Surface exposure dating and geophysical tomography of the royal arches meadow rock avalanche, Yosemite Valley, California
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Published version
OA Location
Author(s)
Pacheco, Marcus
Plattner, Alain M
Stock, Greg M
Rood, Dylan H
Pluhar, Christopher J
Type
Journal Article
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 10Be exposure ages of boulders
within the deposit indicate that the rock avalanche occurred at 16.1 ± 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 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 × 105 m3 and
9.7 × 105 m3
, 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.
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 10Be exposure ages of boulders
within the deposit indicate that the rock avalanche occurred at 16.1 ± 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 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 × 105 m3 and
9.7 × 105 m3
, 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.
Date Issued
2020-09-17
Date Acceptance
2020-08-10
Citation
Frontiers in Earth Science, 2020, 8, pp.1-12
ISSN
2296-6463
Publisher
Frontiers Media
Start Page
1
End Page
12
Journal / Book Title
Frontiers in Earth Science
Volume
8
Copyright Statement
© 2020 Pacheco, Plattner, Stock, Rood and Pluhar. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) http://creativecommons.org/licenses/by/4.0/. The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
License URL
Identifier
https://www.frontiersin.org/articles/10.3389/feart.2020.00372/full
Subjects
0403 Geology
0404 Geophysics
0406 Physical Geography and Environmental Geoscience
Publication Status
Published
Date Publish Online
2020-09-17