GEOLOGIC-HAZARD MAPPING IN BRYCE CANYON NATIONAL PARK, UTAH

The extraordinary geological character of Bryce Canyon National Park in southwest Utah attracts more than 2 million visitors annually. Geologic processes that shaped this dramatic landscape are still active today and can be hazardous to visitors, employees, residents, and infrastructure. To provide the National Park Service with geologic-hazard information for future park management, the Utah Geological Survey conducted a geologic-hazard investigation of a 265 square-mile (686 km2) area centered on the park. Available geologic, hydrologic, soil, and geotechnical information were used to identify where geologic hazards may exist and where site-specific geotechnical/geologic-hazard investigations are recommended to protect health, welfare, and safety. This study provides maps and information for 14 geologic hazards: rockfall, landslide, flooding/debris flow, shallow groundwater, surface faulting, liquefaction, collapsible soil, piping and erosion, wind-blown sand, soluble rock, corrosive soil and rock, expansive soil and rock, shallow bedrock, and radon.

Erosional geologic processes dominate the Bryce Canyon region. Mass wasting (rockfalls, landslides, and debris flows) along and below the steep eastern escarpment of the Paunsaugunt Plateau (Pink Cliffs) create the principal geologic hazards with which visitors, park employees, planners, residents, and public safety personnel must contend. Rockfall hazard is particularly acute along parts of Bryce Canyon’s increasingly visited Navajo Loop Trail and other under-the-rim trails that access the rapidly eroding Pink Cliffs. New geologic mapping completed for this study shows that landslides are common throughout much of the study area where clay-rich Cretaceous-age strata crop out on slopes. Headward erosion and canyon entrenchment via stream scouring (primarily floods) along the Paria River and its tributaries also present a widespread hazard to humans and infrastructure within the study area. Although large earthquakes are relatively rare in the Bryce Canyon area, strong ground shaking, surface faulting, and liquefaction are possible. New geologic mapping shows that Quaternary-age deposits are locally displaced by the Paunsaugunt fault in the Bryce Canyon area and that the fault should be considered hazardous.