EXTENSIVE EOLIAN ACTIVITY DURING OIS 4-3 AS RECORDED BY PALEOFLOOD EVIDENCE AND REGIONAL SEDIMENTATION PATTERNS OF LOESS on THE COLUMBIA PLATEAU, WASHINGTON STATE

Stratigraphy and age control of late Pleistocene loess and associated glacial outburst flood deposits in the Channeled Scabland, Washington State, United States, provide evidence that (1) extensive flooding of the Channeled Scabland occurred during oxygen isotope stage (OIS) 4 and (2) the geomorphic response of regional eolian deposition that occurred during OIS 4 and 3 greatly exceeds eolian sedimentation that followed Scabland flooding during OIS 2. A regionally extensive loess layer is stratigraphically bracketed by the OIS 2 glacial outburst flood deposits (classic Bretz flood deposits, ca. 18 to 14 ka) and deposits of the penultimate episode of giant outburst floods. We conclude that the older loess, similar to loess deposited after OIS 2, is derived from fine-grained sediments deposited by glacial outburst flooding. Luminescence ages of the loess that range from 77–46 ka, the inclusion of the ~50ka Mt St Helens set C tephra, and the degree of soil development indicate that this penultimate episode of Scabland flooding (related to glacial outburst floods associated with the advance of the Cordilleran Ice Sheet) occurred during OIS 4, with the bulk of deposition of loess during OIS 3. Regional trends in the loess layer thickness derived from OIS 4 flooding greatly exceed regional trends in loess layer thickness derived from OIS 2 flood deposits. For example, loess deposited after OIS 2 is 2-4 m thick near proximal loess source areas but loess deposited during OIS 4 and OIS 3 is 3 to 8 m thick. Moreover, the topographic position of the soil that caps the OIS 3 loess marginal to Scabland flood channels suggests that the magnitude of flooding, at least in some flood channels, may have been greater during OIS 4 relative to flooding during OIS 2.