LATE QUATERNARY SEDIMENTATION ON THE LEIDY CREEK FAN, NEVADA-CALIFORNIA - GEOMORPHIC RESPONSES TO CLIMATE-CHANGE

Well-dated surface and subsurface deposits in semiarid Fish Lake Valle y, Nevada and California, demonstrate that alluvial-fan deposition is strongly associated with the warm dry climate of the last two intergla cial intervals, and that fans were stable and (or) incised during the last glaciation. Fan deposition was probably triggered by a change fro m relatively moist to arid conditions causing a decrease in vegetation cover and increases in flash floods and sediment yield. We think that this scenario applies to most of the other valleys in the southern Ba sin and Range. Radiocarbon, tephra, and a few thermoluminescence and c osmogenic ages from outcrops throughout Fish Lake Valley and from core s on the Leidy Creek fan yield ages of > 100-50 ka and 11-0 ka for the last two periods of alluvial-fan deposition. Mapping, coring and shal low seismic profiling indicate that these periods were synchronous thr oughout the valley and on the proximal and distal parts of the fans. F rom 50 to 11 ka, fan deposition ceased, a soil formed on the older all uvium and the axial drainage became active as runoff and stream compet ence increased. Slow deposition due to sheet flow or aeolian processes locally continued during this interval, producing cumulic soil profil es. The soil was buried by debris-flow sediment beginning at about 11 ka, coincident with the onset of relatively dry and warm conditions in the region. However, ground-water discharge maintained a large freshw ater marsh on the valley floor throughout the Holocene. Pulses of depo sition during the Holocene are recorded in the marsh and fan deposits; some pulses coincided with periods of or transitions to warm, dry cli mate indicated by proxy climate records, whereas others may reflect lo cal disturbances associated with volcanism and fires. Within the marsh deposits, much of the elastic material is probably desert loess. In a ddition, the deposition of coppice dunes within the fan deposits coinc ides with two dry periods during the late Holocene.