10,000 PEBBLES OF THE COLUMBIA RIVER BASIN—A PROVENANCE ASSESSMENT

Abstract

Lithologic classification of gravel collected from a broad array of Miocene to modern deposits in the Columbia River basin enables general characterization of gravel composition and clarifies river history. We analyzed 47 samples, most from the area of the Columbia River Gorge where the Columbia River transects the Cascade arc, but also including samples from Snake River basin in eastern Oregon and southern Idaho and middle Columbia River reaches in eastern Washington. Samples were collected from modern or late Quaternary deposits of the Columbia, Snake, Salmon, Clearwater, John Day, and Deschutes Rivers. Older sampled deposits include gravel beds from the Troutdale Formation, Dalles Formation, Alkali Canyon Formation, Ellensburg Formation, Ringold Formation, Clarkston Heights gravel, as well as Missoula-flood and Bonneville-flood deposits. Sample mass averaged 30 kg, from which we classified all clasts larger than 16.5 mm, resulting in classified clast counts averaging 215 (range 74 to 424) per sample. A total of 10,093 clasts were classified into 18 rock types. 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.