SEDIMENTOLOGICAL AND GEOMORPHOLOGICAL PERSPECTIVES ON GRAINSIZE DISTRIBUTIONS: HOW SEDIMENTS GET THEIR SIZE IN THE SIERRA NEVADA, CALIFORNIA

Traditionally, grainsize distributions are used to infer sedimentological paleoenvironments and past flow regimes. Rather than just being an indicator of flow, new work on sediment production views grainsize as a product of both physical and chemical processes. Yet, the relative roles of physical breakdown and/or chemical weathering (i.e., kaolinization and secondary mineral production) in producing a deposit’s resulting grainsize distribution remains poorly understood. In order to quantify the physical and chemical effects that act to produce a grainsize distribution, we couple sediment residence times with geochemical mass balances and grainsize distributions to describe the evolution of grainsize along hillslopes within the Feather River basin of the Sierra Nevada, California. Our results show that particle sizes increase with both hillslope slope gradient and soil-depth. The average grainsize of particles at the soil-saprolite boundary increases with slope from 78 to 181 to 275mm. Using Zr as an immobile element, hillslope sediments appear virtually identical to the underlying saprolite, which indicates that sediment production and the mechanisms that determine particle size distribution is mostly a physical process. Additionally, turnover times calculated from slope-based erosion rates for the basin (Riebe et al., 2000) indicate that once in the soil, particles physically breakdown at rates of 0.9, 10, and 27mm/kyr (in order of increasing slope) along the three measured hillslopes (above, at, and below the river’s knickpoint, respectively). The fact that the particles in the three hillslopes are breaking down at different rates means that the higher-sloped colluvium is fining faster, even though it has an overall coarser grainsize than soils on shallower slopes with higher degrees of chemical weathering. Taking this as one of the first steps towards quantifying the relative roles of physical and chemical weathering on grainsize distributions, we should be able to one day use grainsize distributions from the sedimentary record to calculate the rates of paleo-soil formation, past sediment production, and previous regimes of chemical and physical weathering.