GROUNDWATER FLOW THROUGH FRACTURES IN CRYSTALLINE ROCK IN THE COWEETA GROUP AT COWEETA HYDROLOGIC LABORATORY, NORTH CAROLINA

Little is known about the distribution and geometry of bedrock fractures in crystalline rocks of the Blue Ridge Mountains of western North Carolina, though these properties likely impact groundwater resources and landslide risk. This study uses a combination of rock core description and geologic mapping to examine possible correlations between bedrock characteristics, such as foliation type and lithology, and fracturing.

The study is located within the Ball Creek drainage in the Coweeta Hydrologic Laboratory in Macon County, NC (Prentiss 7.5’ quadrangle) where the main rock unit is a migmatitic garnet biotite gneiss. Four unoriented cores are distributed along a transect approximately 1 mile long, from an elevation of 3700 to 2300 feet, and represent approximately 0.5 miles of across-strike distance. Soil and saprolite reach between 4 and 12 feet below surface, consistent with differing geomorphologic settings between core sites. Below saprolite, foliation type, inclination, composition, and fracture characteristics have been documented every 6 inches. Foliation type and compositional variations have been used to define five mappable units.

Zones of intense weathering between 2 and 12 feet long exhibiting higher fracture densities occur throughout the depth of the cores and are interpreted as zones of groundwater flow through fractures. Work to date suggests that fracture density does not appear to have a strong correlation with foliation dip amount, though changes in mineral assemblages weakly correlate with higher fracture density. During ongoing geologic mapping, outcrop fracture density is mapped and outcrops assigned to one of the defined fabric map units in order to investigate these relationships in pseudo-three dimensions.