Mineralogical variation in paleoflood deposits in the Walla Walla Valley, Washington: A case study for the development of Portable X-Ray Fluorescence Spectrometry (PXRF)

Tamara Carley

Whitman College, Walla Walla WA 99362

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The source of catastrophic paleofloods that sculpted the Channeled Scablands of central Washington is a topic of ongoing debate.  This research attempted to identify floodwater sources by linking the bulk chemistry of slackwater sediments to their constituent minerals whose age and chemistry may indicate their bedrock source.  A Portable X-ray Fluorescence Spectrometer (PXRF) was used to assess elemental variation between discrete rhythmite layers in the Walla Walla Valley. Findings indicated that compositions of flood deposits pre-dating the Late-Wisconsinan glaciation vary significantly from sediments deposited by more recent events.  One Pre-Late Wisconsinan deposit has an average Zr content of 84 ppm, strikingly lower than the 200 ppm value typical of Late-Wisconsinan sediments.  The degree of elemental variation among discrete Late-Wisconsinan rhythmites exposed at Burlingame Canyon is also noteworthy, with Zr ranging from 165 ppm to 282 ppm.  The significance of these elemental variations was deemed robust after careful evaluation of the precision of field and lab measurements of the sediments using the PXRF; for K, Ca, Ti, Mn, Fe, Rb, Sr and Zr, the PXRF measures abundances at the ppm level to within 6% analytical precision.  Beds with anomalous elemental abundances (Zr, Ti, Rb, Sr, K, Ca, Ba) were targeted for muscovite separation for subsequent 40Ar/39Ar dating.  Compositional data and mineral ages will be considered in conjunction to develop a more-comprehensive understanding of the sediment source region.