New stratigraphic markers in the late Pleistocene Palouse loess: novel fossil gastropods, absolute age constraints and non-aeolian facies

Four stratigraphic sections in the southern part of the Columbia Basin preserve a sequence of aeolian and non-aeolian sediments ranging in age from 9·43 to >47·0 ¹⁴C ka based on accelerator mass spectrometry radiocarbon dating of fossil molluscs, geochemistry of Cascade Mountain-sourced tephra and association with formally recognized pedostratigraphic units (the Washtucna and Old Maid Coulee soils). Study sections are interpreted as representing concurrent deposition of loess and distal Missoula Flood rhythmites in valleys tributary to main drainages backflooded during the Missoula Floods, and formation of carbonate and iron-rich soils. Sediments belong to the formally recognized L-1 and L-2 loess units established for the Palouse loess, which were deposited in the Columbia Basin subsequent to events of glacial outburst flooding. Sediments associated with the Mount Saint Helens set S and set C tephras in the study sections preserve a fauna of five species of gastropod mollusc which have not been reported previously from sediments of late Pleistocene age in the Palouse region. The fossils comprise two distinct faunules stratigraphically separated by the Mount Saint Helens So tephra. Accelerator mass spectrometry radiocarbon dating of the fossils collected above the tephra in two of the sections yielded ages of 12·48 ± 0·06 and 9·43 ± 0·05 ¹⁴C kyr. These ages suggest that independent determinations of the 13·35 ¹⁴C kyr age of the So tephra in other areas where Missoula Flood sediments are preserved are probably accurate, and help to refine the age of the latest events in the most recent sequence of catastrophic glacial outburst flooding.