DETECTION OF FAULTS AND FAULT TRACES IN THE SHENANDOAH VALLEY, VIRGINIA USING LIDAR IMAGERY

A limited area of LIDAR imagery taken with 5-m cell-size spacing in the eastern Shenandoah Valley reveals a linear trace near the town of Harriston in central Virginia. The shaded relief image produced from the LIDAR shows a linear trace that passes northwestward (N20W) from Paine Run for at least 6 km across Tertiary and Quaternary alluvium. The alluvium overlies lower Paleozoic Shady Dolomite and Waynesboro Formations. These rocks are largely weathered to saprolite, having undergone extensive dissolution; numerous sinkholes have propagated upward through the alluvium. At one location, sheared saprolite of the Waynesboro Formation suggests a vertical fault; the saprolite is extensively oxidized and water seeps are present in the outcrop. We interpret the linear trace to be a fault (called here the Harriston fault) of unknown age. The fault could be of Paleozoic or Mesozoic age and its surface trace a result of subsidence caused by ground water circulation along the fault in the underlying karst. Alternatively, the fault could be much younger and topographic expression in the alluvium a result of Pleistocene or younger movement. The linear patterns in the LIDAR imagery suggest a compressional stepover in a left-lateral strike-slip fault system. The northwest direction of the Harriston fault trace and steep orientation of the observed fault surface are consistent with focal mechanisms of monitored earthquakes deeper than 8 km in the central Virginia seismic zone. The LIDAR imagery should be a useful tool in the Shenandoah Valley to delineate faults and for exploration of ground water along buried faults, and detection of potential pollution problems, a concern for agriculture and industry.