Abstract

The prehistoric but geomorphically pristine North Long John rock avalanche of the Inyo Mountains piedmont, California, formed by the catastrophic collapse and disintegration of a 500 × 1000 m range-front bedrock slab. This failure rapidly produced and transferred ~25 million cubic metres of new sediment to the piedmont, where it was deposited in a trough between two coalesced alluvial fans. The avalanche consists of nearly monolithologic (aplitic), unstratified, very angular, muddy, cobble, pebble gravel with boulders concentrated at the top and outer margins. The deposits are clast supported except in the lower central zone. Coarse clasts exhibit crackle-breccia fabric, and the voluminous equant pebbles are the disaggregated products of these clasts. The avalanche deposits are distributed in a U-shaped body with paired lateral levees 10-60 m high that extend 1560 m from the range front to a 108 m high distal snout. Each levee contains three segments that, along with the snout, overlap and extend progressively farther downslope from the inside of each other in a telescoped pattern. The most proximal deposits are the remnant sole sheared off where the avalanche intersected the piedmont, whereas the remainder was deposited as an ensuing grain flow. The radial alignment of the long axes of coarse clasts in the upper levees indicate that particle-particle interaction took place during flowage, and that clasts were pushed towards the margins. Part of the northern second levee borders a sharp and vertical, 10-18 m high scarp cleanly sliced through older fan deposits. This scarp was cut by the leading edge of the avalanche, which rapidly bulldozed away ~0.5 million cubic metres of fan deposits in the flow path. A southward cross-tilt resulted from the avalanche turning south due to interference with higher fan deposits on the north side. Friction from this interference restricted avalanche runout to 1.6 km, 25-33% of the distance predicted by empirical data from case studies lacking flow-path obstructions.

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