REVISED STRATIGRAPHIC AND STRUCTURAL HISTORY OF PALEOGENE BASALT-RHYOLITE COMPLEX EXPOSED IN THE PAISLEY HILLS, SOUTH-CENTRAL OREGON

New mapping and U/Pb geochronology have helped refine the stratigraphy and geologic history of Paleogene volcanic formations present in the Paisley Hills, Lake County, Oregon. Two formations are recognized: a lower, mafic, Paisley Fm, and an upper, siliceous, Chewaucan River Fm. The lower part of the Paisley Fm is predominately mafic mudflows dated from 37-35 Ma. It is altered to chlorite + epidote adjacent to small 33.5 Ma monzonite intrusions. The upper Paisley Fm is mostly lava flows and some lahars, and has yielded a single 35 Ma age near its base. The Paisley Fm measures more than 2.6 km thick along the canyon SW of Paisley. A laterally extensive 28.5 Ma dacite flow marks the transition from the Paisley Fm to the overlying Chewaucan River Fm.

The Chewaucan River is composed of a lower set of highly welded, mostly qz-phyric tuffs that were erupted and intruded between 29-26 Ma. Normal faults that offset these units are intruded by only slightly younger quartz-phyric stocks. These relationships may reflect emplacement at the edge of an active caldera. The upper Chewaucan River is a series of 50-100 m thick, lightly welded ~24-23 Ma siliceous ash flow tuffs that rest unconformably on the lower Chewaucan River. The youngest strata are enigmatic ‘tuff-breccia’ beds that we interpret to be phreato-magmatic sandstones. One occurrence yields zircon age peaks at 24.5 and 7.3 Ma. Bedding within a potentially correlative outcrop has been rotated near vertical adjacent to the large, west-dipping Paisley Hills fault, suggesting at least some of the ~1.6 km offset occurred as part of the post-~7 Ma basin and range faulting in this area.

Rocks from both formations have SiO₂ contents from ~47-70 wt.%. Basalt lavas of the upper Paisley Fm have generally evolved compositions (e.g., Mg# <60, MgO<6 wt.%). Basaltic rocks within the overlying lower Chewaucan River Fm are the least evolved, with Mg#=58-62, MgO=7.5-8.5 wt.%, and Cr=250-320 ppm. Na₂O and K₂O increase to 5.5 and 4.5 wt.%, respectively, with increasing SiO₂. Intermediate and felsic rocks of the Chewaucan River Fm have generally higher K₂O than rocks of the Paisley Fm having similar SiO₂ contents.