GEOLOGIC MAPPING IN THE INNER COASTAL PLAIN OF VIRGINIA: DISCOVERY OF NEW, REGIONALLY SIGNIFICANT CENOZOIC FAULTS

The Virginia Division of Geology and Mineral Resources is conducting new geologic mapping in the Coastal Plain east of Richmond. The mapping is part of a long-term STATEMAP project focused on the Richmond Metropolitan Statistical Area (MSA). In the Providence Forge, Roxbury and Tunstall 7.5-minute quadrangles, recent work reveals several down-to-the-east faults. They are covered by Late Pliocene Bacons Castle Formation sediments which appear to be undisturbed. The faults trend north-northeast and are syndepositional with the Yorktown Formation, based on significant thickening of the Yorktown on the downthrown side. Although we have not yet documented the dip of the faults within the map area, the Stafford and Brandywine fault systems to the northwest and Dutch Gap and Old Hickory faults zones to the southwest are high-angle reverse faults. Borehole data indicate that displacement along the faults ranges from 15 to over 120 feet and is often not uniform along the extent of a fault. The Providence Forge Fault, which extends across the entire lengths of both Providence Forge and Tunstall quadrangles, coincides with previously published gravity and magnetic anomalies, aligning with the western margin of a possible buried Mesozoic basin. The Malvern Hill Fault, located on the western side of the Roxbury quadrangle, shows much less displacement than the Providence Forge Fault. Detailed field mapping in this area is important because these faults and others yet to be discovered may affect groundwater flow dynamics (and models) in regionally significant aquifers in sediments above the Potomac Formation.