NEW GEOLOGIC MAPPING IN LOGAN VALLEY WEST AND MAGPIE TABLE QUADS IN THE NE HARNEY BASIN CORRIDOR, EASTERN OREGON, PROVIDES IMPORTANT LOCAL DATA FOR THE MAGMATIC HISTORY OF THE STRAWBERRY VOLCANICS

Current mapping efforts and earlier work (e.g. Steiner & Streck, 2014, GEOLSO, https://doi.org/10.1144/SP385.12) in the NE portion of the Harney Basin, Oregon, has revealed many previously unclassified and unknown mid-Miocene rhyolitic units and tuffs related to the Strawberry Volcanic field. This sheds light on the geologic history of the NE margin of the Harney Basin and the regional volcanic framework. This region was mapped originally by reconnaissance in the 1960s, with many units poorly delineated, often blanketing whole quadrangles as one unit, Ts – Strawberry Volcanics.

Field distinguishing criteria to map rhyolites include abundance and size of phenocrysts, ranging from aphyric to ~20% phenocrysts of mostly plagioclase, whether biotite and/or hornblende occur, as well as stratigraphic position and other lithological characteristics. Analytical data confirmed most unit designations devised by field work, but also led to the separation of units that were lithologically indistinguishable, such as aphyric obsidian. In general, these rhyolites range in composition between 70-77 wt% SiO₂ and 0.65-3.7wt% FeO*. ⁴⁰Ar/³⁹Ar ages of rhyolites, mostly on samples from neighboring quads, are between 16.2-14. 4 Ma. Both quadrangles also contain significant amounts of andesite to basaltic andesite lavas, ranging from aphyric to containing ~10% phenocrysts, a local pyroclastic deposit, the tuff of Milk Spring, and several regional ignimbrites, the Devine Canyon and Rattlesnake Tuff. The tuff of Milk Spring is particularly intriguing as scattered deposits within the mapping area and adjacent western quads suggest an eruptive volume of ~5 km³. It was likely associated with caldera formation from an unmapped caldera that may lie just north of the mapped quads as suggested by welding, thickness, and lithic size evidence.

Activity throughout the mapping area occurred at the tail end of main phase Columbia River Basalt volcanism, with the Monument Dike Swarm occurring several km west and north of the two quadrangles. Accurate mapping of the volcanic units of these two quadrangles helps us better understand the relationship between the Strawberry Volcanic field and Columbia River Basalts, as well as the regional hydrology.