10,000 PEBBLES OF THE COLUMBIA RIVER BASIN—A PROVENANCE ASSESSMENT
In sum, 65 percent of the clasts were volcanic, mostly mafic volcanics of the Columbia River Basalt Group. Yet the varied presence of other rock types, such as siliceous volcanic rocks, intrusive igneous types, and metamorphic rocks, including quartzites and greenstones, enabled statistically distinct groupings of samples, as assessed by nonmetric multidimensional scaling and cluster analysis. In particular, volcanic-rich fluvial gravel in the Miocene Alkali Canyon and Dalles Formations has strong affinity to the modern Deschutes and John Day rivers of central and eastern Oregon. Quartzites and metamorphic rocks signal Columbia River and Snake River provenance, and greenstones, probably of the Seven Devils Group, populate samples of modern and ancient Snake and Salmon rivers.
The composition and distribution of gravel deposits shows no trace of the Columbia River or Snake River in the eastern Columbia River Gorge until sometime after the 5.4 Ma culmination of Dalles Formation deposition, a finding consistent with Miocene gravels of Columbia River affinity tracing a more northerly route and entering the modern valley in the central gorge near Hood River.