COSMOGENIC BERYLLIUM-10 EXPOSURE DATING OF PROBABLE COSEISMIC ROCK AVALANCHES IN YOSEMITE VALLEY, CALIFORNIA

The floor of Yosemite Valley preserves ten extremely large rock fall deposits, here termed rock avalanches, which are up to several million cubic meters in volume. These deposits extend far beyond the base of active talus slopes onto the valley floor and have occurred since the retreat of Last Glacial Maximum glaciers circa 15-17 ka. The hazard associated with rock avalanches depends in large part on the frequency of their occurrence, which was previously unknown for Yosemite Valley. We employed cosmogenic beryllium-10 surface exposure dating of 42 large (>10 m³) boulders embedded within the rock avalanche deposits. These deposits are ideal targets for cosmogenic beryllium-10 exposure dating because they (1) are essentially instantaneous failures, (2) excavate deep-seated quartz-rich granitic rocks, and (3) experience little to no post-depositional modification or erosion. Boulder exposure ages within each deposit show remarkable consistency, with as little as 3% variation between samples. Inheritance from pre-failure exposure is only clearly present in one of 42 samples. Mean exposure ages for the various rock avalanche deposits indicate that failures occurred at 1.0, 1.8, 2.3, 3.7, 4.4, 6.4, and 11.6 ka. Statistical analysis suggests that at least three of the deposits represent two or more failures from the same location, separated in time by hundreds to thousands of years; this finding is significant in part because field evidence for these distinctions is extremely subtle, highlighting the value of exposure dating as a mapping tool. Synchronous rock avalanches (within the uncertainty of the exposure ages, or <200 yrs) at different locations within the valley appear to have occurred at 3.7 ka, and possibly at 2.3 ka, suggesting coseismic triggering. Age correlations tentatively identify large earthquakes originating from the eastern Sierra Nevada and western Nevada as possible triggers for these rock avalanches. In Yosemite Valley, cosmogenic beryllium-10 exposure dating of rock avalanche deposits provides robust age data essential for accurately mapping these catastrophic events and assessing the hazards associated with them.