USE OF SOIL STRATIGRAPHY TO DECIPHER THE 30 KA HISTORY OF HILLSLOPE COLLUVIUM DERVIED FROM WEATHERED BASALT IN NORTHERN CALIFORNIA

The role of soils in both hillslope evolution and the production of surficial sediment is still poorly understood. Application of luminescence dating and terrestrial cosmogenic nuclides to elucidate general rates of both the production and erosion of the soil mantle is increasing; however, many studies commonly lack information about either soil morphology or stratigraphy. Knowledge of soil-stratigraphy is important in evaluating hillslope dynamics because both the degree of soil formation and the presence of buried or truncated soils provide evidence of the relative age of periods of slope stability and erosional slope transport, as well as evidence of slope transport processes. We use evaluation of soil stratigraphy, for a small forested hillslope (<500 m of slope length) located in the Cascades of northern California, to elucidate both the timing and processes controlling ~ 30 ka of hillslope evolution. Stratigraphic interpretation is reinforced with soil profile development index (PDI) age estimates, tephrochronology, and He³ nuclide exposure dates. Soils formed along hilltop ridges are well developed and reflect deep in-situ weathering (>2-3 m) of the basalt bedrock. PDI age estimates and He³ exposure dates indicate that these soils are ~100 ka, implying a long period of relative surface stability until about 30 ka, when erosion of the hilltop soils began. Evidence for changes in stability and onset of soil erosion is the presence of several buried soils downslope of the hilltop. These buried soils have formed in sediment derived from erosion of the hilltop soils (i.e. previously weathered soil matrix and basalt cobbles). The oldest buried soil indicates slope stability occurred again between ~30-10 ka, followed by one or two additional intervals of downslope transport of sediment between ~10-2 ka. The surface soil appears to be an active layer of sediment transport that cuts across all stratigraphy. Soil properties indicate that the primary method of downslope transport is likely due to tree throw and faunal burrowing. Onset of slope instability at ~ 10 ka appears to be related to changes in vegetation from a shrub steppe to a pine dominated forest and an increase in local channel incision at ~10-12 ka. Results indicate that soil stratigraphy can provide a key record of slope evolution.