PRODUCTION, STORAGE, AND TRANSPORT OF SEDIMENT IN ARID BASINS DURING PERIODS OF HOLOCENE CLIMATE CHANGE

Recently, there has been much interest within the scientific community on sediment storage and slope stability on hillslopes. We are examining soil and colluvial thickness across desert mountain basins to better elucidate sediment production, storage, and mobilization in semi-arid to arid basins and how soil stability is linked with sediment yield and aggradation of alluvial fan sequences. We analyzed two lithologically varied basins that are geographically similar, located in the Providence Mountains, Mojave Desert, CA. Alluvial fans have been studied in a variety of piedmont settings, but there have been few studies of soil-geomorphic processes governing sediment production, stability, and mobilization of sediment out of mountain drainage basins. Previous research has focused on the idea that alluvial fan formation occurs during periods of paleoclimate change when plants and soil are “stripped” from slopes during the transition from a “wet” to “dry” climate. In the Providence Mountains extensive alluvial fan sequences linked with known periods of global climate change have formed from these basins over the past 100,000 years. The alluvial fan sequence in the Providence Mountains has been studied in great detail, previously mapped and dated soils in proximal fan positions were used to approximate soil ages within the two basins. Soil pits were strategically placed in the basins to acquire age estimates, soil samples were collected for lab analysis from both basins. Initial results indicate that the soils in these basins are very young, possibly as young as late Holocene (<4ka). Field analysis has shown that the granitic basin on average has a thin covering (<20 cm) of weakly developed soil, primarily consisting of physically weathered bedrock with a strong eolian contribution. The limestone basin has a surprising amount of colluvial accumulation (~40 cm) on the hillslopes and contains abundant silt primarily from eolian sources; these soils are also weakly developed. Mapping soil-colluvial thickness of both basins shows differing sediment thickness on formations within the basins indicating that the pattern of weakly developed soil is extensive. Based on preliminary work it appears that there was at least one period of extensive sediment stripping off the hillslopes after the Pleistocene/Holocene transition.