Near-Surface Geophysical Tomography of the Royal Arches Meadow Rock Avalanche in Yosemite Valley, California.

Abstract:

Since the retreat of glaciers after the Last Glacial Maximum, rock avalanches have occurred intermittently in Yosemite Valley, California. We investigated the distal portion of the oldest-known of these, the Royal Arches Meadow rock avalanche. Cosmogenic ¹⁰Be exposure ages of three boulders on the deposit indicate that the rock avalanche occurred at 16.0 ± 0.3 ka, which places this event immediately after the valley deglaciation. The interface between the rock avalanche deposit and the underlying glaciofluvial sediments therefore provides an elevation marker of the valley floor at the time of deposition. To investigate the depth to this interface, we collected eight Ground Penetrating Radar (GPR) and five Electrical Resistivity Tomography (ERT) profiles across the rock avalanche. Both methods are sensitive to the contrast in physical properties inherent to the interface between the granitic avalanche deposit and the underlying glaciofluvial sediments. From multiple interfaces identified in our GPR data, we determined the base of the rock avalanche deposit by using GPR-constrained ERT inversion.The elevation of this approximately horizontal interface is 1,207 ± 1m, 10 m below the modern ground surface, indicating ~10 m of postglacial sediment aggradation at this location. Based on topographic expression and depth to basal contact, we obtained a minimum volume estimation of 9.6 x 10⁵ m³ for the distal portion of the deposit, nearly three times larger than would be estimated from surface expression alone. Our findings inform reconstruction of the sedimentation history and landscape evolution of Yosemite Valley, and confirm that geophysical methods are valuable tools for three-dimensional investigations of rock avalanches, particularly those buried by younger sediments.