ABSTRACT

A multiphase debris flow triggered by an afternoon thunderstorm was deposited 22 August 1984 on the Dolomite alluvial fan of Owens Valley, California. Two tracts 10-300 cm thick accumulated during this event, together covering 68,000 m² (26% of the fan) with ~ 50,000 m³ of sediment. Both tracts consist of numerous paired boulder-rich levees 100-300 cm high on the proximal fan, and multiple attached lobes 20-100 cm thick on the distal fan. Each levee-lobe pathway was built by an individual surge, with flow-path switching caused by clogging of the frontal bouldery snout of the ensuing surge. The levees consist of clast-supported to matrix-supported, muddy, pebbly, cobbly, boulder gravel displaying a well-developed, radially aligned a-axis clast fabric. The lobes mostly consist of matrix-supported, muddy, pebbly cobble gravel. The clast-supported lobe margins are enriched in cobbles displaying a strongly preferred slope-parallel a-axis fabric along the sides that changes to a slope-perpendicular fabric at the snout. The distal ends of most of the clast-rich lobes are overlain and offlapped by clast-poor debris-flow lobes of granular mud 10-30 cm thick. The central zones of the levees and most lobes were surficially washed by recessional-stage water flows that moved winnowed granules, sand, and mud off the fan.

The radial change from levees in the proximal fan to lobes distally is morphologically expressed as two intersecting slope segments, the upper one sloping 9-12° and the lower one 3-5°. This direct relationship between the constituent forms and slope values also is characteristic of pre-1984 deposits, suggesting that the 1984 event is typical of the primary processes constructing the Dolomite fan. The pronounced radial change from levees to lobes, and its manifestation as a slope inflection, was caused by the selective loss on the upper fan of most of the boulders from the passing debris-flow surges. Boulders were sorted from the surges because of their greater buoyancy, which caused them to be preferentially concentrated in the part of the flow that subsequently was sheared off as levees. This selective boulder removal depleted the flow of the clasts that provided lateral support, facilitating flow expansion and the onset of lobe development. Thus, the resultant facies assemblage is a function of the specific sediment grain-size suite available in the catchment colluvium, and the transport characteristics of this mix when transformed into a debris flow by the rapid addition of heavy rainfall during infrequent thunderstorms.