GEOLOGIC MAPPING FOR RECOGNITION AND CLASSIFICATION OF SLOPE MOVEMENTS IN THE LOWER GREYS RIVER VALLEY, LINCOLN COUNTY, WYOMING

The Greys River of western Wyoming is characterized by a rugged strike-parallel valley and is located in the eastern Idaho/Wyoming overthrust belt. This portion of Wyoming is characterized by numerous mass movements of differing ages. Recent movements are of critical importance because they have a demonstrated capability of disrupting surface transportation corridors. Heavily traveled highways along the nearby Snake and Hoback River valleys, for example, have suffered major slope-movement related problems. The surficial geology of portions of three 1:24,000-scale quadrangles comprising the lower Greys River drainage basin was mapped in order to delineate and classify existing mass movements and to develop slope movement susceptibility maps for the area. Geologic mapping, coupled with aerial photo analysis identified the various types, styles, ages and stabilities of existing slope movements based on their geologic and geomorphic characteristics. The classification system used is a composite of several previously published classifications. Each slope movement category provides information on the style, composition, type of movement, relative age and stability of slope. For example, one such mapping category is a “Composite Debris-laden Earth Slide/Earth Flow, DL II”. “DL” refers to a dormant-relict movement and “II” indicates a slope with inactive movements. An analysis of the mapped quadrangles indicates that 60% of the area has experienced mass movements. Three key indicators of slope stability were identified. These include bedrock type, steepness of slope and existence of previous mass movements on the slope. Using these three characteristics, a statistical analysis was performed to create a series of 1:24,000-scale “Slope Movement Susceptibility Maps” for the lower Greys River drainage basin. Map units on these derivative maps subdivide the region into predictive units that delineate areas of high, medium and low-probability with regard to mass movement susceptibility.