ZONE OF POTENTIAL ROCK SLOPE INSTABILITY IN WATAUGA COUNTY, NORTH CAROLINA

The North Carolina Geological Survey began landslide hazard mapping in Watauga County in September 2006. One focus of the mapping was to evaluate the extent of the potential for rock and weathered-rock slides evidenced by unstable rock slope excavations observed along the Linville Falls shear zone (LFSZ) in the Town of Boone (Loren Raymond, personal communications, 2005; Trigon Engineering Steep Slope Map of Boone, 2006).

Our investigation confirmed that unstable rock slope excavations lie within a west-northwest striking, 14 km long, 0.5 km wide zone that straddles the northern contact between the Grandfather Mountain Window and the LFSZ. Here, northerly striking, steeply dipping, low grade metasediments and metavolcanics of Late Proterozoic Grandfather Mountain Formation (Zgmf) underlie a high relief area bounded to the north by a low relief area of gently north-dipping, protomylonitic, Middle Proterozoic Cranberry Gneiss. Field investigations assessed the history of past slope failure, degree of weathering, degree of mylonitization and brittle deformation, and fracture intensity. Rock fabrics common to this zone include ductile mylonites and phyllonites transitional with, and overprinted by, cataclastic and brecciated phyllonite, and steeply dipping, fault-parallel, slickensided surfaces, and other closely spaced fractures. This zone also parallels the trace of a >180 km long regional lineament.

Examples of rock slides and weathered-rock slides observed in cut slopes in this zone include a rock slide southwest of U.S. Highway 321, and two weathered-rock slides along N.C. Highway 105. The rock slide near U.S. Highway 321 occurs where intersecting high-angle fractures, fault planes and a mylonitic foliation bound the detached rock block in Zgmf phyllonite, metasiltstone and metaarkose. Weathered-rock slides near N.C. Highway 105 are primarily in partially- to completely-decomposed, fractured phyllonite and mylonite of the Zgmf.