A NEAR-SURFACE GRABEN ON THE DELMARVA UPLANDS NEAR CORDOVA, MD, AND ITS NEOTECTONIC IMPLICATIONS

Universal LiDAR coverage for the eastern Maryland counties reveals a previously unrecognized faulted graben. Across the “dimple and dune” topography, from St. Joseph Church to Cordova, MD, a 3m linear scarp cuts NW-SE across upland headwaters of tributaries to Tuckahoe Creek and the Wye River. A parallel 3 m relief scarp one mile to the SW bounds the interior of a linear valley more than 8 km long. GPR profiles, and augering to 30m depths on transects across the valley indicate that both scarps are near-surface faults. Faulting under the St. Joseph scarp is up on the NE and faulting under the parallel scarp is up on the SW. The scarps bound a graben filled with channel gravel of the late Tertiary Pensauken Formation. The gravel in the graben is commonly 15 m thick and an order of magnitude less on the uplifted sides of the structure. Fault control of the channel gravel is both syn- and post-depositional. The structurally controlled channel gravel deposit is a significant local aquifer for irrigation. Bore holes below the gravel indicate that earlier fault history includes the offset of underlying middle Miocene nearshore and marine deposits of the Calvert Formation.

Subsequent headward erosion of the tributaries of Tuckahoe Creek cut across the earlier fault scarp relief which has been muted by slope processes. The youngest superimposed surficial deposits and geomorphology are inset along the toes of the scarps. Deformation of these thin units is not apparent on GPR profiles.

The NW-SE trend of the graben is parallel to the dominant joint set/fracture trend encountered across the MD/VA Chesapeake Region as documented in the literature. Latest fault movement appears to pre-date the superimposed cold climate surficial deposits and features. Thus, latest movement may have occurred during MIS 3 and the initial dilation of the last regional glacial fore-bulge. Continued studies will focus on dating deposits, sharpening cross cutting relationships, and looking for deeper tectonic control. The high resolution of LiDAR may facilitate a morphosequence interpretation of origins and modifications for the geomorphology and the surficial deposits.