

The stratigraphy of Potomac River estuarine sediments off Westmoreland State Park, Virginia was spatially characterized using sub-bottom CHIRP data and sediment cores as part of a larger project to constrain Late Holocene sea level rise in the region. Specifically, our goal was to quantitatively define the Holocene sub-bottom units and modern estuarine sediments and calibrate reflector depths to local mean sea level via the USGS benchmark at Westmoreland State Park. We utilized sub-bottom CHIRP data, high-resolution bathymetry, geophysical processing, and sediment cores. Core impedance values were used to calculate reflection coefficients, and to correlate the core lithology with the acoustic record.

Five distinct continuous and high amplitude reflectors were identified in the geophysical data, from the sediment water interface (A) to acoustic basement (E). Reflectors A, B, and C, and the associated Seismic Facies 1 and 2 (SF1, SF2), represent Holocene sediments. Locally, Reflectors B and C display a hummocky appearance; Reflector B is generally horizontal and truncates into Reflector C at locations in which Reflector C reaches ~1.2m below the sediment surface. Reflectors C and D represent erosional surfaces as interpreted from down-cut features within the reflectors, and truncation of internal reflectors of the units below. SF1 and SF2 onlap the irregular surface of Reflector C, and thus represent the late Holocene transgression.

Sediment cores were recovered at select locations relative to the seismic stratigraphy in order to ground-truth the units. Core KC1 (2.8m) terminated in Seismic Facies 3 penetrating Reflectors A (0m), B (1.2m) and C (2.3m); its core catcher contained Crassostrea virginica shells in a fine-grained, well sorted quartz sand. A second core location was selected where Reflector C was close to the sediment surface, and the core reached refusal at ~1.2m. Cores KC2 (2.45m) and KC3 (3.07m) at the third location terminated in SF2 penetrating only Reflectors A (0m) and B (0.8m); its core catcher, representing SF2 contained estuarine mud with no Crassostrea virginica shells. Radiocarbon dates on bivalves sampled from the vertically-referenced reflectors facilitate timing of seismic facies transitions during past millennia.