HR: 1340h
AN: H53C-1277    [Abstracts]
TI: Lithologic Influence and Experimental Variability in Gravel Abrasion: Implications for Predicting Rates of Downstream Fining of River Bed Sediments
AU: * Farrow, J W
EM: joe\_zephinus@yahoo.com
AF: Department of Geosciences, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132 United States
AU: Sklar, L S
EM: leonard@sfsu.edu
AF: Department of Geosciences, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132 United States
AB: The question of what controls the occurrence and rate of downstream fining of bed-material sediments remains a fundamental unsolved problem despite over a century of field, experimental and theoretical investigations. Downstream fining rates are commonly modeled as exponential or power-law functions of travel distance. Much recent work has focused on the relative influence of particle abrasion and differential transport, however, no general method has been developed for explicitly accounting for the influence of rock strength in parameterizing fining models. Here we report preliminary results of laboratory tumbling experiments in which we are investigating the influence of variable rock durability, both between and within distinct lithologic units, on rates of particle abrasion. We consider three separate questions: 1) can rock tensile strength be used to predict differences in bulk fining rates across a wide spectrum of rock types; 2) does variability in rock durability among individual gravel clasts of the same lithologic composition and initial grain size lead to patterns of downstream evolution of grain size distributions that differ significantly from the predictions of simple fining models; and 3) how large is the uncertainty in abrasion coefficients determined by laboratory tumbling, as determined by replicate experiments with identical initial conditions? We use a horizontal axis, 25-cm diameter, steel barrel tumbler, driven by a mechanical transmission with excellent control of rotational velocity. Rock samples were collected from units of the Franciscan Formation in the Redwood Creek Watershed of Marin County, California, and from sedimentary and intrusive volcanic rocks of the Henry Mountains, in southeastern Utah. We collected clasts predominantly from hillslope source areas, to focus our attention on the durability of gravel as it enters the river network. We use the `Brazilian' tensile splitting test to measure the strength of 50-mm diameter core samples obtained from the same localities as gravel source areas. We determine mass loss by weighing individual gravel clasts, and calculate abrasion coefficients after at least five or six runs, with a typical total `travel' distance of 4-6 km. We find a systematic variation in gravel abrasion rate with rock tensile strength, as well as an evolution of the grain size distribution that is quite sensitive to lithology. For initial conditions of both uniform-sized and log-normally distributed size mixtures, we observe development of skewed and bimodal distributions. We interpret consistent development of positively skewed distributions as reflecting differential pre-existing fracture density among otherwise identical clasts. Abrasion of sedimentary rocks tended to produce a large proportion of sand and silt, presumably due to failure between sediment grains. In contrast, microcrystalline rock, including chert and serpentinite, produced more fine gravel by clast splitting and trended toward a bimodal grain size distribution. Finally, we quantify significant uncertainty in experimental abrasion coefficients for replicate runs with the same initial weight distribution and lithology. Although the data are well fit with a power law relationship, the 95% confidence interval for the estimate of the abrasion exponent ranges from plus or minus 20% to 75% of the mean of three replicates. We conclude that predictive models of downstream fining by abrasion need to incorporate the lithologic sources of both systematic and stochastic variability in the rate of bulk mass loss and the resulting evolution of grain size distributions.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1869 Stochastic processes
DE: 1886 Weathering (1625)
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting