COSMOGENIC NUCLIDE EXPOSURE DATING OF ROCK AVALANCHE DEPOSITS IN YOSEMITE VALLEY, CALIFORNIA

Rock falls commonly occur from the glacially-steepened walls of Yosemite Valley, with smaller rock falls up to hundreds of cubic meters in volume occurring on an annual basis. Larger rock falls up to tens of thousands of cubic meters in volume occur less frequently, but have been documented in the past 150 years. Deposition of these rock falls is typically limited to the active talus slopes beneath the cliffs. However, the floor of Yosemite Valley preserves at least six extremely large rock fall deposits, here termed rock avalanches, 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. We mapped these rock avalanche deposits in the field and in ArcGIS, using airborne LiDAR data that resolves individual boulders. Minimum exposed volumes range from hundreds of thousands to several million cubic meters. To assess the frequency of rock avalanche occurrence, we employed cosmogenic beryllium-10 surface exposure dating of large (>10 m³) boulders embedded within the deposits. Rock avalanches in Yosemite are ideal targets for cosmogenic exposure dating, as they are essentially instantaneous failures that excavate deep-seated quartz-rich granitic rocks, and once they are deposited on the flat valley floor they are essentially immune to subsequent modification. Exposure ages show remarkable consistency across the width of the deposits, with as little as 3% variation between samples located tens to hundreds of meters apart. Mean exposure ages indicate that failures occurred at 1.0, 1.8, 2.3, 3.7, 4.4, 6.4, and 11.6 ka. At least three of the deposits appear to represent two or more failures, separated in time by hundreds to thousands of years. 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 as triggers for at least some of the rock avalanches. These unique and robust age data provide key information for quantifying recurrence intervals for rock avalanches in Yosemite Valley.