SIZE, AGE, AND GEOMORPHIC CONSEQUENCES OF AN ANCIENT ROCK AVALANCHE IN HOP VALLEY, ZION NATIONAL PARK, UTAH

Zion National Park preserves a rich geological record of landslide-dammed valleys in its deeply incised topography, with eleven hypothesized Holocene landslide dams identified in the relatively small geographic area. Previously, all landslide-dammed valleys were thought to contain lakes, but geological evidence suggests that not all held significant standing water. Additionally, isolated radiocarbon dates have been produced at several locations in up-valley sediments, but only one of the ancient landslide deposits has been dated. This inconsistent nomenclature and indirect dating undermines our ability to quantify the geomorphic impact and frequency of large landslides in a popular national park. Here we investigate the size, age, and geomorphic consequences of an ancient, valley-damming rock avalanche deposit at the mouth of Hop Valley in the Kolob Canyons section of Zion National Park. Topographic reconstructions, based on 1-m lidar, geologic mapping, and field observations, indicate the original deposit was ~1.5 km long with a maximum thickness of ~110 m. New ages from cosmogenic nuclide surface exposure dating indicate a single-event failure around the mid-Holocene transition. Stratigraphic sections of accumulated up-valley sediments, calculated sedimentation rates, and paleoclimate reconstructions suggest that the deposit primarily dammed sediment, rather than water, producing an extensive alluvial plain. Evidence also exists that the native Virgin Anasazi people utilized the resulting flat bottomed valley prior to their withdrawal from the region. In the modern Hop Valley system, a perennial, disappearing stream slowly erodes the up-valley sediments, while only rare flash-flood events scour and incise the rock avalanche deposit. This study helps clarify the geomorphic impact and hazard of large valley-blocking landslides in Zion National Park.