HR: 1330h
AN: V42B-0354    [PDF]
TI: Plinian Newberry Pumice and Related Eruptions of Newberry Volcano, Central Oregon, USA
AU: * Kuehn, S C
EM: kuehn@geology.csustan.edu
AF: California State University, Stanislaus, 801 W Monte Vista Ave, Turlock, CA 95382
AB: The Newberry Pumice deposit forms a highly elliptical, eastward-extending lobe, and distal deposits are known from as much as 550 km downwind in Idaho. The 25 cm isopach has an aspect ratio of 7.5:1. Pumice and lithic isopleths also have a strongly elliptical pattern for smaller clasts. Larger clasts (e.g. lithic fragments greater than about 10 cm in longest diameter) have a more nearly circular distribution. The maximum crosswind range for a 3.2 cm lithic fragment is 7 km, and the maximum downwind range is 27 km. These data indicate a maximum column height of about 25 km and a tropopause wind velocity far in excess of 30 m/s. The three largest ballistic lithic fragments at a site on the south rim of Newberry caldera have 1.0, 1.12, and 1.3 m median diameters. The maximum observed thickness of 7-8 m is known from the same locality. The vent location is probably about 1.2 km from this location but could be as close as 0.5 km. Initial velocities of about 130 m/s and 95 m/s are needed for the measured blocks to reach distances of 1.2 and 0.5 km respectively. Particle size and lithic content vary significantly with stratigraphic height. As many as seven ash layers may be observed in the upper part of the deposit. In general, the upper part of the deposit has smaller clasts and a greater lithic content. It also has a different distribution direction with an azimuth of 75-78\deg versus 82\deg for the lower portion. This change in trajectory indicates that wind directions shifted during the eruption and/or that lower altitude winds had a different direction than higher altitude winds. The Newberry Pumice was preceded by at least one small precursory eruption and followed by the Paulina Lake ashflow and Big Obsidian flow. On the south rim of Newberry caldera, the Paulina Lake ashflow deposit includes pyroclastic surge beds overlain by a nonstratified deposit of one or more pyroclastic flows. The basal contact is irregular and suggests erosion. Seven radiocarbon dates for the ashflow deposit span 1,270+/-60 to 2,054+/-230 $^{14}$C years BP. The mean limiting age is 1320+/-30 $^{14}$C years BP (1,260+/-40 2$\sigma$ calibrated age). Nineteen dates on wood and charcoal found at or near the base of the Newberry Pumice span 1,510+/-80 to 2,210+/-140 $^{14}$C years BP. The mean limiting age is 1,525+/-40 $^{14}$C years BP (1425+/-95 2$\sigma$ calibrated age). A sequence of deposits at the south shore of Paulina Lake records at least two eruptions prior to the ashflow with significant time elapsed between eruptions. Either could be the Newberry Pumice.
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting