THE CROOKED RIVER CALDERA: IDENTIFICATION OF AN EARLY OLIGOCENE ERUPTIVE CENTER IN THE JOHN DAY FORMATION OF CENTRAL OREGON

Recent geologic mapping by DOGAMI in central Oregon has identified an early Oligocene caldera, temporally equivalent to large ash flow sheets exposed in the John Day Formation. The Crooked River caldera, centered near Prineville along the northern margin of the High Lava Plains, is a northeast trending volcano-tectonic depression, measuring ~30 km in length by ~20 km across. The preserved core of the caldera consists of a > 300 m thick section of early Oligocene zeolitized pumice-lithic tuff inset into a dissected Eocene section of overlapping andesite and dacite porphyry domes and flows that preserve a pervasive east-west structural fabric. Intracaldera tuff can be traced from Ochoco Reservoir on the southeast margin to a correlative lithic-rich tuff facies at Smith Rock on the northwest margin. The caldera-fill tuff is ringed and intruded by large (50–80 km²) fields of late Oligocene peralkaline rhyolite lava flows and domes exposed at Powell Buttes, Gray Butte, Grizzly Mountain, Barnes Butte, and Ochoco Reservoir. Two, densely welded, pumice and glass shard-rich, outflow tuff cooling units can be traced south of the caldera margin to Prineville Reservoir. Outflow tuff facies rest unconfomably upon an older, tilted section, of early Oligocene ash flows (~32 Ma) and an alkali basalt flow dated at 30.1 Ma. Post-collapse rhyolite domes and flows that intrude or cap caldera-fill have radiometric age dates that range from 28.8 Ma to 25.8 Ma. These bracketing radiometric age dates constrain the largest caldera forming eruption(s)? between 30 and 29 Ma. Previous workers have correlated both the Gray Butte rhyolite and tuff of Smith Rock to member G of the John Day Formation; they reported radiometric ages that range from 29.53 to 29.61 Ma for a welded ash flow tuff at the base of member G that is exposed a short distance north of Smith Rock.

Although situated near the northern boundary of the High Lava Plains (~ 50 km NE of Newberry Volcano) caldera units are remarkably only locally faulted and warped along a northeast trending axis. Resolution of syn- versus post-eruption deformation of caldera units in this area is unclear. The overlying middle Miocene Prineville basalt is locally faulted; olivine basalt situated above the 7.05 Ma Rattlesnake Tuff in the Crooked River canyon south of Prineville shows no evidence of tectonic deformation.