Debris-flow deposits in alluvial fans on the west flank of the White Mountains, Owens Valley, California, U.S.A.

Abstract

Sections measured in incised channels on 10 alluvial fans show that debris-flow beds, mostly 30–200 cm thick, are more important than stream-flow deposits in construction of fans along the west flank of the White Mountains. The debris-flow beds have a matrix-supported fabric with a sandy mud matrix that comprises about 40% of individual beds. Complete grain-size analyses of 19 debris-flow beds show that they average 50% gravel, 25% sand, 11% silt, and 4% clay. Inverse grading at the base of most beds is interpreted as due to a layer of high-shear stress beneath an overlying semi-rigid, high-strength plug that supported cobbles and boulders. Sticks and logs embedded in the plugs are oriented parallel with flow directions, reflecting the laminar-viscous motion of the plugs. In the shear layer, the average inclination of discoidal clasts is subhorizontal, varying from 2 to 7° in the up flow direction. In the plugs, discoidal clasts have a subhorizontal fabric with average dips of 5–13°. Compared to the shear layer, plugs have more scatter in clast orientation with numerous clasts dipping at 60–90°. Levees along the margins of debris-flow lobes contain concentrations of the larger clasts in the flows. In the leeves, discoidal clasts have an average up flow dip of 21–31° with substantial variability in orientation similar to the plugs. Maximum clast size in individual debris-flow beds is fairly constant down fan until the flows spread and thinned on the sandflat at the fan toe. Clast lithologies show that debris flows originate on steep slopes underlain by granitic and metavolcanic rocks at high elevations in the canyons. In contrast, stream flows obtained most of their clasts from talus slopes on metasedimentary rocks near the apices of the fans. Debris flows are generated during intense rainstorms in the spring and summer when landslides in the water-saturated regolith move down slope, shear, dilate, and by adding water are transformed into debris flows which then move with surging laminar motion along canyon flows to the fans. The recurrence interval for debris flows is about 320 years as evidenced by 14C dates on plant material buried beneath debris-flow beds.