GEOLOGIC AND GEOPHYSICAL MAPPING OF PRE-CRETACEOUS BASEMENT ROCKS BENEATH THE ATLANTIC COASTAL PLAIN, VIRGINIA TO SOUTHERN NEW JERSEY

Analyses of borehole samples and their distribution are integrated with regional aeromagnetic and gravity data to produce an interpretive geologic map of rocks beneath the Atlantic Coastal Plain from Virginia to southern New Jersey, with insights into crustal evolution, resource potential, and influence on passive-margin structure, coastal-plain aquifers, and neotectonics. A preliminary geospatial database of >400 boreholes includes basement elevations and lithology, where available, as well as sources and reliability. Maps of the aeromagnetic field and its derivatives are used to depict lithologic domains and structural features in the basement. Mafic igneous and metamorphic rocks have highly variable magnetizations, whereas their densities are uniformly high. This permits a synergy of both fields. Different filters, derivatives, shading, and color models enhance the effects of differing geological sources. For example, the aeromagnetic tilt derivative (gradient tilt angle) map locates edges, enhances subtle features by equalizing amplitudes of all anomalies, and is useful for mapping fault structures. The tilt derivative with gravity contours superimposed is particularly useful for interpreting crustal structure. Our interpretive geologic map shows Proterozoic to Paleozoic crystalline rocks and early Mesozoic rift basins beneath the Atlantic Coastal Plain in relation to borehole locations, seismic profiles, and top-of-basement structure contours. From west to east, buried extensions of Appalachian Piedmont terranes, a concave-east crustal boundary and mafic complex (Sussex terrane), predominantly greenschist-facies Neoproterozoic and younger metamorphic and plutonic rocks of the Chesapeake zone, and amphibolite-facies metamorphic and plutonic rocks of the Cape May (Mesoproterozoic?) and Hatteras (Neoproterozoic) zones are identified. Early Mesozoic rift basins in the region are poorly constrained by aeromagnetic and gravity data with local exceptions (e.g., part of Taylorsville basin) but are variably delineated by boreholes and seismic profiles. Map relations of basement units, structure contours, and coastal plain units indicate basement structural inheritance and recurrent movement on coastal plain faults, some into the Pleistocene.