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  • Late Quaternary sediments in the Willamette Valley, Oregon, include a unit of chiefly silt and fine sand, the Willamette Silt Formation. Previous workers have (1) Assigned the Willamette Silt different ages which range from Sangamon interglacial to late Wisconsin glacial, (2) Proposed that the Willamette Silt was deposited in a ponded body of water which was produced by (a) a tectonic rise in sea level, (b) icejam dams, (c) a eustatic rise in sea level, or (d) a "hydraulic dam", (3) Attributed the Willamette Silt to (a) one catastrophic flood- -the Spokane Flood- -in the Columbia River Valley, (b) "normal" Columbia River floods supplemented by locally derived sediments, or (c) multiple large Columbia River floods. The petrography and stratigraphy of the Willamette Silt and the geomorphology of the Willamette Silt surface have been investigated in the north Willamette Valley. The results of these investigations and the results of a radiocarbon dating program provide data for a new interpretation of the nature and sequence of late Quaternary geologic events in the Willamette and Columbia Valleys. Petrography establishes: (1) that the Willamette Silt was not derived from the local Willamette Valley provenance, (2) that sediments mineralogically similar to the Willamette Silt are present in the Columbia Valley and in the Yakima and Walla Walla Valleys of southeastern Washington, and (3) that the Willamette Silt contains no more locally derived heavy minerals than Columbia River sediment contains. Stratigraphy indicates: (1) that the Willamette Silt unconformably overlies weathered Tertiary bedrock and older Quaternary glacial outwash on the valley margins, (2) that the silt conformably overlies locally derived fluvial-lacustrine sediments in deep central parts of the valley and conformably overlies young glacio-fluvial fan gravels, (3) that deposition of the silt was accomplished during at least 40 large Columbia River floods into the Willamette Valley, (4) that these floods carried heavy loads of chiefly silt and fine sand, (5) that the silt, at least at lower altitudes, was emplaced in a continuously ponded water during a geologically short time span, and (6) that ponded water was not stable at any altitude for long periods of time and that it drained rapidly from the Willamette Valley. Geomorphology of the Willamette Silt surface shows:(1) that deposition of the Willamette Silt was followed by rapid stream entrenchment, (2) that a climactic Columbia River flood, larger than previous floods, entered the Willamette Valley after subaerial weathering and erosion of the Willamette Silt. Stratigraphy, geomorphology, and radiocarbon dates show: (1) that the last Cascade Mountains glaciation that resulted in a significant aggradation by Willamette River tributaries began more than 34,410 ± 3,450 years before present (B.P.), (2) that the last climactic flood occurred about 19,000 years (B. P.), and (3) that the age of the Willamette Silt is between 19,000 years (B.P.) and 34, 410 ± 3,450 years (B.P.). The writer proposes: (1) that flooding was caused by repeated failures of the Lake Missoula glacial ice darn northeast of the channeled scablands, (2) that floods, which sequentially eroded scabland tracts, scoured the Palouse Formation sediments from the scabland tracts, and redeposited part of the sediments in the Columbia River Valley and in major tributary valleys, and (3) that ponded water was maintained in the Willamette Valley by more rapid aggradation in the Columbia Valley than in the Willamette Valley.
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