Paleontology and Stratigraphy of late Pleistocene-early Holocene Palouse Silt, southeastern Washington

Jim Riesterer

Department of Geology, Whitman College, Walla Walla, WA 99362

Several sites in southeasten Washington representing the broadly defined Palouse Silt were studied to determine the paleontological, sedimentological, and stratigraphic significance, and to attempt to establish a stratigraphic framework.

Ages of various sites were determined to range from 36.5 ka to 6.85 ka (late Wisconsin to early Holocene) based on tephra geochemistry. Ages of non-tephra-bearing sites were estimated by a combination of relative age criteria, including stratigraphic position, paleosoil morphology, and lithologic similarity.

Grain-size analysis of hundreds of samples representing dozens of outcrops show a consistent trend toward a uniform mean grain diameter and a low standard deviation, suggesting that the mechanism for deposition was the same for all outcrops studied. Examination of microscopic surface textures of silt grains suggests that wind was the primary mechanism. most sites showed extensive bioturbation.

The fauna of the study sections comprises a diverse group of mammalian taxa representing the Order Rodentia (ground squirrels, pocket gophers, voles, and kangaroo rats), and the Order Carnivora (Family Canidae; wolf or coyote). Numerical analysis of small mammal teeth show insignificant variation between fossil mammals and those that currently inhabit the region. Mammalian data suggest that climate changed markedly (from drier to wetter) near the 36.5 ka horizon, and then remained stable through the early Holocene (6.85 ka), when the climate reverted to a wetter regime.

Also present in the study sections are at least three species of gastropod that are at present undescribed and that may represent new taxa. The flora preserved in study sections consists of the seeds of Lithospermum ruderale, a plant that currently inhabits drier areas in the region. 

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