Alluvial fan and catchment initiation by rock avalanching, Owens Valley, California

Abstract

The North Long John alluvial fan of the Inyo Mountains piedmont, Owens Valley, CA, was catastrophically initiated by a prehistoric (early Holocene?) rock avalanche. This avalanche resulted from the collapse and disintegration of the central part of a 1.1×2.0 km range front bedrock facet comprising the divide between the catchments of two large, adjoining alluvial fans. Failure rapidly produced and transferred ∼25 million m 3 of new sediment to the piedmont, where it was deposited in the trough between two coalesced fans. These deposits are of unstratified, angular, muddy, bouldery and cobbly pebble gravel present in a U-shaped form. This form consists of lateral levees 10 to 60 m tall that lead 1.6 km from the range front to a 108+ m high distal snout. The avalanche sediment comprise the initial platform for, and still dominate the nascent North Long John fan. Rock avalanche failure also created a 100+ m deep concave scar in the range front that now serves as the fan catchment. The slopes of this catchment are mantled by colluvial sediment generated from intense shear along the avalanche detachment surface. Thus, the avalanche served as an alluvial fan `starter kit' by building the fan platform, and by creating a range front catchment lined with sediment. Because of this unique and recent origin, the North Long John fan and its catchment are in an early stage of development, lacking features such as old surfaces and a well-developed drainage net typical of the older, adjoining alluvial fans and catchments. Post-avalanche fan activity has been dominated by the deposition of ∼4 million m 3 of sediment by debris flows. These flows, including one in 1984, were instigated by the concentration of flashy thunderstorm precipitation across the colluvium of the nascent catchment. Debris flows have built a conical tract that extends 1 km from the fan apex, and partially buries the older avalanche deposits. The debris flow tract consists of muddy, pebbly, cobbly, boulder gravel in levees 50–200 cm tall in the steep (12–16°) upper segment, and boulder-free lobes in the lower-sloping (5°) distal segment. This tract terminates in a `pond' behind the avalanche snout, where recessional debris flow pebbly mud has been trapped. This pond eventually filled and was breached by a deep channel that now transects the snout of the rock avalanche. A second debris flow tract 300 m in radius has prograded outward from the end of the avalanche bypass channel. The facies of this tract are identical to those of the proximal fan. An abundance of colluvial sediment mantling the steep slopes of the catchment indicates that the North Long John fan will continue to enlarge by thunderstorm-induced debris flows.