Depositional processes of the gravelly debris flow deposits, South Dolomite alluvial fan, Owens Valley, California

Abstract

Coarse-grained debris flow was instigated in the South Dolomite alluvial fan of the Inyo Mountains piedmont, Owens Valley, California, by a concentration of flashy thunderstorm precipitation in 1984. The debris flow extends about 650 m from the fan apex to the Owens Lake, and forms levee deposits along the sides of the passing tract in the steep (6–26°) upper-fan (proximal) segment and lobe deposits at the flow terminus in the less steep (3–6°) lower-fan (distal) segment. Both the levee and lobe deposits consist of clast-supported, unstratified, poorly sorted, pebble-to-boulder-grade gravels. The length of the levees is much longer than that of lobe and the clasts exhibit flow-parallel [a(p)] long-axis orientation in the levees but flow-transverse [a(t)] orientation in the lobe, suggesting that the levees were formed by “pushing aside” mechanism, i.e., lateral (or oblique) accumulation of clasts from the surge heads. On the other hand, the lobe deposit was formed by sourceward incrementation of successive unsaturated debris flow surges. Detailed analyses of the grain-size distribution indicate distinct inverse grading of the clasts both in lobe and in levee deposits with respect to maximum clast size and graphic mean grain size. The inverse grading is mainly attributed to clast collision, with the upward displacement of large clasts due to dispersive pressure combined with kinematic sieving. It is notable however that the graphic mean size of the levee matrix shows normal grading, which can be explained by the combined effect of hindered settling and fluid turbulence. The bimodal grain-size distribution of the levee matrix suggests an incomplete mixing of coarser-grained surge head materials and finer-grained surge body and tail materials.