Abstract

Landsliding has historically been one of the most damaging geologic hazards in western Wasatch County, Utah. Significant historical landsliding has occurred in the Dutch Hollow, Snake Creek, and Lake Creek drainages and in upper Provo Canyon. As part of a geologic-hazards mapping program, we have mapped 361 slope failures in a 250-square-mile (650-km²) study area. The landslides involve 20 geologic units, including Mississippian to Tertiary sedimentary rocks, Tertiary extrusive and intrusive igneous rocks, and unconsolidated Quaternary glacial and alluvial deposits. Geologic unit, slope inclination, and age of existing landslides are the most important factors for defining relative landslide hazard in western Wasatch County. Based on morphology, 79 percent of the landslides are late Holocene. Eighty-three percent of the late Holocene landslides are shallow debris slides and 17 percent are deep-seated slumps. Pleistocene alluvium, Pleistocene alluvial-fan deposits, and the Permian-Pennsylvanian Oquirrh Formation and Weber Sandstone are particularly susceptible to debris sliding, whereas Pleistocene glacial deposits, the Tertiary Keetley Volcanics, and the Pennsylvanian-Mississippian Manning Canyon Shale are particularly susceptible to slumping. The Tertiary Keetley Volcanics and Pleistocene glacial deposits are susceptible to both deep-seated slumping and shallow debris sliding.

We used slope inclinations of late Holocene landslides, which we infer occurred under climatic conditions similar to the present, to develop criteria for delineating areas of relative landslide hazard. We determined that the minimum critical slope inclination above which late Holocene landsliding has typically occurred varies for individual geologic units and ranges from 15 percent to 50 percent (9°-27°). Approximately 85 percent of existing landslides in the study area have slopes steeper than the determined critical slopes, and are therefore susceptible to reactivated movement.

As the county’s growing population expands into previously undeveloped areas, further potentially damaging landsliding can be expected. We are using the results of this study to compile landslide-hazard maps for land-use planning. Site-specific hazard evaluations will still be necessary, however, to address local factors affecting slope stability. The results of this study indicate there is a strong correlation among geologic material, mode of slope failure, and slope inclination that must be considered in site-specific studies for effective landslide-hazard reduction in western Wasatch County.