Paper No. 9
Presentation Time: 8:00 AM-12:00 PM
THE GEOLOGY OF THE HOWARDSVILLE 7.5’ QUADRANGLE, WESTERN PIEDMONT AND EASTERN BLUE RIDGE PROVINCES, CENTRAL VIRGINIA
The Howardsville 7.5’ quadrangle is located on the boundary between Proterozoic Blue Ridge rocks and early Paleozoic Piedmont rocks in central Virginia . The Neoproterozoic Lynchburg Group, part of the Iapetan rift sequence, crops out in the northwest corner of the quadrangle and includes metamorphosed arkosic sandstones. Metagabbro intrudes the Lynchburg Group. Metabasalt and metasedimentary rocks of the Catoctin Formation overlie the Lynchburg Group. Candler Formation phyllites of Cambrian age overlie the Catoctin. Rocks of the western Piedmont, which are separated from the Blue Ridge sequence by the Bowens Creek Fault, include interbedded meta-arkose, meta quartz wacke and meta felspathic wacke. The Mesozoic Scottsville and Midway Mills basins, half-grabens formed during the rifting of Pangea, overlie the Blue Ridge and Piedmont units. The larger Scottsville basin is bounded in the west by a normal fault and is typically composed of pebble to boulder conglomerate with a fine-grained red sandstone matrix. Alluvium deposited by the James and Rockfish Rivers overlies bedrock at the surface.
Structurally, the eastern Blue Ridge is a NE-SW striking sequence with foliations dipping primarily to the southeast. Metabasalt outliers dipping both SE and NW occur southeast of the Blue Ridge sequence. Bedding within the Scottsville and Midway Mills basins dips shallowly NW. Foliations within the metasedimentary rocks in the Piedmont dip SE. Crenulation and shear banding are present in micaceous layers, and NW-SE striking joints are typically subvertical in both provinces.
Previous studies have identified rocks both to the northwest and southeast of the James River as Candler Formation. One purpose of this study is to distinguish the metasedimentary rock units in the Blue Ridge and Piedmont provinces. Phyllites within the Candler Formation in the eastern Blue Ridge are composed primarily of white mica, chlorite, and quartz, and sometimes chloritoid. Metasedimentary rocks in the western Piedmont are meta-arkoses and wackes interbedded with thin layers (~1 mm) of white mica.