Sedimentology and age constraints of Pleistocene outburst flood-related deposits in the Palouse Hills, southeastern Washington
May 11, 2011
Geology - Environmental Studies
The well-documented late Wisconsinan Missoula Floods have recently been shown to be only the final sequence in a process that has been ongoing for at least the second half of the Pleistocene. An outcrop in southern Walla Walla County, Washington shows complex stratigraphic relationships, including potential flood-cut unconformities that suggest a history related to earlier catastrophic glacial outburst floods. The outcrop contains 11 distinct units defined by changes in composition, the presence of soils, top-truncated clastic dikes, clasts of exotic lithologies, and obvious unconformities. The basal unit is a two-meter thick, poorly consolidated quartz sand with large-scale cross-bedding indicating eolian deposition; no source of quartz is known nearby. Above this are several units of massive brown micaceous silt, separated by moderately to well-developed caliche horizons. The silt/caliche units generally consist of composition, sorting, and grain size comparable to the Palouse Loess. Three layers of tephra are present within the massive silt units, two of which are identifiable. These tephras have been geochemically matched to the Pringle Falls tephra from Long Valley Caldera (age 165,000-210,000 ybp) and the Summer Lake NN Bed from Newberry Caldera (greater than 200,000 ybp). A date for the basal quartz sand from Optical Stimulation Luminescence (OSL) is pending. A well-preserved vertebrate fauna was found in the outcrop including partial remains of a medium sized carnivore, an artiodactyl, a mustellid (?), several rodents, and a reptile (?). Multiple distinct erosional surfaces in the outcrop are consistent with catastrophic flooding events. The mean grain size and lack of sedimentary structures suggest that eolian processes deposited most of the sediments between episodes of flooding. The source for the underlying quartz sand, also interpreted as eolian, is uncertain. Overall, the characteristics of this sequence of sediment are consistent with other pre-Wisconsinan flood sites in southeastern Washington.
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