OUTBURST FLOOD DEPOSIT FROM WILLIAMSON RIVER CANYON, OREGON: FAILURE OF A PYROCLASTIC DAM

Pyroclastic flows from the eruption of Mount Mazama entered and blocked the 7.3 km long, bedrock-lined Williamson River canyon approximately 30 km southeast of the source. Eventually, the dam failed and released a lake it had impounded. Remnants of the outburst flood deposit are preserved from within the lower reach of the canyon and for at least 4 km downstream. The deposit overlies pyroclastic flow deposits on its north side and hydrovolcanic tuff and siltstone bedrock on its south side. Pumice boulders resting on the surface and a flow-direction aligned pumice bar indicated at least part of the post-flood surface is preserved. The deposit has two parts: boulder core and sand-rich deposits. The boulder core extended 2.5 to 2.7 km from within the mouth of the canyon and contained boulders up to 3.4 m in intermediate axis diameter. Sand (passing -1.0 to 3.75 phi) comprised of weakly vesiculated pumice, dense glass, and crystals overlies and flanks the boulder core near the mouth of the canyon. Sand density collected on 0.25 (0.70-1.35 g/cm³), 1.25 and 2.00 (1.2 to 1.4 g/cm³), and 3.75 (0.85 – 1.30 g/cm³) phi sieves was relatively constant within the first 2 km of the mouth of the canyon. Sparse pebbles of basalt, hydrovolcanic tuff, and pumice are present. Locally, sand comprised 95 to 100% of samples by weight. Sand-rich deposits are up to at least 1.5 m thick, but thickness of the boulder core is poorly known and probably is greater than 6 m. Beyond 2.7 km of the canyon, grain size distribution was commonly bimodal with pumice pebbles and pumice/crystal medium-grain sand. Dense glass and crystals became less common, and pumice vesiculation increased in sand. Variability in density increased for all sand fractions (0.25 phi 0.4-1.3 g/cm³; 1.25 phi 0.42-1.42 g/cm³; 2.0 phi 0.5-1.4 g/cm³; 3.75 phi 0.6 to 1.4 g/cm³) and some samples contained 35 to 43% by weight pumice pebbles. At 4 km from the canyon the outburst flood deposit was at least 1.3 m thick and varied from pumice pebble rich (north side) to poor (south side). Silt coats grains of all sizes in all samples. This coating is extensive near the canyon and is black to dark brown. The extent of coating becomes less and is light brown with increased distance. The presence of this coating allowed the flood deposit to be distinguished from pyroclastic flow deposits.