RECENT ADVANCES OF GEOLOGIC MAPPING IN THE VIRGINIA COASTAL PLAIN

The Virginia Division of Geology and Mineral Resources has completed new geologic mapping between Richmond and Williamsburg over the past several years. The National Cooperative Geologic Mapping Act funding partially has supported our work.

In the Richmond area we have extended one reverse fault (up on the east side) in the Dutch Gap fault zone several miles north of Virginia Route 10. In Richmond where the James River becomes tidal and turns to the south, numerous high-angle and north-trending brittle faults are found in the Petersburg granite. Together, these faults are likely responsible for the abrupt change in direction of the James River at Richmond. Eastover Fm. clays are now known to extend at least to the west side of Richmond. The contact between the Yorktown and Eastover is obscure in the Williamsburg 30- by 60-minute quadrangle as these sediments in borings are rarely fossiliferous. The upper sandy beds of the Eastover Fm. become thinner to the west until they are absent in the Providence Forge area; there and to the west, we are able to map a true lithologic contact between the Eastover and overlying Yorktown formations. Marine Yorktown sands are absent in the western part of the Dutch Gap quadrangle so that the lower Bacons Castle gravel sits directly upon Eastover clays there and westward to the James River. To the north, marine Yorktown sands grade westward to correlative fluvial-estuarine sediments mapped as Tpsg on the 1993 Geologic Map of Virginia. In the Richmond-east area, the marine-estuarine sands are sandwiched between gravelly sediments, characterized by abundant heavy minerals, and correlate to mined sediments at the Old Hickory and other heavy mineral deposits in Dinwiddie and Sussex counties. In the eastern part of our mapping and lying above marine Yorktown sediments is a widespread and thin sheet of downward coarsening sandy mud that we currently correlate with the Sedley Fm. In the Gloucester area these sediments have been displaced and thickened by outer-rim growth faults related to the Chesapeake Bay Impact Crater. Fluvial-estuarine sediments of Late Pliocene and Pleistocene alloformations maintain a very close correlation to morphology as portrayed on the smaller-scale 1993 Geologic Map of Virginia. Our geologic maps now include delineation of sediments in the bottoms of major tidal rivers.