| Description |
Zion National Park preserves a rich geological record of landslide-dammed canyons in its deeply incised topography, with 11 Holocene landslide dams identified in the relatively small area. However, geological evidence suggests that not every landslidedammed canyon produced a lake. Here we investigate the geomorphic and sedimentologic consequences of an ancient, canyon-damming rock avalanche deposit at the mouth of Hop Valley in Zion National Park, describing its size, age, and failure conditions. Topographic reconstructions indicate the original deposit was ~75 million m3 and 1.5 km long with a maximum thickness of 180 m. New ages from cosmogenic nuclide surface exposure dating indicate a single-event failure at 6.7 ± 0.74 ka. The rock avalanche dammed a maximum of ~60 million m3 of sediment to transform Hop Valley from a steep, v-shaped bedrock gorge to a lush, flat-floored canyon. Stratigraphic sections of accumulated up-valley sediments, calculated sedimentation rates (averaging 8.2 m/ky), and paleoclimate records suggest the deposit primarily dammed sediment, rather than water, to produce an extensive alluvial plain. Evidence also exists that the native Virgin Ancestral Puebloans utilized the flat-floored valley. In modern Hop Valley, a perennial, disappearing stream slowly erodes the upvalley sediments, while only rare floods incise the rock avalanche deposit. This study helps clarify the geomorphic and sedimentologic impact of landslide dams in canyon topography, with controls exerted by mass movement volume, stream order, stream gradient, and streamflow. |
