Page 4 - gt1606

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.
of Geology and Geophysics, 115 South 1460 East, Salt Lake City,
Utah 84112, USA; Jordan Aaron, University of British Columbia,
Dept. of Earth, Ocean and Atmospheric Sciences, 2020-2207 Main
Hall, Vancouver, Canada V6T 1Z4; Marcus Christl, Laboratory of
Ion Beam Physics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich,
Switzerland; and Susan Ivy-Ochs, Laboratory of Ion Beam
Physics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland,
and 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 cosmo-
genic 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 simu-
lations 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 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
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.
INTRODUCTION
To the visitor viewing Zion National Park for the first time,
there is a tantalizing similarity in shape between the sheer-
walled gorge of Zion Canyon and the Yosemite Valley …
The impression is so pronounced that the explanation of
this similarity is a daily task for the members of the natu-
ralist staff. —R.K. Grater (1945, p. 117)
Zion National Park, Utah, USA, receives millions of visitors
annually, but few appreciate that the tranquil and inviting flat
valley floor of Zion Canyon set amidst towering sandstone cliffs
owes its origin to a large, prehistoric landslide (Fig. 1A). Many
visitors identify deposits of smaller slides as they enter Zion
GSA Today, v. 26, no. 6, doi: 10.1130/GSATG269A.1.
* Corresponding author e-mail: jeff.moore@utah.edu
4
Canyon (e.g., one that damaged the main road in 1995), climbing
a winding roadway through blocky rock avalanche debris incised
by the Virgin River (Fig. 1B). The scale of the Sentinel rock
avalanche, however, evades easy perception—with visible deposits
>2 km long, 1 km wide, and up to 200 m thick, the slide is approx-
imately five times larger than the largest historic, non-volcanic
landslides in North America (Grater, 1945; Pankow et al., 2014).
Remnant clay beds and fossil mollusks preserved throughout the
canyon reveal a period when Sentinel Lake occupied Zion Canyon
for several centuries until eventually filling with sediment
(Hamilton, 1976).
Catastrophic rock avalanches represent an extreme-magnitude
natural hazard. Case histories illustrate the devastating conse-
quences as millions of cubic meters of rock travel kilometers
distance in only seconds, reaching peak velocities of ~100 m/s
with flow-like characteristics (e.g., Crosta et al., 2004; Dunning
et al., 2007). Beyond the immediate hazard, however, rock
avalanches also have long-lasting geomorphic and ecological
consequences, blocking river valleys and controlling local base-
level for millennia, while facilitating human habitation and culti-
vation of otherwise steep terrain (Korup, 2006; Hewitt et al.,
2011). This juxtaposition of modern-day hazard and geomorphic
transformation is ideally captured in Zion Canyon; however, the
related effects of transient landscape disturbance can be found in
a wide range of environments (Korup et al., 2010), and the ecolog-
ical effects of altered river courses may be especially relevant in
Previous efforts to date the Sentinel rock avalanche relied on
radiocarbon from charcoal found in lacustrine and alluvial sedi-
ments. Hamilton (1976) determined an age of 3.4–4.5 cal. k.y. B.P.
(recalculated calibrated 1� range using IntCal13; Reimer et al.,
2013) for charcoal found in post-lake sand overlying clay beds.
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
and 10 m below ground level, respectively (Doelling et al., 2002)
(see Fig. 2A). Most recently, Hamilton (2014) obtained a lumines-
cence age of 4.3 ± 1.3 ka for sand between lacustrine clays near the
top of the lake sequence. Taken together, these ages imply a nearly
4000-yr lifespan for Sentinel Lake. However, calculations of
modern sediment flux for the Virgin River suggest that the lake
filled with sediment in only 600–800 yr (Hamilton, 1976, and our
new value described herein).
In this paper, we report new mapping of Sentinel rock
avalanche deposits and select lacustrine sediments. We approxi-
mate the topography of Zion Canyon before and after the slide to
generate refined estimates of the rock avalanche volume and to
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.