Timing, distribution and climatic implications of late quaternary eolian deposits: northern Columbia Plateau, WA

Kurt Alan Dalman

Eolian sedimentation on the northern Columbia Plateau, WA is intimately tied to last glacial maximum (LGM) outburst flooding. Post-LGM, L1, loess on the northern plateau never exceeds 3 m while coeval loess on the southern plateau reaches thicknesses of 8 m (Sweeney, 2004). Similarly, post-LGM mass accumulation rates (MARs) for loess on the northern plateau are one to two orders of magnitude less than those of the southern plateau. Relatively thin loess deposits on the northern plateau are a product of source-limiting factors. These factors include: 1) a lack of fine-grained material within source deposits due to the bypassing of fine-grained particles over the northern plateau during glacial outburst flooding, 2) the presence of coarse-grained deposits stratigraphically above fine-grained deposits preventing eolian remobilization of fine-grained deposits, 3) the erosional removal of loess and soil during glacial outburst flooding, which left large portions of the region bare bedrock and unsuitable for vegetative growth and sediment accumulation. Glacial outburst flooding on the northern Columbia Plateau deposited a diverse mixture of clay- to boulder-sized sediment. The fine-grained sediment fraction is held in deposits associated with narrow passageways along the flood path and in back flooded tributary valleys. The coarse sediment was deposited within and at the terminuses of flood channels and formed gravel bars and alluvial fans. Adequate fine-grained material required to produce the relatively thin loess deposits of Wilbur Plateau and east of Quincy Basin is present within Quincy Basin. The Columbia River Valley, Grand Coulee, Wilson Creek Valley and Lind Coulee were also identified as likely local sources of loess on the northern plateau. The stratigraphy of the northern Columbia Plateau sediments gives insight into the outburst flood and climatic history of eastern Washington. Tephras and paleosols are major stratigraphic markers on the plateau. Tephras indicate outburst-flooding episodes occurred following the Mount St. Helens "S" eruption (ca. 15.4 ka) and preserved Glacier Peak tephra suggests extensive eolian activity prior to and following ca. 13.2 ka. MARs determined from paleosols and tephra data suggest cold and dry glacial episodes marked by accelerated eolian activity during and following glacial ice retreat.

Timing, distribution and climatic implications of late quaternary eolian deposits: northern Columbia Plateau, WA

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