HOLOCENE LOESS IN THE AMARGOSA DESERT, INYO COUNTY, CALIFORNIA

Although aeolian dust commonly accumulates in vesicular A horizons beneath desert pavements, desert loess is rarely found in American deserts. Winds blowing southward across Franklin Lake playa in the Amargosa Desert are deflected by Eagle Mountain, which has resulted in the accumulation of 0.5-1.5 m of desert loess. The fine sandy silt consists of 55%-65% silt and contains quartz, feldspar, calcite, and amphibole with abundant soluble salts. A thin basal unit in two excavated pits is OSL-dated at 8,700 to 10,100 yr BP. Below the loess is thin, fan gravel. Strongly oxidized sand below the gravel dated at 21,170 yr BP indicates that little-to-no deposition occurred during pluvial conditions of OIS 2. The bulk of loess accumulation began about 4,500 yr BP and continued to about 1,800 yr BP with short depositional breaks recorded in different pits at different times. The uppermost 10-12 cm thick loess unit exhibits characteristics similar to Av horizons. The base of this unit is about 1100 yr BP, and the uppermost 5-6 cm of sandy silt accumulated during the past 600-640 years.

Basal loess age correlates with regional drying out of playas at the end of the Younger Dryas. The main accumulation of sandy silt and salts blown off Franklin Lake playa correlates with the time that ENSO activity dramatically increased in the Pacific Ocean near the end of the 5^th Millennium. Deposition of the uppermost loess took place during the Medieval Warm Period and continued up to the present. Interestingly, little to no loess was deposited during the Holocene climatic optimum. Drier playa conditions during that time likely created unfavorable conditions for dust mobilization, whereas the increased frequency of wet—dry episodes associated with strong ENSO activity provided abundant fine-grained sediment for aeolian transportation. Loess accumulation in the Amargosa Desert correlates with nearby Holocene dune activity at Ash Meadows and at Mesquite Lake playa.