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Dynamics and legacy of 4.8 ka rock avalanche that dammed Zion Canyon, Utah, USA

GSA TODAY | JUNE 2016  Jessica J. Castleton, Jeffrey R. Moore*, University of Utah, Dept.   Canyon (e.g., one that damaged the main road in 1995), climbing
                       of Geology and Geophysics, 115 South 1460 East, Salt Lake City,      a winding roadway through blocky rock avalanche debris incised
                       Utah 84112, USA; Jordan Aaron, University of British Columbia,       by the Virgin River (Fig. 1B). The scale of the Sentinel rock
                       Dept. of Earth, Ocean and Atmospheric Sciences, 2020-2207 Main       avalanche, however, evades easy perception—with visible deposits
                       Hall, Vancouver, Canada V6T 1Z4; Marcus Christl, Laboratory of       >2 km long, 1 km wide, and up to 200 m thick, the slide is approx-
                       Ion Beam Physics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich,         imately five times larger than the largest historic, non-volcanic
                       Switzerland; and Susan Ivy-Ochs, Laboratory of Ion Beam              landslides in North America (Grater, 1945; Pankow et al., 2014).
                       Physics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland,     Remnant clay beds and fossil mollusks preserved throughout the
                       and Dept. of Geography, University of Zurich, Winterthurerstrasse    canyon reveal a period when Sentinel Lake occupied Zion Canyon
                       190, 8057 Zurich, Switzerland                                        for several centuries until eventually filling with sediment
                                                                                            (Hamilton, 1976).
                       ABSTRACT
                                                                                              Catastrophic rock avalanches represent an extreme-magnitude
                         The Sentinel rock avalanche blocked the mouth of Zion              natural hazard. Case histories illustrate the devastating conse-
                       Canyon, Utah, USA, over a distance of 3.3 km and created a large     quences as millions of cubic meters of rock travel kilometers
                       lake that filled the canyon floor with sediment, transforming this   distance in only seconds, reaching peak velocities of ~100 m/s
                       iconic desert landscape. However, key questions remain regarding     with flow-like characteristics (e.g., Crosta et al., 2004; Dunning
                       the size, timing, and dynamics, as well as the geomorphic effects    et al., 2007). Beyond the immediate hazard, however, rock
                       of this prominent landslide. Reconstructing topography before        avalanches also have long-lasting geomorphic and ecological
                       and after the failure, we calculate an original deposit volume of    consequences, blocking river valleys and controlling local base-
                       286 million m3 with maximum thickness of 200 m. New cosmo-           level for millennia, while facilitating human habitation and culti-
                       genic nuclide surface exposure ages of 12 boulders from across the   vation of otherwise steep terrain (Korup, 2006; Hewitt et al.,
                       deposit reveal a mean age of 4.8 ± 0.4 ka and are consistent with    2011). This juxtaposition of modern-day hazard and geomorphic
                       single-event emplacement. Results of 3D numerical runout simu-       transformation is ideally captured in Zion Canyon; however, the
                       lations agree well with mapped deposit boundaries and thickness,     related effects of transient landscape disturbance can be found in
                       affirming our hypothesized failure scenario and indicating an        a wide range of environments (Korup et al., 2010), and the ecolog-
                       average runout velocity of 50 m/s. Following partial breach of the   ical effects of altered river courses may be especially relevant in
                       landslide dam, we estimate that water levels stabilized for ~700 yr  desert canyons of the Colorado Plateau.
                       until the lake filled with sediment. Deposited lacustrine clays
                       reveal a period when Zion Canyon was filled by the 3 km2 Sentinel      Previous efforts to date the Sentinel rock avalanche relied on
                       Lake extending more than 7 km upstream. Today the Virgin River       radiocarbon from charcoal found in lacustrine and alluvial sedi-
                       incises alluvial and lacustrine deposits still stranded behind       ments. Hamilton (1976) determined an age of 3.4–4.5 cal. k.y. B.P.
                       remnants of the rock avalanche dam, attesting to the long-lasting    (recalculated calibrated 1� range using IntCal13; Reimer et al.,
                       geomorphic and ecological impacts of large landslides in steep       2013) for charcoal found in post-lake sand overlying clay beds.
                       desert landscapes.                                                   The Utah Geological Survey (UGS) reported ages of 7.2–9.0 and
                                                                                            7.0–8.3 cal. k.y. B.P. for charcoal found in lacustrine clay at 4 m
                       INTRODUCTION                                                         and 10 m below ground level, respectively (Doelling et al., 2002)
                                                                                            (see Fig. 2A). Most recently, Hamilton (2014) obtained a lumines-
                            To the visitor viewing Zion National Park for the first time,   cence age of 4.3 ± 1.3 ka for sand between lacustrine clays near the
                            there is a tantalizing similarity in shape between the sheer-   top of the lake sequence. Taken together, these ages imply a nearly
                            walled gorge of Zion Canyon and the Yosemite Valley …           4000-yr lifespan for Sentinel Lake. However, calculations of
                            The impression is so pronounced that the explanation of         modern sediment flux for the Virgin River suggest that the lake
                            this similarity is a daily task for the members of the natu-    filled with sediment in only 600–800 yr (Hamilton, 1976, and our
                            ralist staff. —R.K. Grater (1945, p. 117)                       new value described herein).

                         Zion National Park, Utah, USA, receives millions of visitors         In this paper, we report new mapping of Sentinel rock
                       annually, but few appreciate that the tranquil and inviting flat     avalanche deposits and select lacustrine sediments. We approxi-
                       valley floor of Zion Canyon set amidst towering sandstone cliffs     mate the topography of Zion Canyon before and after the slide to
                       owes its origin to a large, prehistoric landslide (Fig. 1A). Many    generate refined estimates of the rock avalanche volume and to
                       visitors identify deposits of smaller slides as they enter Zion      comment on failure kinematics. We then use cosmogenic nuclide
                                                                                            surface exposure dating to provide the first direct date of the rock
                                                                                            avalanche deposit and constrain the age of Sentinel Lake.

     GSA Today, v. 26, no. 6, doi: 10.1130/GSATG269A.1.
     * Corresponding author e-mail: jeff.moore@utah.edu

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