BASEMENT-ROOTED COASTAL PLAIN FAULTS AND THEIR GEOMORPHIC EXPRESSIONS IN VIRGINIA AND MARYLAND

Industry seismic-reflection data from the Potomac River reveal the presence of numerous coastal plain fault zones that show offset on the top of the pre-Cretaceous basement rocks. Some of these faults coincide with geological boundaries within the basement, and others occur between them. Geologic maps of the basement based on boreholes, aeromagnetic, and gravity data were compared with top-of-basement structure contours, and with structure-contour and isopach maps of the Cretaceous and Cenozoic coastal plain units. Further comparison with physiographic maps indicates that the major rivers locally change direction where they cross underlying structural boundaries. For example, meander bends and deflections of the James River locally coincide with the eastern boundary of the Petersburg Granite (southward turn at Fall Zone), and with the boundaries of the Taylorsville rift basin, Sussex terrane, and Chesapeake block in the underlying basement. Major bends in the Potomac River coincide with western and eastern boundaries of the Goochland terrane, and smaller bends coincide with the boundaries of the Taylorsville rift basin, Sussex terrane, and Chesapeake block. The Rappahannock and Patuxent Rivers have similar meander loops or sharp bends where they cross structural boundaries in the basement. The Chickahominy and Choptank Rivers also locally change course where they cross proposed fault boundaries. The geomorphic expressions of these fault systems suggest the possibility that river-channel deflections in other parts of the coastal plain locally may coincide with underlying faults, including some that are rooted in the basement. The seismic data show a much more detailed and complex top-of-basement structure than that represented by structure contours based solely on borehole data.