Page 6 - gt1606

Figure 2. (A) Geological map of the study area
GSA TODAY | JUNE 2016 River. We also estimate that ~5 million m3 have been added to the
deposit in the sandy debris cone at the base of the source area (Fig. were predominantly Navajo Sandstone of each coloration facies,
2A), although the timing of this addition is uncertain.
The rock avalanche source is revealed by the craggy, unweath-
ered east face of the Sentinel; a series of broken pinnacles help
delineate the scarp. The form of the original cliff, however, is not
well constrained. We model the source form loosely after nearby
Abraham Peak, which shares the same stratigraphy and regional
joint patterns. We allow the source area to extend to 2150 m above
sea level and assume common slope angles for the different lithol-
ogies (Fig. 3B). The kinematics of the release are similarly
unknown. Based on topographic reconstruction, we place the
lowest point of the failure surface near the base of the Kayenta
Formation, which features alternating beds of sandstone and
shale. Rotational or translational failure through the Kayenta
likely exploited one or more weak shale layers. Our recreated
source has a volume of 223 million m3, yielding an estimated
volume increase of 28% as intact rock was converted to debris
(typical fragmentation volume increase is ~25%; Hungr and
Evans, 2004). The relatively flat and hummocky surface of the
rock avalanche deposit, combined with a lack of internal erosional
or depositional surfaces visible in outcrops, suggest single-event
catastrophic failure and emplacement.
Surface Exposure Dating
We sampled 12 boulders from across the rock avalanche deposit
for cosmogenic 10Be surface exposure dating (Fig. 2B; GSA
1 GSA Supplemental Data Item 2016070, including cosmogenic nuclide surface exposure dating sample details and results, as well as animation of the simulated Sen-
tinel rock avalanche, is online at www.geosociety.org/pubs/ft2016.htm. You can also request a copy from GSA Today, P.O. Box 9140, Boulder, CO 80301-9140, USA;
gsatoday@geosociety.org.
6
(modified from Doelling et al. [2002] in the
landslide area). Units: Qa—Quaternary alluvi-
um; Qt—Quaternary talus (Qts—sandy talus
cone at base of source); Qsd—deposits of the
Sentinel rock avalanche (in places with >1 m of
alluvium or colluvium cover); Jts—Temple Cap
Formation; Jnw—Navajo Sandstone, white;
Jnp—Navajo Sandstone, pink; Jnb—Navajo
Sandstone, brown; Jkt—Kayenta Formation,
Tenney Canyon Tongue; Jnl—Navajo Sand-
stone, Lamb Point Tongue; Jk—Kayenta Forma-
tion; Jms—Moenave Formation, Springdale
Sandstone; Jmw—Moenave Formation, Whit-
more Point Member; Jmd—Moenave Forma-
tion, Dinosaur Canyon Member. Orange
squares are select exposures of Sentinel Lake
clay beds; star shows location of Utah Geological
Survey (UGS) core. Circles are cosmogenic
exposure age sample locations. Contour interval
is 30 m. (B) Results of cosmogenic surface expo-
sure dating showing individual ages and errors,
mean age (solid line) of 4.8 ka, and associated
standard deviation (gray bar). Samples with
open circles were deemed outliers (see text).
(C) Estimated extents of Sentinel Lake at its ini-
tial high-stand (water level ~1420 m) and later
stable level (~1345 m).
Supplemental Data Repository1 Table S1). The boulders, which
exhibited a range of bedding orientations and had average dimen-
sions of several meters. We avoided boulders that were excessively
friable or showed evidence of slabbing, excavation, or movement
after deposition (Ivy-Ochs and Kober, 2008). Iron concretions in
several sampled boulders are weathering resistant, projecting 1–2
cm above the surrounding rock and allowing estimates of boulder
surface erosion since deposition. Sample preparation followed the
procedures of Ivy-Ochs et al. (2006). 10Be/9Be AMS measurements
were performed at ETH Zurich (Christl et al., 2013). Sample
10Be/9Be ratios were normalized to the ETH in-house standard
S2007N, which is calibrated to 07KNSTD. The weighted mean
full-process, measured blank 10Be/9Be ratio of (3.6 ± 2.6) × 10−15
was used for blank corrections. We calculated exposure ages with
the CRONUS online calculator (Balco et al., 2008) using a spall-
ation production rate of 3.93 ± 0.19 atoms g−1 a−1 of the northeast
North America calibration data set (Balco et al., 2009) and a time-
dependent spallation production model (Lal, 1991; Stone, 2000).
Several of the quartz mineral separates had high B contents,
leading to large AMS uncertainties for a few samples. Final expo-
sure ages are shown in Figure 2B; input data and calculation
results are reported in Supplemental Table S1 (see footnote 1).
Exposure dating revealed that boulders from across the surface
of the slide were deposited simultaneously, indicating a single-
event, massive, and catastrophic rock slope failure. We identified
three outliers in our dating results: boulder Z1, which had large