25 Mesozoic-Cenozoic Orogenesis Along the Pacific Northwest Cordilleran Margin

Thursday, 3 April 2025
1:30 PM - 5:30 PM
Granada/Hermosa

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

Paper number: 25-2
Program: Theme Sessions
Time: 1:50 PM – 2:10 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 57, No. 4

doi: 10.1130/abs/2025CD-409908

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