Portland State University. Department of Geology
Scott F. Burns
Term of Graduation
Date of Publication
Master of Science (M.S.) in Geology
Geology, Stratigraphic -- Pleistocene, Paleopedology -- Oregon -- Wasco County, Volcanic ash, tuff -- Oregon -- Wasco County, Geology -- Oregon -- Dalles
1 online resource (x, 175 pages)
A roadcut on Highway 197, three kilometers southeast of The Dalles, Oregon, exposes a sequence of Quaternary sediments and five buried paleosols. The sediments, paleosols, and associated tephras at this site provide evidence of a Quaternary history of catastrophic flooding in the Columbia Basin extending back at least 700 ka and of an early eruption (ca. 600 ka) of Mount Adams. Four sedimentary units are exposed in this cut: Holocene loess, late Wisconsin Missoula Flood slackwater deposits, five pre-late Wisconsin catastrophic flood slackwater deposits bearing well developed paleosols, and late Tertiary Dalles Formation volcaniclastics. All but the oldest are predominantly silts and fine sands. Several of the paleosols contain scattered gravel of varied lithologies, including granite, consistent with a catastrophic flood origin.
Each paleosol contains a Bk or K horizon ranging in carbonate development from Stage II to Stage IV; therefore, each period of soil development represents between 20,000 and 100,000 years. The latest-Pleistocene (between 15 and 12 ka) Missoula Flood deposits and overlying loess exhibit very limited soil development with virtually no pedogenic carbonate. Each of the paleosols must consequently represent a much longer period of soil formation. Thus, several floods must have occurred prior to the late Pleistocene.
Several of the paleosols contain abundant volcanic ash. Paleosol 3 contains numerous rounded pumice clasts up to 1.4 cm in diameter. Microprobe analysis of this pumice resulted in a similarity coefficient of 0.95 with the Dibekulewe tuff of Nevada, age circa 600 ka and source unknown (Sama-Wojcicki and others, 1985). The high similarity coefficient correlates the pumice at The Dalles with the Dibekulewe tuff and implies that the sediments deposited with the pumice are of middle Pleistocene age. If these are catastrophic flood deposits, then this type of flooding was occurring prior to the late Wisconsin. Based on chemistry, location and known geologic record, a probable source of the Dibekulewe tuff, which is much coarser at The Dalles than in Nevada, is early activity at Mount Adams in the Southern Washington Cascades.
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Cordero, David Irving, "Early to Middle Pleistocene Catastrophic Flood Deposits, the Dalles, Oregon" (1997). Dissertations and Theses. Paper 6284.