PUBLIC DOMAIN AIRBORNE LIDAR GROUND MODEL AND GEOLOGIC MAP DATA: AN UNDER-UTILIZED RESOURCE FOR BEDROCK FRACTURE NETWORK CHARACTERIZATION

Surface images derived from LiDAR data is not a new mapping technique. However, statewide airborne LiDAR datasets – such as that in NC – are as yet an under-utilized resource for bedrock geology studies. The combination of public domain LiDAR and geologic map data, with targeted field work, offer exposure-independent constraints on bedrock fracture networks at the scale and resolution needed for assessment of geologic resources and land use planning, even where bedrock exposure is poor and vegetation cover is heavy, such as in western NC.

In the Panthertown Valley area (PV) of SW NC, bedrock geology is dominated by a km-scale, upright, open antiform of felsic gneiss overlain by biotite gneiss, gently refolded about a subvertical axis. Felsic gneiss in the fold core is incised by 0.5 km-wide valley lineaments (two follow the ~N-S and NE-SW segments of the fold axis; others have a NW-trending cross-joint geometry). Other than the exfoliation fractures forming the valley walls, fractures are rare in outcrop. Major (>1 km long) lineaments identified in a ~664 km² LiDAR ground model image (NC, 2005) form NNW and NE-trending sets and a lesser E-trending set. This encloses the PV focus area (83 km²), where abundant lineaments (<1.5 to >1130 m in length) occur in NE and NNW orientation sets as well; however, the NNW set is more abundant, and an E set is not observed.

Although orientations of these topographic- and LiDAR-derived lineament sets are similar, their geomorphic expression in PV suggests this bedrock was mechanically anisotropic during the regional fracture-forming event. Most topographic relief occurs along the fold axis where it is closest to NNW or NE, and at high angles to the fold axis where it is oriented NE. This study found no evidence of variation in mineral abundances, grain size/distribution, or rock fabric, that correlates with all the topographic lineaments. It is inferred that micro-fracturing and/or closely spaced mesoscopic fractures increased surface area and/or infiltration rates allowing these zones to weather faster. The geomorphic expression in PV could therefore be the result of pre-existing weaknesses developed during folding that localized fracturing during a later regional strain event, meaning that the bedrock fracture network in the PV area comprises NNW- and NE-trending, subvertical, fractures.