STRATIGRAPHY AND STRUCTURAL EVOLUTION OF LYNCHBURG GROUP ROCKS IN THE AMHERST 7.5-MINUTE QUADRANGLE, VIRGINIA

Detailed mapping within the Amherst 7.5-minute quadrangle has recently been completed through the Virginia Division of Geology and Mineral Resources. The entire width of the Neoproterozoic Lynchburg Group (LG) is well exposed in the southeastern half of the quadrangle. Lithologic map units of the LG consist of uppermost volcaniclastic metagraywacke, cross-biotite phyllite, cross-biotite graphitic slate, quartzite, metagraywacke, and a lowest most sequence of metagraywacke, phyllite, and mafic and felsic meta-igneous rocks. These units correlate well with regional stratigraphic models proposed by Nelson (1962), Wehr (1983), and Wang (1991). Interlayered volcanic and intrusive felsic, mafic, ultramafic, and ultrabasic meta-igneous rocks occur throughout the LG, but are most abundant in the lowest most sequence. These igneous rocks contain riebeckite, associating them with plutons of the Neoproterozoic Crossnore Plutonic Suite, and provide a minimum age of ca. 680 Ma for deposition of the LG.

The LG experienced multiple periods of deformation, metamorphism, foliation development, and faulting during the Neoproterozoic to Paleozoic. Earliest foliations in LG rocks parallel penetrative ductile foliation in Mesoproterozoic basement rocks along and near the contact, and may be Neoproterozoic in age. Early Paleozoic deformation consists of a penetrative foliation defined by upper greenschist- to epidote-amphibolite-facies mineral assemblages, and a major fault that separates the stratigraphic package into an upper and lower sequence. West of the fault, rocks were deformed by at least three penetrative events, whereas rocks east of the fault are affected by two events. Dominant overprinting lower greenschist-facies assemblages, foliation, and splay-faults in the easternmost LG may be Middle Paleozoic in age, and associated with dextral-reverse motion along the Bowens Creek fault to the east, synchronous with docking of the Smith River allochthon and Carolina terrane farther east.