COLUMNAR STRUCTURES AND THE INFLUENCE OF MULTIPLE PLANAR DISCONTINUITIES IN METAMORPHOSED RHYOLITE IN THE CENTRAL PIEDMONT OF NORTH CAROLINA

Outcrops of very fine-grained vitric metarhyolite in the Cambrian Tillery and Cid formations in the Carolina terrane of central North Carolina near the Uwharrie River locally have columnar morphologies. These greenschist facies rocks occur in southeast-verging, inclined to overturned, moderately plunging to periclinal folds. Axial planar cleavage dips steeply northwest. In some outcrops, sets of prismatic metarhyolite columns have distorted hexagonal cross-sections and are interpreted as strained colonnades. They are overprinted by cleavage and transected by tectonic joints. Mapping the extent of these primary features is essential in interpreting the pre-deformation setting of the volcanic centers in this arc, but extracting those features is challenged by multiple planar discontinuities in the host rocks. These discontinuities include variably spaced cleavage, two or more regional joint sets, systematic cracks from mechanical weathering, cleavage-guided exfoliation, unroofing surfaces, and in some rocks, planar layering. The sparse outcrops that show distinctive columnar joints generally have few overprinting planar discontinuities. Most metarhyolite exposures are on higher ridges in the survey area and, upon weathering, form extensive crack networks of planar and curviplanar fractures that may appear as parallel sets. Cleavage, tectonic joints, and cracks carve metarhyolite exposures into angular, often prismatic colluvial blocks. These blocks, however, do not typically have the distinct morphologies of columnar joints. Orientation cluster analysis of Schmidt plots for all fracture (joints, cracks, exfoliation) measurements was applied to search for groupings that might reflect the presence of columnar joints. While as many as six clusters can be identified, two being regional joints, the distribution is too fuzzy to yield useful results. In addition, the reference set of columnar joints and presumably all similar primary structures are strained and rotated by folding. Locating the extent of columnar joints in this area through fracture orientation analysis may be limited. Spatially, however, the most demonstrative columnar joint outcrops are near the base of the metarhyolite unit, where it overlies metamorphosed argillites in the Tillery Formation.