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Figure 1. (A) View northeast (looking upstream
Numerical runout simulation helps confirm the hypothesized
single-event, catastrophic failure scenario. Long-lasting geomor-
phic and ecological effects contributing to the iconic setting of
Zion National Park attest to the diverse impacts of large rock
avalanches in steep desert landscapes.
SENTINEL ROCK AVALANCHE
Deposits of the Sentinel rock avalanche are deeply incised by
the Virgin River, providing exceptional exposures over a distance
of ~2 km (Figs. 1 and 2). We observed large-scale remnant stratifi- Virgin River underneath the slide (Fig. 3A), we approximated the
cation reflecting the composition of the source: Kayenta
Sandstone from the base of the source is generally found along the
basal portion of the deposit, while Navajo Sandstone from the top
of the source forms the upper part of the deposit (for details of
these lithologies see Doelling et al., 2002). Moreover, the two
materials exhibit strong textural differences caused by rock
avalanche emplacement: the Kayenta Sandstone, with higher clay
content, is highly deformed but frequently retains remnant centi-
meter-scale sedimentary structure, while the massive and rela-
tively homogenous Navajo Sandstone exhibits compact and
shattered, clast-supported deposits. Navajo Sandstone in Zion
Canyon appears white in the upper part of the formation, transi-
tioning to pink and brown below through diagenetic leaching of
iron (Nielsen et al., 2009). Boulders of each diagenetic facies are for this volume compared to other terrestrial events (Lucas et al.,
found within the deposit (Fig. 1B).
To quantify the volume of the rock avalanche, we reconstructed
topography beneath the deposit and immediately after failure.
Our reconstruction is based on field assessments, exposed
into Zion Canyon) over the surface of the
Sentinel rock avalanche deposit; incised gorge
of the Virgin River at far right. Qsd—Sentinel
rock avalanche deposits; Qts—sandy talus (see
Fig. 2). (B) Rock avalanche deposits exposed by
river incision, showing constituent rock types
assessed from remote mapping; height of
exposure is ~150 m. The upper part of the
deposit consists primarily of Navajo Sandstone
debris, which is characteristically shattered
and compact, while the basal portion of the
deposit consists of Kayenta material that has
been deformed and tilted but often retains
small-scale structure. See Figures 2 and 3 for
composition of the source.
outcrops, and extrapolation from nearby slopes. A key marker is a GSA TODAY | www.geosociety.org/gsatoday
~30 m thick bed of Springdale Sandstone, a member of the
Moenave Formation, which dips gently (~2°) to the northeast (Fig.
2A) (Doelling et al., 2002). We assume the failure surface did not
penetrate this layer, because in-place outcrops are exposed at the
base of the rock avalanche deposit. Another key element in our
reconstruction is a large bedrock arm flanking the southwestern
end of the deposit; here we mapped bedrock along the river gorge
and under thin colluvium on two knobs above rock avalanche
debris (see Fig. 2A). Aided by long-profile extrapolation of the
topography below deposits of the Sentinel rock avalanche (Fig.
3B). To reconstruct the top of the slide debris, we extrapolated
existing surfaces across the incised Virgin River gorge.
Subtracting the reconstructed basal topography from the top-
of-slide debris surface, we calculated a mean and maximum
deposit thickness of 95 m and 200 m, respectively (see Fig. 4F).
The original deposit was 3.3 km long and 1.4 km wide, covered an
area of 3 million m2, and had an estimated total volume of 286
million m3 (volume presumed accurate to within ±20% from trial
solutions using alternate topographies). A minimum fahrboesc-
hung angle (i.e., the ratio of fall height to path length along flow
lines) of 20° for the Sentinel slide indicates relatively low mobility
2014). This may be related to the cross-valley flow orientation.
Comparing the original volume of rock avalanche deposits to the
volume of material found today, we estimate that approx. 131
million m3, or ~45%, of debris has been eroded by the Virgin
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