ABSTRACT

Rocks of the John Day Formation (middle Oligocene-lower Miocene) in eastern Oregon are composed of fragments derived from volcanic vents and include air-fall tuff, fluvial tuff, lacustrine tuff and ignimbrite. The majority of the John Day rocks, however, are massive tuff that resemble loess deposits. The median diameters and sorting coefficients of these rocks are similar to both loess and fine-grained volcanic ash, but plots of a ratio using median diameter and sorting coefficient

([Md]/[Md + ])

suggest that the massive John Day rocks are most like air-fall tuffs. Their resemblance to wind-raised and wind-deposited loess deposits is attributed to soil-forming and soil-mixing agents that operate during the slow deposition of dust; namely, frost-heaving, plant-wedging and soil-living animals. The interpretation that these massive rocks are mainly air-fall tuff is commensurate with paleobotanical evidence that suggests a warm temperate climate, rather than a dry climate that could produce dust storms necessary to deposit several hundred feet of structureless volcanic dust.