Long quaternary record in eastern Washington, U.S.A., interpreted from multiple buried paleosols in loess

Abstract

Loess that is from a few meters to 75 m thick covers an area of more than 20,000 km 2 on the Columbia Plateau in Washington, Idaho and Oregon. The region of deepest loess is in eastern Washington and is called “The Palouse”. The Palouse is downwind of the Channeled Scabland, through which massive outburst floods from Glacial Lake Missoula flowed repeatedly during glacial stages of the Pleistocene. The deepest stratigraphic section yet studied is in a remnant of the once continuous loess cover that is surrounded by Scabland channels. A normal over reverse magnetic polarity zonation in this section supports an age of more than 790,000 yr, but 15–35 m of loess may be present beneath this section, so a total of 1.5-2 million yr is possible. Upward fining of texture in some layers in the loess at this site and stratigraphic evidence at nearby sites suggest that at least some pulses of loess deposition were triggered by episodes of cataclysmic floods. Nineteen or more individual paleosols can be recognized in 26 m of section based on field morphology, physical and chemical properties, and micromorphology. The paleosols consist of calcic, petrocalcic and duripan horizons, many of which are associated with cambic horizons. Less strongly developed soils in this and other sections have been obscured by partial overlap of soil development in an episodically rising loess landscape. Paleosols in the loess at other sites reflect a dry-to-moist climatic gradient from west to east across the region during the Pleistocene that was grossly similar to today's. Accumulation of pedogenic carbonates and silica dominates in a western zone where the present-day mean annual precipitation is less than about 450 mm. Translocation of silicate clays and leaching of carbonates dominate in a central steppe zone where precipitation is between 450 and about 700 mm. Fragipans are common in an eastern zone along the steppe-forest transition at present-day precipitation of over 700 mm. Reconnaissance study suggests that these climatic-pedogenic zones have been somewhat stable during many of the episodes of soil development preserved in the loess, although some paleosols have different features from the majority of the paleosols in that zone, e.g., paleosols with fragipans in a sequence of paleosols with argillic horizons, which suggests that some episodes of soil development took place under different climatic conditions.