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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 67 (1997)No.
5. (September), Pages 792-804
Rock-Avalanche Elements of the Shadow Valley Basin, Eastern Mojave Desert,
California: Processes and Problems
S. Julio Friedmann
ABSTRACT
An impressive record of mass wasting is preserved in the Shadow Valley
basin, California, chiefly rock-avalanche breccias and gravity-driven glide
blocks. The rock-avalanche breccias show commonly described features including
penetrative fragmentation and preserved stratigraphy inherited from their
source terrane. Basal contacts may have substantial (5-60 m) relief and
a well developed mixed zone. A variety of transport indicators show predominant
transport from an eastern source to a western depositional site across
a variety of facies. Structural reconstructions suggest that Shadow Valley
rock avalanches had unusually long run-outs that are not obviously due
to variations in substrate, drop height, or mechanism of avalanche initiation.
Many features observed in the Shadow Valley rock-avalanche breccias
demonstrate significant internal and basal shear, as well as a prolonged
interaction with depositional substrate. As such, the basal contacts were
not frictionless, and basal shear stresses probably were transmitted through
the breccia mass during transport. If so, then many common features of
megabreccia deposits, including long run-out, inherited stratigraphy, and
"crackle" or "jigsaw" textures, may be consistent with intergranular actions
and collisions. Granular mechanics may be sufficient to produce long run-out
and these other features, suggesting that no special mechanism is required
to produce Shadow Valley-type deposits. Recent two-dimensional numerical
models and new observations of the physics of granular m dia support this
conclusion.
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