PALEOCLIMATIC IMPLICATIONS OF FOSSILIFEROUS LATE QUATERNARY EOLIAN AND FLUVIO-LACUSTRINE SEDIMENTS, SOUTHEASTERN WASHINGTON

Exposures of the Palouse loess and other late Quaternary sediments in southeastern Washington have yielded five species of terrestrial gastropod, five species of mammal (Order Rodentia), and one plant species (Lithospermum ruderale). Ages of fossil-bearing outcrops, based on the geochemical identity of several tephras, ranges from 47,000 to 6,875 ybp. Correlations are established based on morphological characteristics of paleosols, continuity of tephras, and apparent stratigraphic position with respect to the tephras and paleosols. Depositional environments include primarily eolian (Ayer Road, Nunamaker Road), eolian and fluvio-lacustrine (McFeely Road, Piper Canyon Road), and catastrophic outburst flood (North Touchet Road). Distribution of facies is controlled by local geomorphology.

The Ayer Road section (Palouse loess, about 47,000 ybp), dominated by ground squirrels and pocket gophers (common to abundant) and voles and rabbits (rare), suggests climatic conditions ranging from semi-humid to semi-arid. The presence of the gastropod genus Columella at Ayer Road may indicate local pockets of cool and moist conditions. The presence of Lithospermum ruderale in this section suggests an arid climatic component. The McFeely and Piper Canyon Road sections, with abundant gastropods, suggest cool and moist conditions; sedimentologic characteristics indicate fluvio-lacustrine deposition here at about 20,000 ybp. The North Touchet Road section, represented by Missoula Flood slackwater sediment (about 13,000 ybp), is dominated by ground squirrels; snails and plants are absent. The Nunamaker Road section (Palouse loess, about 6875 ybp), dominated by ground squirrels and pocket gophers, with rare kangaroo rats and an increase in relative abundance of sagebrush voles, suggests a shift to more arid conditions, similar to those of the region today, in early Holocene time.