MINERALOGICAL PROVENANCE OF PALEOFLOOD DEPOSITS IN THE WALLA WALLA VALLEY, WASHINGTON

The source of catastrophic paleofloods that sculpted the Channeled Scablands is a topic of ongoing debate. This research takes a mineralogical approach to constraining the source of the normally graded slackwater sediments (rhythmites) deposited in the Walla Walla Valley. More than 40 distinct rhythmites from the Late-Wisconsinan Glaciation are exposed at Burlingame Canyon near Touchet, Washington. Slackwater deposits that predate the Late-Wisconsinan Glaciation are also exposed at various roadcuts and quarries around the Walla Walla Valley. Detrital minerals in the floodwater deposits, like zircon and rutile, record information can connect sediment to a specific region.

A Portable X-Ray Fluorescence Spectrometer (PXRF) is used to identify elements and their variable abundance in the coarse basal units of rhythmites. Elements (specifically Zr and Ti) are studied as a proxy for mineral abundance (zircon and rutile, respectively). We have found significant variation in elemental composition between 10 discrete Late-Wisconsinan rhythmites at Burlingame Canyon. Zr abundance ranges from 165 ppm to 282 ppm between beds. Titanium abundance ranges from 5,558 ppm to 8,568 ppm. Differences in elemental abundance are even more striking when the Late-Wisconsinan rhythmites at Burlingame Canyon are compared to older slackwater sediments in the Walla Walla Valley. 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 same Pre-Late Wisconsinan bed has an average rutile content of 983 ppm while the Late-Wisconsinan beds surveyed have an overall average rutile count of 6,468 ppm.

Rhythmites with anomalous concentrations of zirconium and titanium are targeted for zircon and rutile mineral separation. These isolated minerals undergo chemical analysis and age-dating techniques. Results provide the means to test hypotheses related to the source-region of the paleofloods.