ADVANCES IN STRATIGRAPHY AND STRUCTURE AT THE FALL ZONE, RICHMOND, VIRGINIA

We have begun a multi-year project in the Richmond metropolitan area through the AASG-USGS STATEMAP Program. Our goal is to produce and compile detailed 1:24K-scale geologic maps along I-64 and I-95 and in other high-growth areas, for mineral resource identification and to assist in land-use planning. We are currently re-examining previously published geology on the Bon Air, Richmond, and Seven Pines 7.5-minute quadrangles (Daniels, 1974; Goodwin, 1980). Our work has been facilitated by an abundance of new exposures recently created by Tropical Storm Isabel and Tropical Depression Gaston.

Coastal Plain stratigraphy ranges from Cretaceous to Holocene, and eastern Piedmont rocks consist of mostly Mississippian Petersburg Granite. New geologic synthesis of the stratigraphy has lead to a subdivision of previously mapped units resulting in (1) an arkose unit of unknown age, (2) sediments that are believed to be near shore equivalents of the classic marine Yorktown Formation of Early Pliocene age, and (3) sediments of the Bacons Castle Formation of Late Pliocene age. The arkose unit typically consists of a quartz and feldspar sand and granule matrix containing scattered and well-rounded quartz pebbles and cobbles. The feldspar in this unit is variably weathered to clay. The arkose is variably indurated and ranges up to 10 feet in thickness; where it has been found, it always overlies the Petersburg Granite. Near shore Yorktown sediments consist of an upper clayey sand and gravel unit and a lower silty clay and sand unit (with a basal gravel), which unconformably overlie Miocene silty clay (Eastover Formation). The Bacons Castle Formation consists of sandy clay overlying widespread gravel, which contains abundant Skolithos-bearing quartzite clasts.

Our updated maps include hundreds of structural measurements in the Coastal Plain sediments and in the Petersburg Granite. Coastal Plain sediments (mainly Miocene clays) contain joint sets that appear to parallel sets in the Petersburg Granite. Several joint sets in Coastal Plain sediments are oriented similarly with streams and landslide scarp faces. We speculate that joints have influenced stream patterns and landslides. This may have broad implications for geologic hazard assessment, land use and transportation planning, construction, and other environmental issues.