PRELIMINARY GEOLOGIC MAP OF PRE-CRETACEOUS ROCKS BENEATH THE MID-ATLANTIC COASTAL PLAIN, VA-MD-DE; INTERPRETATION OF GEOLOGIC, AEROMAGNETIC, AND GRAVITY DATA

Piedmont lithotectonic units and structures of the Roanoke Rapids, Goochland, Chopawamsic, and Potomac terranes, Baltimore Complex, Petersburg Granite, and Taylorsville basin extend beneath coastal-plain sediments and are mapped using borehole, aeromagnetic, and gravity data. Farther east, the arcuate Sussex terrane contains mafic and ultramafic rocks that coincide with high-amplitude magnetic and positive gravity anomalies. The Sussex terrane is part of a crustal boundary (suture?) zone separating NE-trending magnetic anomalies of extended units to the west from a pattern of swirling, partly NW-trending anomalies of terranes to the east that have no surface exposures. From the Sussex terrane eastward to Chesapeake Bay, greenschist-facies metavolcanic and granitoid rocks of Neoproterozoic age resemble those of the Roanoke Rapids and other peri-Gondwanan volcanic-arc terranes. Earlier concepts of a “Chesapeake terrane” lumped these Neoproterozoic magmatic rocks in eastern Virginia with unrelated Mesoproterozoic (Grenvillian?) basement at Cape May, N.J. The Chesapeake Bay impact structure straddles a boundary between the greenschist-facies Neoproterozoic magmatic rocks and amphibolite-facies schists and gneisses to the east. The age and stratigraphic correlation of graphitic sillimanite-mica schist and calc-silicate gneiss from southern Delmarva Peninsula are undetermined. Subgreenschist-facies gray and black shale clasts in impact breccias are derived from an unknown Paleozoic(?) sequence. Circular to elliptical gravity lows commonly represent granitoid plutons, as confirmed by a geothermal test well at Portsmouth, Va. Magnetic rims around a few circular gravity lows suggest chill margins or contact aureoles. Part of the Taylorsville rift basin correlates with a gravity low and long-wavelength magnetic anomalies, consistent with a thick sedimentary section. However, the buried early Mesozoic Queen Anne, Toano, and Norfolk rift basins lack distinct magnetic and gravity anomalies, and are variably constrained by limited boreholes and/or seismic data. Steep faults offset the top of pre-Cretaceous basement and coastal-plain units overlying buried extensions of the Spotsylvania and Hylas fault zones, as well as lithotectonic boundaries to the east, suggesting tectonic heredity.