PALEOGENE BASALT-RHYOLITE COMPLEX EXPOSED IN THE PAISLEY HILLS, SOUTH-CENTRAL OREGON

New geologic mapping and geochronology have established that two previously undescribed and unnamed Paleogene volcanic formations are present in the Paisley Hills, Lake County, Oregon. The lower layers, informally named the Paisley Hills Formation, are a succession of basalt flows and interbedded mafic lahars at least 3 km thick. The base of this section is not observed. Highly altered (chlorite+epidote) volcanics are the structurally deepest units exposed and are overlain by the basalts. The basalts, in turn, are overlain by an additional ~2 km of variably welded, siliceous ash-flow tuffs that we have informally named the Chewaucan River Formation. The basal part of this siliceous section contains at least three small eruptive centers. The uppermost beds are water-lain tuff.

We obtained 21 new U/Pb zircon ages on these strata in the Paisley Hills and adjacent areas. The altered volcanics yield ages of 37 Ma. Rhyolitic layers interbedded within the mafic flows bracket rest of the section the to be 35-29 Ma. The overlying Chewaucan River Formation ranges in age from 28 to 23.5 Ma. The oldest Paisley Hills Formation is ~4-5 Ma older than lithologically similar units in southwest Oregon and northern California, but is the same age as the basal Western Cascades section near Eugene, OR. The Paisley Hills Formation also overlaps in age with the upper Clarno Fm. that occurs in NE Oregon. Preliminary chemistry indicates the Paisley Hills mafic flows are calc-alkaline like the Cascades, not tholeiitic like the Clarno Fm. It is possible, therefore, that this section represents a remnant of the early Cascades arc, although it is older than, and lies ~150 km east of the nearest Western Cascades arc volcanics. The chemistry of the siliceous units has not yet been characterized.

The Paisley Hills are onlapped by flows of the 16-17 Ma Steens basalt and the 7-8 Ma Winter Rim basalt, demonstrating the Hills have been a prominent topographic feature since at least the middle Miocene. Large offset, down-to-the-west normal faults displaced the Paleogene section prior to onlap of the Winter Rim flood basalts. Younger Basin-and-Range normal faults cut the eastern flank of the Paisley Hills and the Winter Rim units.