Environmental controls on the evolution of alluvial fans in Buena Vista Valley, North Central Nevada, during late Quaternary time

Abstract

Alluvial fans in Buena Vista Valley, north-central Nevada, formed during at least four time periods during the late Quaternary as evidenced by four temporally and spatially constrained fan deposits: Qf1, Qf2, Qf3, and Qf4, from oldest to youngest. The stratigraphic relation between Qf3 and remnant beach ridge deposits of pluvial Lake Lahontan, the latter of which forms a paleoclimatic proxy for this region, indicate that climate and climatic change are primary controls on the evolution of fans. Qf3 deposition commenced during or immediately following the highstand of Lake Lahontan, between 13.5 and 12.5 ka, and continued as lake levels plummeted to levels below the sill elevation of the Buena Vista subbasin. We interpret Qf3 to be the result of a dynamic interplay between hillslope vegetation, soils, and hydrology as they are conditioned by climate and climatic change. Accelerated pedogenesis, because of aerosolic silts and clays from exposed lake sediments, coupled with decreased density of vegetation reduced infiltration capacity of the hillslope soils in interscrub areas. In contrast to fans in more arid climates, the increased runoff was not associated with increased coarse-grained sediment yield and major fan aggradation. Discharge from sediment-deficient basins entrenched older, proximal fan deposits and transported the sediment downfan to form secondary fans. The process–response of Qf3 to climatic change in north-central Nevada is especially illustrative of the impact of climate on landscapes in that: (1) it results from changes in basin sediment and/or water yield that occur in less than 500 years, and (2) it differs from responses in arid climates to the south and more semiarid climates to the north. The magnitude of vegetation change, including type and density of vegetation and the relative proportion of interscrub area, is particularly critical to water and sediment yield. Unit Qf2, an early late Pleistocene deposit, may also have been deposited during a transition from a wetter to drier climate. The synchronism of aggradation, stability, and entrenchment on all fans along the perimeter of Buena Vista Valley in the late Quaternary is further support for the dominance of climatic control on fan process; on the other hand, the impact of other extrinsic variables on fan process, including tectonism, base level, and basin lithology, all of which vary along the perimeter, is negligible.