The La Grande–Owyhee eruptive axis in eastern Oregon is an ~300-km-long, north-northwest–trending, middle Miocene to Pliocene volcanic belt located along the eastern margin of the Columbia River flood basalt province. The eruptive axis extends from Elgin on the north to Jordan Valley on the south and is juxtaposed between the Chief Joseph dike swarm on the east and the Monument dike swarm and the middle Miocene Strawberry volcanics on the west. Numerous volcanic vents, from which a diverse assemblage of tholeiitic, silicic, calc-alkaline, and alkalic lavas erupted, are contained within or directly adjacent to the La Grande, Baker, and Oregon-Idaho grabens along the length of the eruptive axis. The volcanic rocks that erupted from and are preserved within the eruptive axis form a stratigraphic link between the flood basalt–dominated Columbia Plateau on the north and bimodal basalt-rhyolite vent complexes of the Owyhee Plateau on the south. Volcanism along the La Grande–Owyhee eruptive axis progressed through six stages beginning in the middle Miocene and continuing through the Pliocene. Stage 1 (16.1–15.5 Ma) was characterized by fissure eruptions that produced the Grande Ronde Basalt. Stage 2 (15.5 Ma) was marked by fissure eruptions of highly evolved, tholeiitic lavas (icelandites) and rhyolites. Stage 3 (15.5–14.7 Ma) was distinguished by caldera-forming eruptions of ashflow tuffs and high-temperature rhyolite lavas. Stage 4 (14.7–13.7 Ma) was marked by fissure eruptions that produced olivine basalts. Stage 5 (13.5–10.0 Ma) was characterized by the eruption of calc-alkaline basaltic andesite, andesite, and dacite lavas. Stage 6 (7–1 Ma) was marked by small-volume alkalic eruptions. Mapped field relations and similar timing between emplacement of the Columbia River Basalt Group and volcanic rocks erupted within the La Grande–Owyhee eruptive axis verify a common temporal link between the two successions. This link indicates that chemically diverse volcanic strata exposed along the La Grande–Owyhee axis need to be considered when developing further detailed petrologic and volcano-tectonic models for the Pacific Northwest during the middle Miocene.

ABSTRACT The John Day Formation of central and eastern Oregon, contains 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. Recent detailed geologic mapping in the John Day and Clarno Formations, however, indicates the presence of at least three large-scale rhyolite caldera complexes centered along the northeast-trending axis of the Blue Mountains. This field guide describes a three-day geologic transect, from the scenic high desert of central Oregon eastward across the axis of the Blue Mountains, that will examine the physical volcanology and geologic setting of the 41.50-39.35 Ma Wildcat Mountain caldera exposed along the crest of the Ochoco Mountains, the 29.56 Ma Crooked River caldera at Prineville, and the 29.8 to 28.1 Ma Tower Mountain caldera near Ukiah.