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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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