T108 Geologic Maps, Digital Geologic Maps, and Derivatives from Geologic and Geophysical Maps (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

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

Paper number: T108-22
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

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.