PROGRESS OF COASTAL PLAIN GEOLOGIC MAPPING FROM RICHMOND TO THE VIRGINIA EASTERN SHORE

New geologic mapping in the Virginia Coastal Plain since 2009, partly funded by STATEMAP, focused on the Williamsburg 100k quadrangle, I-64 corridor, and the Richmond area. Hundreds of auger borings throughout the James-York Peninsula resulted in the identification of new faults and lithostratigraphic discoveries.

The Dutch Gap Fault was extended northward to just south of I-64. Max throw is about 80 feet with down-to-the-west movement that was syn-depositional to the Bacons Castle Formation. This fault may partially account for the north-south orientation of the James River east of Richmond. The Malvern Hill Fault, has a max throw of 40 feet (down-to-the-east movement) and extends from Malvern Hill to about 6 miles north of Bottoms Bridge. En echelon and cross-faults add to overall complexity. Faulting is syn-depositional to the Yorktown and Bacons Castle formations. The Providence Forge Fault has a max throw of 130 feet (down-to-the-east movement). It has been traced from the James River to the Pamunkey River. We do not know the dip orientations of these fault planes; however, repetition of section in a few borings would suggest reverse faulting. Numerous normal faults have been found within the vicinity of the outer rim of the Chesapeake Bay Impact Crater (Gloucester and York County, and Newport News). Movement on these faults range from the late Pliocene (syn-depositional to Yorktown) through early Pleistocene.

A new Pleistocene terrace, the Elsing Green, is bounded by a scarp toe of about 38 feet, has flats ranging between 28 and 35 feet and has been dated by using ¹⁰Be (132 ka) and amino acid (125 ka) age techniques. The Moore House Member of the Yorktown Formation has been mapped from its type section westward into Surry, Charles City and New Kent counties. In the Richmond area, Pliocene fluvial sand and gravel overlie nearshore marine sands and clay (Tpsg and psg on regional maps); this sequence has been associated with the marine Yorktown sediments to the east. Lower marginal marine sediments everywhere overlie the Eastover and Yorktown and below the Bacons Castle, extending eastward to James City County. These slightly muddy fine- to coarse-grained sands and lenticular-bedded clays within Tpsg are named the Cold Harbor Formation; they range up to 35 feet thick and contain a consistent but low grade (1-2%) of heavy minerals.