LITHOLOGIC AND THICKNESS VARIATIONS IN THE MIOCENE-PLIOCENE YORKTOWN FORMATION, INNER COASTAL PLAIN VIRGINIA

The Virginia Division of Geology and Mineral Resources (DGMR) is using subsurface borehole data and 3D modeling to gain an understanding of mostly-concealed structure and stratigraphy along I-64 between Richmond and Williamsburg. This work has improved our understanding of the nature and variability of the Yorktown Formation, a fossil-rich, shallow marine transgressive-regressive sequence. Modeled variations in subsurface elevation and thickness have revealed the presence of two previously-unknown, down-to-the-east, high-angle faults. The Yorktown Formation generally exhibits a consistent eastward thickening across the coastal plain, except in the vicinity of these faults, where thickness departs from the regional trend. The Providence Forge and Malvern Hill faults both display stratigraphic thickening of the Yorktown Formation in the downthrown block, indicating that fault motion was syndepositional with sedimentation during Yorktown time. The lithologic character of the Yorktown Formation also changes near the faults. West of Williamsburg, the Yorktown Formation is primarily a shelly, fine-to medium grained quartz sand (Rushmere and Sunken Meadow Members) and reaches a maximum thickness of 125 feet at Providence Forge, Virginia. West of the faults, the Yorktown Formation becomes a medium- to coarse-grained sand with abundant Ophiomorphia and other burrows, with some flaser bedding; thickness ranges from 5- to 15-feet. Further west near Mechanicsville, the Yorktown Formation consists of a basal muddy conglomerate of 1- to 2-inch diameter pebbles overlain by lenticular to wavy bedded, clayey, silty sands or fine-to-medium grained sand with a consistent abundance of heavy minerals, up to 70 feet thick. This sequence is overlain by fluvial muddy sand and gravel, representing a marine regression at the end of Yorktown deposition.