2009 Portland GSA Annual Meeting (18-21 October 2009)
408 Thursday, 15 October 2009
7:30 AM-6:00 PM, Oregon Convention Center:

Paleogene Calderas of Central and Eastern Oregon: Sources of Widespread Ash-Flow Tuffs in the John Day and Clarno Formations

Primary Leader: Jason D. McClaughry
Leader(s): Martin J. Streck, Karyn A. Patridge, Caroline L. Gordon, Mark L. Ferns
Field Trip Description: The Paleogene John Day and Clarno Formations of central and eastern Oregon contain a widespread assemblage of ash-flow tuffs, previously considered to be the eruptive products of small, isolated volcanic centers coincident with the modern day Cascade volcanic arc. Recent detailed geologic mapping in these Paleogene rocks has identified at least three large-scale rhyolite caldera complexes centered along the northeast trending axis of the Blue Mountains in central and eastern Oregon. These calderas are volcanic vents much larger than the small, isolated eruptive centers envisaged by preceding workers and are comparable in size to some of the largest calderas known worldwide. Over the course of the three day trip, field sites will explore stratigraphic and geochemical aspects of the ~29.5 Ma Crooked River caldera at Prineville, the ~40 Ma Wildcat Mountain caldera exposed along the crest of the Ochoco Mountains, and the ~28.5 Ma Tower Mountain caldera exposed near Ukiah in eastern Oregon. Highlights include examination of spectacular examples of ash-flow tuff deposits exposed at Smith Rock, Steins Pillar, and Dale and associated rhyolite domes and flows.
Field Trip will span: 3 days
Sponsor(s): Oregon Department of Geology and Mineral Industries; Portland State University

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