DIGITAL TECHNIQUES APPLIED TO NEW MAPPING IN THE VIRGINIA PIEDMONT

New geologic mapping along the eastern edge of the Chopawamsic terrane in central Virginia is proceeding using a variety of digital mapping techniques. Field stations and contacts are screen-digitized on georegistered digital raster graphs (DRGs) of topographic maps using AutoCAD Map® software. Field observations are stored in an MS Access® database and linked by station number to digitized point locations. Customized Visual Basic® programs allow automated plotting of attributed point data from other databases (e.g., archival sample analyses) and from other investigators. Raster images of historic geologic maps from the early to mid-20^th century at a variety of scales are presented at a uniform scale to allow work to focus on areas of need. Scanned and digitized aeromagnetic maps are being used to trace geophysical anomalies and relate them to geologic features. This integrated approach has shed new light on a number of geologic problems, including the nature of the terrane boundary. The Spotsylvania lineament, a sublinear geophysical anomaly associated with a major ductile fault zone in northern Virginia, can be traced southwestward into central Virginia along the eastern edge of the Chopawamsic belt. The rocks in this zone, locally mapped decades ago as "Wissahickon Formation", "biotite schist" and "augen gneiss", are now recognized as mylonites and porphyroclastic high-strain gneisses. The Elk Hill Complex of Brown (1937) is now interpreted to be a thrust slice of metavolcanic rocks of uncertain affinity, fault-separated from the Chopawamsic terrane. Eastward, ductile fault rocks are interleaved with map-scale bands of sillimanite-grade metapelitic rocks. Digital technology has expedited the integration of multiple data types and will enhance the utility of map products forthcoming from this project.