Response of alluvial fan systems to the late Pleistocene to Holocene climatic transition: contrasts between the margins of pluvial Lakes Lahontan andMojave, Nevada and California, USA

Dated shorelines of late Pleistocene pluvial Lakes Lahontan (Great Basin De sert, northwest Nevada) and Mojave (Mojave Desert, eastern California) prov ide timelines for the assessment of alluvial fan sedimentation at the lake margins during the late Pleistocene to early Holocene. Two sets of alluvial fan systems have been mapped: the Stillwater fans, feeding Lake Lahontan; and the Zzyzx fans, feeding Lake Mojave. Their contrasting morphologies sug gest different responses of the two fan systems to late Pleistocene to earl y Holocene climatic change. At the time the Stillwater fan systems underwen t minimal sedimentation, with the catchment hillslopes apparently stable. T he Zzyzx fans experienced major changes in water and sediment supply from t he catchment hillslopes. There was a major phase of hillslope debris-flow a ctivity, followed by fanhead trenching and distal fan progradation. Both ar eas were wetter and colder in the late Pleistocene than they are today, but during the transition to the Holocene the Zzyzx area was more likely to ex perience intense rains associated with the monsoonal penetration of warm mo ist tropical air into the Southwest. Vegetation reconstructions for the lat e Pleistocene to the early Holocene suggest that catchment hillslopes in th e Mojave supported a desert shrub vegetation, but those in the Stillwaters supported juniper woodland and grasses at low elevations and pine at higher elevations. Contrasts in hillslope vegetation cover together with storm ac tivity may account for the different responses of the alluvial fans to clim atic change during the Pleistocene to Holocene climatic transition. After t he fails in lake levels of Lakes Lahontan and Mojave in the early Holocene, both areas underwent aridification, resulting in reductions in hillslope v egetation cover. Increased storm runoff led to fanhead trenching and distal progradation of the alluvial fans. Variations in fan style at that time ma y relate primarily to base-level conditions resulting from different gradie nts on the exposed lake shores. (C) 1999 Published by Elsevier Science B.V. All rights reserved.