COASTAL PLAIN FAULTS ROOTED IN CRYSTALLINE BASEMENT ACROSS THE SALISBURY EMBAYMENT, VIRGINIA AND MARYLAND

Compilation of seismic, core, and borehole data across the western part of the tectonic downwarp known as the Salisbury embayment reveals numerous faults that have top of crystalline basement offsets ranging from 6 to 80 m. Most of these faults offset the overlying Cretaceous and Tertiary coastal-plain units. Additional fault zones are interpreted where coastal-plain strata steepen abruptly in dip or show dramatic thickening, thinning, or truncation, especially where these changes are vertically stacked. Some Cretaceous and Cenozoic high-angle reverse and normal faults, and folds, are rooted in pre-Cretaceous basement structures. Most of the faults strike NE and dip NW, producing horst and graben structures across the Salisbury embayment. They demonstrate prolonged Cretaceous and Cenozoic reactivation of Paleozoic and early Mesozoic fault systems, with some faults active into the Pleistocene. Only a few faults strike NW as in the James River structural zone (up to the SW).

The western part of the Stafford fault system is spatially associated with the late Paleozoic Spotsylvania fault zone along the boundary between the Goochland and Chopawamsic terranes, and in places, coincides with the Fall Zone. The Brandywine fault zone, a series of NW- and SE-dipping, reverse and normal faults, coincides with a buried extension of the Hylas fault zone along the boundary between the Goochland terrane and Taylorsville Triassic basin. Faulting occurs within the various terranes with increased faulting associated with the eastern boundary of the Taylorsville basin and a zone along the eastern margin of the Sussex terrane and western flank of the Chesapeake block to the east. Eastward beneath Chesapeake Bay, faults decrease in apparent density and amounts of offset across a gravity low interpreted as a granite pluton.

Recurrent fault movements are evident by older strata being offset more (e.g., top of basement up to 80 m, top of Paleocene up to 30 m). These faults were active during the Cretaceous and the Tertiary. The latest movements are small, 1-7 m offsets of Pliocene and Pleistocene terrace deposits. These faults affected the distribution and preservation of stratigraphic units and have modern geomorphic expressions that locally correlate with large meanders of coastal-plain rivers.