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 of these, the Royal Arches Meadow rock avalanche, which has been partially buried by sediment aggradation. Cosmogenic 10 Be exposure ages of boulders within the deposit indicate that the rock avalanche occurred at 16.0 +/- 0.3 ka, immediately after deglaciation and thus prior to most aggradation. 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 identify the elevation of 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 contrasts between the granitic avalanche deposit and the underlying sediments. By constraining ERT inversions with GPR interfaces that are continuous across the profiles, we identified a single interface, interpreted as the basal contact of the rock avalanche, that separates resistive material from conductive material underneath. The elevation of this approximately horizontal interface is between 1,206 m and 1,209 m, roughly 10 m below the modern ground surface, indicating ≈10 m of sediment aggradation since deglaciation. Based on topographic expression and depth to this contact, we determined a minimum volume estimate of between 8.1*10^5 m^3 and 9.7*10^5 m^3 , nearly three times larger than what would be estimated from surface expression alone. Our findings allow reconstruction of the sedimentation history of Yosemite Valley, inform hazard and risk assessment, and confirm that geophysical methods are valuable tools for three-dimensional investigations of rock avalanches, particularly those buried by younger sediments.
Highlights
Rock avalanches - streams of rapidly moving rock debris resulting from catastrophic failure of bedrock slopes - are among the most powerful geologic forces on earth, rapidly eroding extremely large masses of rock and causing dramatic and long-lasting landscape changes (e.g., Evans et al, 2006; Hovius and Stark, 2006; Korup et al, 2007; Hewitt et al, 2008; Hermanns and Longva, 2012)
We identified Beta as the interface between the rock avalanche deposit and the underlying sediments, which we interpret as the basal contact of the rock avalanche and the floor of Yosemite Valley at the time of the rock avalanche event
Our choice of Beta is based on the following observations: (1) of all three interfaces identified in the Ground Penetrating Radar (GPR) data, Beta led to the clearest separation between electrically resistive and conductive material in GPR-constrained Electrical Resistivity Tomography (ERT) inversions (Figures 5, 6); and (2) in the GPR data, Beta separates a unit of visible scattering from a unit with little scattering underneath (Figure 4)
Summary
Rock avalanches - streams of rapidly moving rock debris resulting from catastrophic failure of bedrock slopes - are among the most powerful geologic forces on earth, rapidly eroding extremely large masses of rock and causing dramatic and long-lasting landscape changes (e.g., Evans et al, 2006; Hovius and Stark, 2006; Korup et al, 2007; Hewitt et al, 2008; Hermanns and Longva, 2012). Rock avalanche volumes are usually determined by field investigations or remote sensing methods, though in both cases accurate volumes can be elusive due to the inherent difficulty in establishing accurate deposit extents and thicknesses, post-event erosion of the deposit, cover by more recent debris, or a lack of sufficiently high resolution remote sensing data (Hutchinson, 2006; Hewitt et al, 2008). This situation is exacerbated in cases where rock avalanche deposits are partially buried by sediment aggradation, with an unknown but potentially substantial portion of the deposit volume hidden from view. Integrating our ground penetrating radar results with electrical resistivity tomography allows us to identify the rock avalanche base (section 3.4)
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