Jessica J. Castleton¹*, Jeffrey R. Moore¹*, Jordan Aaron², Marcus Christl³, Susan Ivy-Ochs^{3, 4}

1 University of Utah, Dept. of Geology and Geophysics, 115 South 1460 East, Salt Lake City, Utah 84112, USA
2 University of British Columbia, Dept. of Earth, Ocean and Atmospheric Sciences, 2020-2207 Main Hall, Vancouver, Canada V6T 1Z4
3 Laboratory of Ion Beam Physics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland
4 Dept. of Geography, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland

Abstract

The Sentinel rock avalanche blocked the mouth of Zion Canyon, Utah, USA, over a distance of 3.3 km and created a large lake that filled the canyon floor with sediment, transforming this iconic desert landscape. However, key questions remain regarding the size, timing, and dynamics, as well as the geomorphic effects of this prominent landslide. Reconstructing topography before and after the failure, we calculate an original deposit volume of 286 million m³ with maximum thickness of 200 m. New cosmogenic nuclide surface exposure ages of 12 boulders from across the deposit reveal a mean age of 4.8 ± 0.4 ka and are consistent with single-event emplacement. Results of 3D numerical runout simulations agree well with mapped deposit boundaries and thickness, affirming our hypothesized failure scenario and indicating an average runout velocity of 50 m/s. Following partial breach of the landslide dam, we estimate that water levels stabilized for ~700 yr until the lake filled with sediment. Deposited lacustrine clays reveal a period when Zion Canyon was filled by the 3 km² Sentinel Lake extending more than 7 km upstream. Today the Virgin River incises alluvial and lacustrine deposits still stranded behind remnants of the rock avalanche dam, attesting to the long-lasting geomorphic and ecological impacts of large landslides in steep desert landscapes.

Manuscript received 12 Oct. 2015; accepted 11 Jan. 2016

doi: 10.1130/GSATG269A.1