GEOLOGY OF THE WHITE GATE, VIRGINIA, 7.5-MIN.-QUADRANGLE: REVISITING LARGE-SCALE LANDSLIDES AND RIDGE CREST SACKUNG

Large landslides of unknown origin occur in the White Gate, Virginia, 7.5-minute quadrangle. The geologic map of this quadrangle, recently completed by the Virginia Division of Geology and Mineral Resources, is one of a series of digital geologic products published in cooperation with the U.S Geological Survey STATEMAP Program. The prehistoric landslides were discovered about 20 years ago and include large-scale rock block slides, rock slumps, and ridge crest sackung or sags. Earlier studies by Schultz and Southworth in 1989 placed these features within the regional geologic framework of the Virginia Valley and Ridge Province. The largest landslides and sackung occur on dip slopes dominated by Silurian age sandstone and quartzite. Ridge crest sackung in western Giles County, Virginia, are tentatively dated Pleistocene in age, based on limited 14C and pollen ages and relative weathering. Both the sackung and landslides in the White Gate, Virginia, area are in close proximity with epicenters of historical low-level seismic events. Whisonant and Watts in 1991 did a slope stability analysis on a large rock-block slide complex northeast of the White Gate area and concluded that seismic shaking similar to the estimated 5.6 magnitude Giles County, 1897 quake would induce landslides of this size in this geologic setting. We propose that additional detailed field mapping, in concert with geophysical and subsurface analysis, should be focused on these features. Such studies could enhance our understanding of why they occur here, if they are the product of changing climate conditions, and what if anything they tell us of the more ancient seismic record in the Giles County area. The publication of the geologic map of the White Gate, Virginia 7.5-minute quadrangle provides a foundation for the reevaluation of these interesting features.