HR: 1340h
AN: V53B-1559    [Abstracts]
TI: Correlation of Tephra Deposits at Newberry Volcano, Oregon: Refining Estimates of Plinian Eruptions
AU: * Duschatko, B C
EM: bcd25@dana.ucc.nau.edu
AF: Northern Arizona University, Department of Geology, PO Box 4099, Flagstaff, AZ 860011 United States
AB: Preliminary field data suggest that some of the 40-50 Pleistocene tephras identified by Kuehn (2002) at Newberry Volcano, central Oregon, may represent different lobes of tephra generated by compositionally zoned, caldera-forming eruptions, rather than discrete eruptive events. Newberry volcano is a large, bimodal shield volcano. The physical, mineral and chemical properties of each tephra deposit were recorded in over 30 test pits on the eastern and northeastern flank of the volcano. The individual components of each tephra deposit were described in 10 cm increments for each pit. The 99-5 tephra, previously thought to represent a discrete eruptive event, is proposed to be part of the Evans Well tephra. Stratigraphic relations from Kuehn dictate that the Evans Well tephra was emplaced during a large plinian eruption between 150,000 and 55,000 yr BP. The 99-5 and Evans Well tephras are composed of coarse, white/light-gray, subangular, fibrous to frothy pumice with rounded vesicles. Pumice diameters range from 2-8 cm and the coarsest pumices are pink or have a pink interior. Both tephras contain mafic and banded pumice throughout, with an increase in concentration at the top of the deposit. Red, black (usually bomb fragments) and purple volcanic lithic fragments are located throughout each of the deposits, with concentrations ranging from 15% (99-5) - 25% (Evans Well). The two tephras nowhere overlap despite their proximity. Other tephra deposits in similar stratigraphic positions at Newberry lack concentrations of mafic and/or banded pumice at the top of the deposit, contain pumices with different physical properties (size, color, vesicularity, inclusions), contain different accidental and juvenile material and have different chemical signatures. Preliminary major, minor, and trace element chemistry analyses show that tephra 99-5 and Evans Well are chemically similar and both tephras show chemical variations with depth. The majority of major and minor elements analyzed in tephra 99-5 are within an acceptable range of values for the corresponding elements in the Evans Well tephra. For example, the SiO2 variations in 99-5 range from 60.1-70.5% and the Evans Well values of 61.9-68.6% fall within that range making the correlation of these two tephras chemically valid. The Evans Well tephra is stratigraphically located between the Pumice Flat eruption, interpreted to have a 40 km plinian column and the caldera-forming Paulina Creek eruption. The stratigraphic position of Evans Well places it in a very active eruptive time period for Newberry Volcano (150,000-55,000 yr BP). Deposits of interpreted Evans Well tephra are found within a large area on the volcano's eastern flanks and in most locations are 2 m thick, similar to deposits from the Pumice Flat eruption. Evans Well, combined with tephra 99-5 may represent a large plinian eruption similar to that of Pumice Flat.
DE: 8428 Explosive volcanism
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005