INTERPRETATION OF A MIDDLE MIOCENE PALEOSOL FROM THE POWDER RIVER VOLCANIC FIELD, NORTHEAST OREGON

Miocene paleosols are preserved in interbeds between volcanic deposits in the Powder River Volcanic Field (PRVF) in northeast Oregon, and can be used to interpret past environments. The PRVF eruptions coincided spatially and temporally with the later eruptions of the Columbia River Basalt Group (CRBG), and while some CRBG paleosols have been described in the literature, PRVF paleosols have not. A prominent paleosol in the Cricket Flats area east of Elgin, Oregon developed atop a ca. 15 Ma volcaniclastic unit, then was buried by an olivine basalt flow by about 15 Ma. This puts the age of the paleosol directly after the middle Miocene climatic optimum, which ended around 15.4 Ma. Field description and interpretation of former soil horizons, thin sections, and clay mineral analysis allow us to infer original soil development, soil pedotype, and the extent of weathering, thereby illuminating Miocene climatic conditions. We will interpret this paleosol in light of existing paleoenvironmental interpretations from this region with the intention of contributing to the terrestrial climate record.