STRATIGRAPHIC ARCHITECTURE OF THE TIDAL POTOMAC AND RAPPAHANNOCK RIVERS ALONG THE COASTAL CLIFFS OF VIRGINIA’S HISTORIC NORTHERN NECK

The Rappahannock and Potomac Rivers are tidally influenced, oligo-mesohaline, partially mixed estuaries that empty into the central, mixed-energy portion of the Chesapeake Bay. Tidal tributaries cut the highly eroded cliffs in the region that comprise the Calvert Cliffs Fm (Miocene) and Tabb Fm (Pleistocene). An EdgeTech SB-216S was used to acquire high-resolution (4-24 kHz) Compressed High Intensity Radar Pulse (CHIRP) sub-bottom profiles to better understand the sedimentary architecture of the Holocene cover as it relates to the underlying acoustic bedrock in the context of late Holocene sea level rise.

CHIRP transects were performed in a 1 km scale grid of 100-200 m along the cliffed coasts of the Potomac River (Rosier and Mattox Creeks) and Rappahannock River (Totuskey Creek) whereas we recognize several prominent architectural elements. The Holocene cover includes laterally continuous heterolithic grey clay and oyster beds that range in depth from 0.6-2.4 m. Prograding wedges of fine sand range in thickness from 0.4-1.2 m and downlap seaward onto the heterolithic facies. The Holocene sediment cover unconformably overlies the acoustic bedrock that includes a series of v-shaped channel incisions (1.2-2.5 m) and fault bounded mounding features (1.1-13.2 m) that can be traced into the present day cliffed coastline.

The acquired inventory of architectural features appears to closely parallel the emergent coastal plain. The Pleistocene fluvial channels of low stand are incised into Pliocene-Miocene deposits. The position of the sub-bottom channels and mounds echoes a variable geomorphic setting complete with shifting fluvial channels and sand bars, typical of tide dominated deltaic, estuarine deposits. The 1-4 meters of Holocene clay and oyster deposits can be attributed to a late Holocene 1-2mm/yr sea level rise spanning the past couple of millennia.