GEOLOGIC FEATURES RELATED TO RECENT DEBRIS FLOWS AND WEATHERED-ROCK SLIDES IN THE BLUE RIDGE AND PIEDMONT OF NORTH CAROLINA

Geologic information collected for the N. C. Geological Survey's slope movement database, and geologic mapping confirms that certain geologic features play important roles in debris flows and weathered-rock slides. At the site scale, structural and lithologic discontinuities influence pathways for infiltration, groundwater seepage, differential weathering, and the development of colluvial catchments. At local scales, geologic structures influence slope angle and hillslope shape. Attention to these features improves the value of geologic and geomorphic maps used to develop predictive models of slope movement. Fracture, bedding and foliation planes are preferential groundwater pathways in metamorphic rocks of the Blue Ridge and sedimentary rocks of the Deep River basin. These discontinuities form pathways leading to the pore pressure build-up that triggers weathered-rock slides, and mobilizes colluvium into debris flows. In plane- and wedge-failure rock slides, differential weathering creates permeability contrasts that lead to increased pore pressures along clay-rich slip surfaces that range in dip from about 12 to 40 degrees. High-angle fractures form separation surfaces and infiltration pathways between slide blocks and contribute to the self-perpetuating nature of weathered-rock slides. Increased fracture intensity near faults in the Deep River basin may play an important role in decreasing slope stability. Intersecting bedrock discontinuities form wedge-shaped accumulation zones for colluvium and deliver bedrock groundwater to the colluvium-bedrock interface at debris flow scarps. At local scales, identifying dip and front slopes are important. In Transylvania County, front slopes formed by dipping high-grade metamorphic rocks generally have greater slope angles than companion dip slopes and appear more susceptible to slope movements. These slopes are identifiable in CIR aerial photography. Analogous hillslopes with weathered-rock slides occur in sedimentary rocks of the Wadesboro basin. Dip slope failures coincident with SE-dipping rocks have long been recognized in the Durham basin. Recent 1:24,000-scale and detailed geologic mapping identified NE-dipping rocks in cross-structures within the Durham basin, and sheds light on combinations of lithofacies and structures susceptible to sliding.