Glacial outburst floods and loess sedimentation documented during Oxygen Isotope Stage 4 on the Columbia Plateau, Washington State

Abstract

Stratigraphy and age control of late Pleistocene loess, associated glacial outburst flood deposits and flood-cut unconformities in the Channeled Scabland, Washington State, United States, indicate a significant Cordilleran ice sheet advance during marine Oxygen Isotope Stage 4. Glacial outburst flood deposits from stage 2 (classic Bretz flood deposits, ca 21 to 13 ka) and related features in the Channeled Scabland overlie a widespread layer of loess that contains buried soils and the Mount St. Helens set C tephra (ca 46 ka). This loess in turn overlies deposits of the penultimate episode of giant outburst floods and an unconformity cut by those floods. Regional trends in the thickness, texture, and overall composition of the older loess are strikingly similar to those from the youngest loess, known to be derived from stage 2 flood deposits. We conclude that the older loess also is derived from fine-grained flood deposits. Luminescence ages, tephrochronology, and soil development rates indicate that the bulk of deposition of the older loess occurred during stage 3, following glacial outburst flooding marked by a regional flood-cut unconformity. The apparent cyclical pattern of cold-climate buried soils, flood deposits, and thick loess accumulations demonstrate that sediment supply renewed by flood episodes is a major control on accumulation of loess on glacial timescales.