GEOLOGIC MAP OF THE COLUMBIA, CALEDONIA, AND SOUTH ANNA 7.5-MINUTE QUADRANGLES, CENTRAL VIRGINIA PIEDMONT - LEGACY MAPS COMPILED AS A GEMS GEODATABASE

Over a century of geoscience research and mapping in the central Appalachian Piedmont has defined terranes, delineated major faults, and established the timing of tectonic events. Details of structure and stratigraphy, however, remain to be resolved through detailed mapping of areas formerly covered only by reconnaissance maps. The Columbia, Caledonia, and South Anna 7.5-minute quadrangles form a west-to-east strip that covers over 450 square kilometers (175 square miles) near the center of the Central Virginia Seismic Zone (CVSZ). Draft geologic maps of these quadrangles were issued by Virginia’s geological survey as separate open-file reports between 2005 and 2016. The maps are now being updated based on recent geochronology, airborne geophysics, and LiDAR, and are being compiled into a single geodatabase using the Geologic Map Schema (GeMS). GeMS, the national standard for digital geologic maps in the USA, offers significant advantages for digital geologic map compilations. The geodatabase includes feature-level metadata for all structure symbols, faults, contacts, and polygons, including source attribution. The mapping resolves a number of questions that previously had only speculative answers. For example, the Carysbrook and Columbia plutons, interpreted by some to be separate plutons, are shown here to have a gradational contact. The structurally complex Quantico-Columbia synclinorium does not have a simple southern terminus near the James River, as depicted on the 1993 Geologic Map of Virginia, but folds back on itself in a secondary synform to form a highly compressed, narrow, completely inverted eastern limb. The map covers the densest concentration of abandoned gold mines in the Gold-pyrite belt and demonstrates that they are aligned near the contact between the Chopawamsic and Quantico formations along the western limb of the synclinorium. The Pegmatite belt, a fault-bounded block of K-feldpsar-rich layered gneisses, terminates in a northeast-plunging antiform. The Elk Hill Complex, composed primarily of amphibolite and mica-rich gneiss, extends northeastward beyond the quadrangle boundaries. Presented together, these maps offer a comprehensive view of the geology of part of the central Virginia piedmont and provide a framework for interpreting the source of seismicity in the CVSZ.