GEOMORPHIC DEVELOPMENT OF A LATE QUATERNARY PAIRED EOLIAN SEQUENCE, COLUMBIA PLATEAU, WASHINGTON

The character of the surficial cover on the Columbia Plateau in eastern Washington largely reflects the influences of Pleistocene glacial outburst flooding and paired (dune-loess) eolian activity. Pleistocene outburst floods periodically flowed across the Channeled Scabland, depositing abundant sediment. Prevailing SW to NE winds aided by a persistent semi-arid climate combined to produce a diverse, predominantly eolian landscape of stabilized to active sand dunes and sand sheets and vegetated loess deposits that mantle a substrate of dissected basalt and older sediment. Proximal to distal changes in the textural character of outburst flood sediment along their flow paths are mirrored in the eolian geomorphology. At flood-proximal sites, fines-depleted, coarse-grained outburst flood deposits have been reworked into extensive sand dunes but only thin, discontinuous loess, whereas at flood-distal sites, coarse-depleted, fine-grained slackwater flood deposits have been reworked into relatively thin, discontinuous sand dunes and sand sheets but thick, continuous loess. Thicker loess accumulations form the region known as the Palouse. Linear ridges of sand-rich loess (greda) that are aligned with prevailing winds are concentrated in the more arid, upwind portions of the Palouse, whereas rolling, sand-poor hills of silt-rich loess dominate the moister, downwind portions of this region. Topographic irregularities in and near sediment sources have influenced loess accumulation by selectively trapping saltating sand, promoting aggradation downwind of thick units of suspended-load silt. Thick loess deposits also have been generated where bioclimatic controls (plant cover density and soil moisture) have enhanced loessial accumulation. The response times to climatic fluctuations of paired sand dune and loess activity are not yet well constrained. Ongoing stratigraphic and sedimentary studies indicate that this data will improve understanding of the modes and character of morphologic change on the Columbia Plateau and in other dryland settings.