EUREKA FLAT: A LONG-TERM DUST-PRODUCTION ENGINE OF THE PALOUSE LOESS, PACIFIC NORTHWEST, USA

Thick and expansive loess accumulations require long-term sources of dust and relatively stable climatic conditions as demonstrated by deposits of the Palouse loess in the U.S. Pacific Northwest. Eureka Flat, a narrow, 80-km-long deflationary plain oriented parallel to prevailing southwesterly winds, has been a source of Palouse loess throughout the Pleistocene. During this time, episodic glacial outburst floods have replenished the supply of fine-grained sediment that mantles Eureka Flat. Wallula Gap, a bedrock canyon incised by the Columbia River, funnels the prevailing winds that flow across Eureka Flat. These winds have persistently re-entrained sand, silt, and clay-rich flood sediment into sand dunes, sand sheets, and downwind loess. The upwind margin of Eureka Flat contains actively deflating fine-grained outburst flood deposits that are discontinuously capped by stabilized to partially stabilized parabolic dunes, sand sheets, and thin sandy loess. The central to downwind margin of Eureka Flat is devoid of dunes and is blanketed by 1.5 m of post-last glacial maximum (LGM) loess. Coring at the downwind end of Eureka Flat has revealed an extensive sand sheet buried by loess. The sand sheet to loess transition is characterized by a gradual change from unimodal, stratified eolian sand, to structureless, bimodal eolian sand and silt with insect burrows and rhizoliths, to unimodal, sandy loess. This stratigraphy marks a bioclimatically driven change from arid eolian sand deposition to less arid loess deposition as the density of dust-trapping vegetation increased. Up to 4.5 m of post-LGM sandy loess mantles topography adjacent to and downwind from Eureka Flat as linear ridges aligned parallel to the prevailing wind. These linear loess ridges occur immediately downwind from dust-producing basins and apparently extend in the wind shadows of topographic obstacles, including basalt knobs. Loess isopach maps reveal a relatively thick finger of loess that extends directly downwind of Eureka Flat for approximately 150 km. Up-building of loess on this finger-shaped accumulation has continued through much of the past 75 ka, illustrating the profound influence of the Eureka Flat dust engine.