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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