Quaternary Chronology of the Palouse Loess near Washtucna, Eastern Washington

Abstract

Four roadcuts in the Palouse loess near Washtucna, southeastern Washington, expose a thick sequence of buried calcic soils and tephra layers which span more than the last 730,000 years. The identification of four tephra layers of known age in the upper part of the loess sequence (Mazama, Mt. St. Helens Set S, and two separate layers of Mt. St. Helens Set C) allow the formulation of a soil chronology for the last 40,000 years. Paleomagnetic stratigraphy resulted in the identification of the Brunhes Normal-Matuyama Reversed polarity epoch boundary in one roadcut, thereby establishing an age of at least 730,000 years for the lower part of the loess section. The soils are described in terms of field characteristics and clay mineralogy. Three broad groupings of soils are distinguished on the basis of carbonate stage of development, stratigraphic position, and age: soils younger than about 30,000 years have stage I and II carbonate; soils which formed between 30,000 and at least 40,000 years B.P. have stage III carbonate; and soils which formed prior to at least 40,000 years ago have stage III and IV carbonate. The clay mineral assemblage of the Palouse loess consists of chlorite and mica with minor amounts of smectite, vermiculite and mixed layer clays. The buried soils do not appear to have clay mineral assemblages which can distinguish among them, although three samples from buried soils contained kaolinite. The stratigraphy exposed in the Washtucna roadcuts and the chronology of loess deposition and soil formation developed herein could provide a framework for the Pleistocene chronology of loess deposits on the Columbia Plateau.