PUMICE FLAT TEPHRA OF NEWBERRY VOLCANO, OREGON: DEPOSIT OF A MIXED-MAGMA PLINIAN ERUPTION

The Pumice Flat tephra (also designated as tephra 9822B) probably represents the largest known plinian eruption of Newberry Volcano, a large bimodal shield located in central Oregon. The estimated age of the eruption is 100-150 ka.

The maximum preserved thickness of 3.4 m and the most complete sequence are both known from site 98-22 located 22 km ESE from Newberry caldera. At other locations studied, much of the deposit has been removed by erosion. At site 98-22, the sequence consists of (1) a 3 cm basal ash overlain by (2) 205 cm of coarse white pumice. The pumice is coarsest at about 50% of the total deposit thickness. Mingled pumice (containing glass compositions from 58 to 73 wt% SiO2) are initially rare, but increase in abundance upward. Contrast between glass compositions in the mingled pumice remains sharp even at a scale of a few microns. Lithic content also increases upward from about 3.5% to 35% by mass. This portion of the deposit is overlain by (4) an 8 cm zone containing white pumice with common accretionary lapilli, (5) an 18 cm thick texturally and compositionally bimodal bed containing coarse white pumice in a matrix of black ash (40% ash by mass), (6) 50 cm of coarse white pumice with a black ash coating, (7) a 1 cm bed containing abundant accretionary lapilli, (8) 23 cm of black scoria in a black ash matrix (30% ash by mass), (8) 12 cm of black scoria without ash, and (9) 65 cm of pale yellow to buff-colored pumice with some silicic ash and few dark scoria.

Preliminary data on overall lithic clast distributions suggest a major dispersal axis to the ESE and a possible secondary dispersal to the NE. The largest individual pumice (48 cm diameter) and lithic (14 cm diameter) clasts are known from a site 10 km ESE of Newberry caldera. Lithic clasts with a diameter of 6.4 cm (average of 5 largest clasts) reach at least 25 km from Newberry caldera along the major axis and probably have an axis-perpendicular distribution at least 12 km wide. These data indicate deposition from an eruption column at least 40 km in height with tropopause winds of about 30 m/s.