Quartzite terrains, geologic controls, and basin denudation by debris flows: their role in long-term landscape evolution in the central Appalachians

A large storm in 1995 that impacted the central Blue Ridge Mountains of Virginia triggered over a thousand debris flows, which deeply incised stream channels in zero-, first-, and second-order basins underlain primarily by granite, metabasalt, and quartzite. This event provided an opportunity to gain insight into geologic controls on debris-flow initiation and rates of basin denudation. Intensive investigations in quartzite terrain indicated that well-developed joints provide for rapid infiltration of rainwater and thus affected debris-flow initiation. Possible mechanisms of slope failure include (1) elevated rates of fracture recharge in steep valleys and/or (2) bedrock bedding planes. Fracture recharge may serve to substantially increase rock pore pressure during high intensity rainfall, especially when coupled with antecedent moisture. The quartzite bedrock tends to break into orthogonal blocks due to jointing. Statistical analysis shows the presence of a minimum of two joint populations that serve as bedrock controls on lateral erosion. Resistant bedding planes, parallel to slope, control vertical erosion below a finer-grained layer of saprolite. The combination of increased recharge and joint orientations in quartzite basins are likely the main factors resulting in the highest measured values of basin denudation from debris flows, relative to other lithologies, in the central Appalachians. Additionally, these joint and bedding planes produce a topographic signature at all scales and, therefore, are an important factor controlling long-term landscape evolution.