Late Quaternary stratigraphy of loess in the Channeled Scabland and Palouse regions of Washington state

Abstract

Loess deposits of late Quaternary age in the Channeled Scabland and western Palouse regions of Washington state contain distinctive sequences of calcic buried soils, loess layers, tephra layers, and stratigraphic unconformities caused by cataclysmic flooding. Four new soil-stratigraphic units, the Sand Hills Coulee Soil, Washtucna Soil, Old Maid Coulee Soil, and the Devils Canyon Soil, are recognized across a 100-km transect. The pedostratigraphic correlations are strengthened by two tephra layers found at several sites that are correlative with the Mount St. Helens set S (radiocarbon ages about 13,000 yr B.P.), Mount St. Helens set C (radiocarbon ages about 36,000 yr B.P.), and by a third one that may be from an earlier (pre-set C) Mount St. Helens eruption. The set S tephra layer separates the Sand Hills Coulee and Washtucna Soils; the set C tephra layer separates the Old Maid Coulee and Devils Canyon Soils; and the pre-set C tephra layer underlies the Devils Canyon Soil. The well-known late Wisconsin episode of Scabland flooding and two older episodes of flooding are marked by regional stratigraphic unconformities that lie between pedostratigraphic units. One unconformity that underlies the Washtucna and Old Maid Coulee Soils was caused by a major episode of Scabland flooding that occurred before 35,000 yr B.P., probably during the early Wisconsin (80,000 to 65,000 yr B.P.). A possible second and older episode of flooding, recognized by an unconformity that underlies the Devils Canyon Soil, may be Illinoian (ca. 130,000 to 200,000 yr B.P.) in age.