POST-LGM EOLIAN HISTORY OF THE NORTHERN COLUMBIA PLATEAU, WASHINGTON AND RELATION TO GLACIAL OUTBURST FLOODING

Sedimentary, stratigraphic, and geomorphic evidence from the Quincy Basin, northern Columbia Plateau, Washington reveals the influence that glacial outburst flooding, source sediment availability, and topography have had on the thickness and distribution of post-last glacial maximum (LGM) eolian deposits. Fine-grained outburst flood sediments in the northern plateau are preserved in thinner and more spatially restricted deposits than on the southern plateau where thicker and more widespread fine-grained slackwater accumulations provided the raw material for up to 8 m of post-LGM, Palouse L1 loess (e.g., near Juniper Canyon, OR and Eureka Flat, WA). Flood deposits in the northern plateau are sand- and gravel-enriched attesting to the proximity of floodwater sources and the tendency for fine-grained silt- and clay-enriched flood sediment to bypass the area. The bulk of eolian sediments on the northern plateau were derived from fine-grained flood deposits that accumulated following deposition of the Mount St. Helens set S tephra (ca. 15,400 cal. yr B.P.). The topography bordering the Quincy Basin has helped modify prevailing SW-NE winds to more easterly orientations as evidenced by the W-E alignments of eolian features. Topographic funneling of prevailing winds by steep-sided canyons incised into the eastern walls of the Columbia River Valley was instrumental in concentrating eolian sand near Moses Lake into the largest dune field on the plateau. L1 loess deposits downwind of the dune field are generally thin (< 1.5 m) and not as well sorted as many L1 loess deposits on the southern plateau, where topographic trapping of saltating grains contributed to thicker deposits and better grain size segregation. The limited fine-grained sediment supply and more effective flood reworking in the northern plateau contributed to thinner loess deposition and much lower post- LGM dust fluxes than those on the southern plateau.