EOCENE AND OLIGOCENE RHYOLITE CALDERAS IN THE JOHN DAY AND CLARNO FORMATIONS OF CENTRAL AND EASTERN OREGON, USA: A NORTHWARD EXTENSION OF THE “IGNIMBRITE FLARE UP” IN THE GREAT BASIN?

The Eocene Clarno and Oligocene to early Miocene John Day Formations of central and eastern Oregon contain a widespread assemblage of both ash-flow and airfall tuffs, yet only a few corresponding caldera sources have been identified in the region. Investigators have long speculated on the sources of tuffs in the John Day Formation and have suggested that these pyroclastic rocks were vented from now buried eruptive centers in or marginal to a nascent Cascade Range. These presumed buried sources were used to build a tectonic model in which John Day volcanism documented a westward jump of an Eocene subduction zone at the end of “Clarno arc” volcanism. Regional geologic mapping in the Clarno and John Day Formations, however, indicates the presence of at least three large-scale rhyolite caldera complexes centered along the northeast trending axis of the Blue Mountains. These include: (1) the 41.50–39.35 Ma Wildcat Mountain caldera exposed along the crest of the Ochoco Mountains, (2) the 29.56 Ma Crooked River caldera at Prineville, and (3) the 29.8 to 28.1 Ma Tower Mountain caldera near Ukiah in northeast Oregon. The Wildcat Mountain, Crooked River, and Tower Mountain calderas are part of a broader sweep of voluminous ash-flow tuff magmatism (ca. 41–23 Ma) preserved in volcanic and intrusive rocks distributed across the axis of the Blue Mountains in central and eastern Oregon, and in correlative rocks extending into the Western Cascades in southwest Oregon, at Hart Mountain in south-central Oregon, and near Potlatch in northern Idaho. These newly recognized calderas and their correspondent ash-flow and airfall tuff deposits distributed well east of previously postulated vent areas document a regionally extensive magmatic episode unrelated to a largely inferred ancestral Cascade Range. These igneous centers may represent a northward extension of the contemporaneous middle Eocene to Oligocene (ca. 43–23 Ma) “ignimbrite flare up” in the Great Basin of the southwest United States.