GEOMORPHIC, SEDIMENTOLOGIC AND BIOCLIMATIC MODELS FOR LOESS ACCUMULATION ON THE COLUMBIA PLATEAU, PACIFIC NORTHWEST

Stratigraphic, sedimentary and geomorphic mapping and analysis of 'paired' sand- and silt-rich eolian facies on the Columbia Plateau of Washington and Oregon indicate that changes in topography, source sediment character, and bioclimate control the nature and thickness of loess deposits. The distribution and texture of eolian sediments in this region (that were largely derived from reworking of Late Pleistocene glacial Lake Missoula outburst flood sediments) are explained by topographic trap, source limiting, and bioclimatic models. In the topographic trap model (observed at Juniper Canyon, Oregon) a deeply incised valley has segregated saltating eolian sand from suspended eolian silt. Sand dunes terminate on the upwind side of the incised valley that has allowed the deposition of the suspension load as thick loess (at least 3m) on the downwind side of the valley. In the source limiting model (Quincy Basin, Washington) silt-depleted source sediment, even in the presence of abundant saltation, cannot provide the volume of silt necessary for thick loess accumulations. Such conditions are common in proximal source areas within the Columbia Plateau. In the bioclimatic model (Juniper dunes and Eureka Flat, Washington), climatic fluctuations control the type and density of vegetation, thus promoting transgression and regression through time of the eolian sand/silt boundary. Stratigraphic transitions from eolian sand (early to mid-Holocene) to eolian silt (late-Holocene) in source-proximal areas demonstrate a change from arid to semi-arid conditions. The three models, applied either separately or in combination, help explain the distribution and thickness of loess in other settings. Saltation and suspension interact to control the sedimentology, stratigraphy, and geomorphology of paired eolian systems. Understanding these interactions is important to correctly reconstruct paleoclimatic fluctuations in long eolian records.