A loess record of pre-Late Wisconsin glacial outburst flooding, Pleistocene paleoenvironment, and Irvingtonian fauna from the Rulo site, southeastern Washington, USA

Abstract

Worldwide, loess-paleosol sequences are archives of terrestrial Quaternary paleoenvironment data. The Rulo site is a newly described site in the Palouse loess of southeastern Washington State, USA. The site reveals a 19m thick deposit of loess and flood sediment, preserving a total of 30m of reconstructed stratigraphic section. Five unconformity-bound sequences of sediment are exposed at the site, the lowest two of which have yielded fossil remains, including the first reported occurrence of flat-headed peccary (Platygonus compressus) in northwestern North America. The unconformities, two diamicts bearing extrabasinal clasts, and a number of clastic dikes provide evidence for pre-Wisconsin glacial outburst floods in southeastern Washington. More than a dozen paleosols are preserved at the site. These soils are relatively unweathered and therefore primarily record periods of slow sedimentation accumulation in the Palouse, rather than changes in weathering rates. A paleosol near the base of the sequence is unusually weathered for Palouse paleosols, and records a period of increased weathering during the Middle Pleistocene. All units above the sand are normal paleomagnetic polarity; additional age control is provided by a tephra in the upper third of the sequence that matches a ca. 220ka tephra from Newberry Volcano, and by stratigraphic correlation of later units to other sites in the Palouse. The Rulo site adds to the record of Irvingtonian fauna in North America, and supports accumulating evidence for pre-Late Wisconsin glacial outburst floods in the Pacific Northwest.