EVIDENCE FOR BASEMENT-ROOTED COASTAL PLAIN FAULTS IN THE POTOMAC RIVER VALLEY AND CENTRAL CHESAPEAKE BAY REGION

Marine and land-based seismic, core, and borehole data along the Atlantic Coastal Plain of the Potomac River Valley and central Chesapeake Bay reveal numerous faults that vertically offset the top of crystalline basement by 6 to 80 m, and most of these faults offset the overlying coastal-plain sediments from ~1 to 30 m. Many Cretaceous and Cenozoic high-angle reverse faults, normal faults, and folds are rooted in early Mesozoic and Paleozoic NE-trending fault zones that underlie this part of the coastal plain and produce horst and graben structures that coincide with interpreted basement structures and terrane boundaries. Many faults dip NW, a few strike NW, and some dip SE. The Stafford fault system (SFS) is a series of en echelon, NW dipping, high-angle reverse faults having vertical offsets up to 61 m, a subsidiary dextral component, and right-oblique as well as down-dip striations. The western part of the SFS coincides with the Spotsylvania high-strain zone, that separates the Chopawamsic and Goochland terranes, and locally coincides with the Fall Zone. A map of top of the crystalline basement suggests a ~30 km northward extension of the Stafford fault system (SFS) into Washington, D.C. and similar maps of the Tertiary suggest several faults are between the SFS and the Brandywine fault zone (BFZ), a closely spaced series of NW and SE dipping, high-angle reverse and normal faults. The BFZ coincides with the western boundary fault of the Taylorsville Triassic basin and the eastern margin of the Goochland terrane. Marine seismic data along the Potomac River and across the Chesapeake Bay reveal faults with similar styles and displacements from the SFS eastward to mouth of the Potomac River. Beneath Chesapeake Bay, the faults appear to be less abundant and have smaller vertical displacements across a gravity low interpreted as a granite pluton. Another concentration of faults coincides with the Sussex terrane boundaries. Recurrent Cretaceous and Tertiary fault movements in the coastal plain are evident from upward-decreasing offsets (top of basement up to 80 m, top of Paleocene up to 30 m). The latest movements produced small offsets (1-7 m) of Pliocene and Pleistocene terrace deposits, affected the distribution and preservation of stratigraphic units, and have modern geomorphic surficial expression.