CHANGES IN LOESS-DUNE DEPOSITION IN RESPONSE TO CLIMATE FLUCTUATIONS ON THE COLUMBIA PLATEAU, WASHINGTON

The sand dune and loess deposits on the Columbia Plateau in Washington are part of a paired eolian system whose spatial distribution and character have been dictated by Quaternary climate change. Genetic linkages between the dunes and loess exist because saltating sand plays a key role in ejecting silt-sized particles from the source bed for suspension transport. Past episodes of increased aridity apparently enhanced dune activity and downwind expansion of dunes and sand sheets while episodes of increased moisture restricted dune mobility, thus affecting where loess accumulated. The boundary between saltation and suspension deposition fluctuated through time creating an interfingered deposit of loess and dune sand. This control on the loess-dune boundary is ‘bioclimatic’ and is influenced by plant density changes that have impeded dune migration promoting aggradation of eolian sand and loess. Eolian facies as seen in core or outcrop change from sand to silt and are interpreted as marking fluctuations in aridity since the late Pleistocene. Previous workers have used pollen and phytolith analysis to document Quaternary climate changes on the plateau, especially the mid-Holocene thermal maximum that occurred from about 7 to 4 ka.

Two sites in south central Washington show dramatic changes in grain size with depth and are underlain by late Wisconsin glacial outburst flood deposits that constrain the maximum age of source-proximal eolian deposits to <13 ka. Juniper Dunes Wilderness area shows a decrease in grain size with depth from a sand-dominated cover sheet grading into loess. At Eureka Flat (east of Juniper dunes), dune activity is limited to upwind, arid portions of the flat. Downwind of the active dunes, a 1 m loess mantle covers well-stratified sand that is interpreted as an eolian sand sheet that may have accumulated during the mid-Holocene thermal maximum. As less arid conditions prevailed during mid to late Holocene, dune activity was limited to the upwind portions of the flat and loess accumulated downwind, covering stabilized sand. Dunes migrating over silt-rich flood slackwater sediment on Eureka Flat apparently have been providing wind blown silt for the past 75 ka and are likely a source for much of the downwind Palouse loess.