| Description |
The Sentinel rock avalanche in Zion National Park is one of the largest catastrophic landslide events recognized in the North American desert southwest. Originating from the western wall of Zion Canyon near its confluence with Pine Creek, the initial collapse removed a nearly 900 m high wall of predominantly Navajo sandstone. Energetic deposition is revealed by the relatively flat and hummocky topography of the debris field, which blocked flow of the Virgin River out of Zion Canyon. We combine new mapping of rock avalanche deposits with reconstruction of past topography to constrain the landslide extent, thickness, volume, and subsequent erosion. We estimate the original debris field covered an area of 3 million m2, was ~3.3 km long where it blocked the Virgin River, and had a volume of 284 million m3. The mean estimated thickness is 93 m, with a maximum deposit thickness of 200 m. Since deposition, erosion by the Virgin River has removed approximately 45%, or 131 million m3 of the Sentinel rock avalanche debris. Cosmogenic nuclide surface exposure dating of 12 boulders from across the surface of the rock avalanche deposit reveals a mean age of 4.8 ? 0.4 ka. Results further show that boulders from across the slide were deposited simultaneously, indicating a single-event, massive and catastrophic failure scenario. Numerical simulation of rock avalanche runout was performed using the 'equivalent-fluid' code DAN3D, and the results show excellent match to our mapped deposit extents and estimated thickness. The simulated rock avalanche crossed Zion Canyon in only ~20 s, with maximum velocities exceeding 90 m/s, ran up the opposing wall, and spread laterally up and down canyon. The Virgin River was dammed by landslide debris, which formed the extensive Sentinel Lake, eventually trapping a vast quantity of lacustrine and alluvial sediment. The cumulative effects reveal the long-lasting and diverse impacts of large rock avalanches in desert canyons of the Colorado Plateau: in addition to representing an extreme magnitude hazard, large landslides events also have wide-ranging ecological and geomorphic effects, here helping create the flat valley floor of Zion Canyon. |
