31-9 Mapping Extrusive Volcanics in South Central Oregon—The Potential for Primary Resources including Lithium, Uranium, and Gold highlights a focus for Geologic Mapping in the Paisley Hills

Session: Best Student Geologic Map Competition (Posters)

Presenting Author:

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Abstract:

New mapping and U/Pb geochronology of the Paisley Hills, in south-central Oregon, has helped refine the stratigraphy and geologic history of Paleogene volcanic formations in the region. The lower, mafic, Paisley Fm and the upper, siliceous, Chewaucan River Fm were first described by Walker, G.W. in 1963 and by Muntzer, J.K. in 1969. No systematic study of these ‘core’ rocks has been published since early work in the 1960s. The lower part of the Paisley Fm is predominantly mafic mudflows that experienced hydrothermal alteration with dates ranging from 37 to 35 Ma, while the upper part of the Paisley Fm exhibits a series of lava flows, lahars, and a felsic unit at the base. A laterally extensive 28.5 Ma dacite flow marks the transition from the Paisley Fm to the overlying Chewaucan River Fm. The lower Chewaucan River Fm is composed primarily of highly welded, quartz-phyric tuffs that erupted and intruded between ~28.5 to 26 Ma, and the upper Chewaucan River Fm is lightly welded ~24 to 23 Ma siliceous ash flow tuffs rests on an unconformity. The youngest volcanics are the Winter Rim Basalts that overlie an unconformity and are emplaced horizontally on the tilted beds of the Chewaucan River and Paisley Formations. Faulting of the described units shows extensional forces akin to those that shaped the Basin and Range occurred in recent geologic time. Due to nearby historic gold and uranium mining and the potential of lithium deposits the State of Oregon Department of Geology and Mineral Industries have determined that mapping of the region is a priority. Mapping of the region was carried out in 2024 under the USGS EDMAP Program. Mapping at 1;24,000 scale was undertaken by three students in key areas of the Paisley Hills, one each in the Paisley, Morgan Butte, and Shoestring Butte 7.5-minute quadrangles under the guidance of PI’s Dr. Gary Gray and Dr. Kenneth Johnson for the project. My focus was mapping a section of the Paisley 7.5-minute Quadrangle which had excellent exposure of volcanic strata along the Chewacan River road. This exposure made it the ideal candidate to build a stratigraphic section, this coupled with descriptions in the field and geochemistry conducted in the lab allowed for a detailed understanding of the region.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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