BEDROCK CONTROLS ON QUATERNARY DEBRIS DEPOSIT MORPHOLOGY, COMPOSITION AND PROCESSES: SWANNANOA MOUNTAINS, OTEEN AND BLACK MOUNTAIN QUADRANGLES, BUNCOMBE COUNTY, NORTH CAROLINA

Quaternary slope movement deposits on the north flanks of the Swannanoa Mountains differ in morphology, debris composition, and process mechanisms from those on the south flanks. We examined bedrock controls on these attributes using landslide and bedrock GIS geodatabases, 1:24,000-scale bedrock maps, stereonet plots, and LiDAR digital elevation models to aid in data analysis.

The Swannanoa Mountains are bounded to the north by the WSW-ENE Swannanoa lineament, and to the south by the WNW-ESE Gashes-Garren Creek lineament. Upper amphibolite facies, Neoproterozoic to Cambrian rocks of the Ashe Metamorphic Suite underlie the area, and include metaconglomerate, metagraywacke, schistose metagraywacke, (sillimanite) garnet-mica schist, amphibolite, and altered ultramafic bodies. The same bedrock map units occur on both flanks, except for a metaconglomerate unit on the south flanks. Foliation orientations define an upright, NE-trending (F2?) antiform that transects the area. Although SE-dipping foliation planes dominate, outcrop scale folds result in variable dip and scarp slope conditions. A dominant fracture population subparallels the Gashes-Garren Creek lineament, with SE-dipping fractures prevalent on the south flanks.

On the north flanks, elongate (length-width ratios 2.4 to 4.7), fan-like deposits occupy footslope coves. Geomorphic surfaces within fans indicate debris flow deposits of varying ages, as do matrix colors and weathering differences of bedrock clasts. Fan source areas are colluvial hollows at the heads of tributary channels with orientations similar to the strike of foliation and fracture planes. On the south flanks broad (length-width ratios 0.5 to 0.8), footslope accumulations are characterized by clast-supported, rock block and boulder deposits. Linear and arcuate scarps are visible in the rough-textured deposit surfaces. Source areas for these composite deposits are 100-830m-long cliff line segments that subparallel dominant foliation and fracture orientations. Rock fall-slide and debris slide processes dominate over debris flow activity. Average slope gradients and convex slope profiles of south flank source areas contrast with lower gradients and concave and planar profiles on north flanks, indicating that the north flanks are geomorphically more mature.