HOW TOUGH IS TUFF: QUANTIFYING HOW RIVER ROCKS ROUND AND FINE DUE TO ABRASION DURING BEDLOAD TRANSPORT OF DIFFERENT GRAIN SIZE DISTRIBUTIONS

Along rivers, bedload transport causes collision and wear of sediment particles, and riverbed sediments exhibit two downstream trends: rounding in which rocks tend to get rounder, and fining where particle size decreases with distance downstream. Sediments supplied by hillslopes to rivers are initially angular upstream, and smooth into rounded gravels downstream as a result of abrasion. The transition from angular to round particles in fluvial environments indicates that a significant fraction of particle mass has been lost due to abrasion, but very little is known about what controls the ubiquitous pattern of rounded river rocks in real rivers. Previous studies have shown abrasion to occur in two phases: the first phase being dominated by rounding, and the second phase resulting in size reduction. However, few studies have investigated what is the role of grain size distribution on wear rates, and questions remain on the role of abrasion in downstream fining. Further, abrasion rates measured in laboratory experiments have been limited to a relatively few types of lithologies. To quantify the response of volcanic rocks to abrasion in the fluvial environment, we performed tumbling experiments on Bishop Tuff which erupted from the Long Valley Caldera in eastern California at 760 ka. Our samples were collected from the densely welded portion of Bishop Tuff exposed within the middle Owens River Gorge. We used a laboratory tumbling mill to quantify abrasion rate as a result of collisions of varying grain size distributions and examine downstream evolution of shape and size of pebbles. Using image-based shape parameters, we show that (1) Bishop tuff exhibits the highest wear rates during the first phase of abrasion, (2) particles tend to retain some micro-angularity despite significant mass loss, (3) fragmentation and chipping are the dominant mechanisms for the observed mass lost, and (4) the wear product consists predominantly of fine silt. Results show that narrow grain size distributions wear slowly, whereas wider grain size distributions yield higher wear rates and rates of fragmentation. Overall, fragmentation is the dominant mechanism of abrasion in the downstream reduction of particle mass in the lithology studied, with important implications for sediment mobility and generation of fine sediments.