Paper No. 179-9
Presentation Time: 4:00 PM
ROCK AVALANCHE RECORDS IN ZION NATIONAL PARK: TOWARD A BETTER UNDERSTANDING OF POTENTIALLY HAZARDOUS EROSION IN ZION CANYON
BILDERBACK, Eric1, HENDEREK, Robyn L.2, ZYATITSKY, Karina1, TYE, Alex3, KROLCZYK, Emma T.4, MAHAN, Shannon4, RITTENOUR, Tammy M.5 and WOOD, John R.1, (1)National Park Service, Geologic Resources Division, Post Office Box 25287, Denver, CO 80225, (2)National Park Service, Zion National Park, State Route 9, Springdale, UT 84767, (3)Earth and Environmental Sciences, Utah Tech University, 225 S University Ave, Saint George, UT 84770-3875, (4)U.S. Geological Survey, Box 25046 Federal Center, Denver, CO 80225, (5)Department of Geosciences, Utah State University, 4505 Old Main Hill, Logan, CO 84322
The August 24
th, 2019, Cable Mountain Rock Avalanche in Zion National Park (NP), Utah caused significant damage to cultural and natural resources and impacted public access to an important historic trail system. While multiple rockfall events occur in Zion NP every year, this approximately 12,000 cubic meter rock avalanche was on the large end of historic rockfall events. Rockfall and rock avalanches from the Navajo Fm or other sandstone units in Zion Canyon largely disaggregate into sand, cobble, and boulder size sediment that can be mobilized by the Virgin River and its tributaries. Unfortunately, from a risk assessment perspective, this process of canyon formation in massive sandstones does not leave intact rockfall deposits for age constraint and frequency analysis that allows for calculation of erosion rates and risk, such as has been applied in another heavily visited, picturesque area, Yosemite Valley, California.
The 2019 Cable Mountain Rock Avalanche left behind a sand matrix supported mobile deposit that has been reworked by rain and snowmelt runoff in the three years since the event. This deposit is a modern example of infrequent (on human timescales), high energy rockfalls that are important to date, predict, and understand from a visitor risk and a canyon erosion perspective. The distal edges of the 2019 Cable Mountain Rock Avalanche came to rest on a landform that has now been recognized as a dissected package of one or more previous rock avalanches deposits. In the spring of 2022, we went to the field on a campaign to try to understand these sediment records through excavations, descriptions, and sampling for luminescence dating. We are also exploring concepts of relative dating using the 2019 record as a modern comparison. Despite the various challenges in understanding high-energy, canyon-forming events in Zion Canyon, the 2019 event record will help focus future geomorphic and geochronologic studies.
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