%0 Journal Article
%T Debris Fan Produced by Failure of Canyon-Blocking Pyroclastic Flows
%A Michael L. Cummings
%J Journal of Water Resource and Protection
%P 328-360
%@ 1945-3108
%D 2024
%I Scientific Research Publishing
%R 10.4236/jwarp.2024.165019
%X Ash-rich pyroclastic flows from the cataclysmic eruption of Mount Mazama (~7700 yr. B. P.), Cascade volcanic arc, Oregon, entered and blocked the narrow, bedrock-lined canyon of the Williamson River approximately 35 to 44 km from the source volcano. The blockage impounded a body of water which then released producing four stratigraphic units in the downstream debris fan. The four stratigraphic units are a boulder core comprised of locally sourced bedrock boulders and three sand-rich units including a fine-grained sand unit, a sandy pumice gravel (±basalt/hydrovolcanic tuff) unit, and a pumice pebble-bearing, crystal-rich sand unit. Hand-drilled auger holes up to ~1.6 m deep were used to obtain samples of the sand-rich units. Units were delimited using surface and down-hole observations, composition and texture, estimated density, statistical parameters of grain size, and vertical and lateral distribution of properties. Overtopping followed by rapid incision into the ash-rich pyroclastic flows progressively cleared the canyon, but a bedrock knickpoint near the head of the canyon limited the volume of debris available for transport to about 0.04 km<sup>3</sup> to 0.08 km<sup>3</sup>. Co-deposition of bedrock boulders and lithic-rich sand was followed by rapid deposition with minimal reworking of remobilized pyroclastics. Continued draining of the impounded lake sent hyperconcentrated flows onto the debris fan depositing pumice-rich gravels that graded upward to crystal-rich sands.
%K Outburst Flood
%K Mount Mazama
%K Debris Fan
%K Canyon Blockage
%K Pyroclastic Flows
%U http://www.scirp.org/journal/PaperInformation.aspx?PaperID=133107