RECENT DEBRIS-FLOW PROCESSES AND RESULTANT FORM AND FACIES OF THE DOLOMITE ALLUVIAL-FAN, OWENS VALLEY, CALIFORNIA

A multiphase debris flow triggered by an afternoon thunderstorm was de posited 22 August 1984 on the Dolomite alluvial fan of Owens Valley, C alifornia. Two tracts 10-300 cm thick accumulated during this event, t ogether covering 68,000 m(2) (26% of the fan) with similar to 50,000 m (3) of sediment. Both tracts consist of numerous paired boulder-rich l evees 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 o f clast-supported to matrix-supported, muddy, pebbly, cobbly, boulder gravel displaying a well-developed, radially aligned a-axis clast fabr ic. The lobes mostly consist of matrix-supported, muddy, pebbly cobble gravel, The clast-supported lobe margins are enriched in cobbles disp laying a strongly preferred slope-parallel a-axis fabric along the sid es that changes to a slope-perpendicular fabric at the snout, The dist al ends of most of the clast-rich lobes are overlain and offlapped by clast-poor debris-how lobes of granular mud 10-30 cm thick. The centra l zones of the levees and most lobes were surficially washed by recess ional-stage water hows that moved winnowed granules, sand, and mud off the fan. The radial change from levees in the proximal fan to lobes d istally is morphologically expressed as two intersecting slope segment s, the upper one sloping 9-12 degrees and the lower one 3-5 degrees. T his 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 f an. The pronounced radial change from levees to lobes, and its manifes tation 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 buoyanc y, which caused them to be preferentially concentrated in the part of the how that subsequently was sheared off as levees. This selective bo ulder removal depleted the flow of the clasts that provided lateral su pport, facilitating flow expansion and the onset of lobe development. Thus, the resultant facies assemblage is a function of the specific se diment grain-size suite available in the catchment colluvium, and the transport characteristics of this mix when transformed into a debris h ow by the rapid addition of heavy rainfall during infrequent thunderst orms.