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 media support this conclusion.

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