MICROSTRUCTURE, STRAIN, AND QUARTZ CRYSTALLOGRAPHIC PREFERRED ORIENTATION FABRICS OF THE ANTIETAM FORMATION, CENTRAL APPALACHIANS, VIRGINIA

The Antietam Formation is a late Precambrian to Early Cambrian orthoquartize exposed along the strike of the Appalachians, stretching from Pennsylvania to Tennessee. It consists of 95% quartz in mainly structureless, massive beds, marked by abundant Scolithus ichnofossils oriented orthogonal to bedding, and represents a nearshore, shallow marine platform sediment reworked from beach sands. Little previous work has examined its detailed deformational history. In central Virginia, the Antietam Formation is exposed in a large north-plunging fold in the Blue Ridge anticlinorium likely formed during Alleghanian convergence. Microstructural observations reveal features typical of lower greenschist facies deformation, including deformation lamellae, extended and folded rutile needles, undulose extinction, and a variably developed grain-shape preferred orientation. Some original detrital grain boundaries are indicated by either rounded shapes or dusty rims. However, most boundaries are bulged or serrated, consistent with grain boundary bulging recrystallization following dislocation creep deformation. We have performed strain analysis using deformed rutile needles within the plastically deformed quartz. These observations yield axial ratios within the XZ plane reaching 1.5. Both layer parallel and layer perpendicular shortening are observed, varying with sample position within the anticline. Quartz crystallographic preferred orientation fabrics (CPO) were obtained using electron backscatter diffraction. Preliminary results indicate a type I cross-girdle pattern, typical of predominantly coaxial deformation. Quartz opening angle thermometry indicates deformation temperatures of 325-350°C. Taken together, our observations suggest the Antietam Formation preserves modest coaxial strains developed primarily by dislocation processes as the unit was folded during Appalachian orogenesis.