PROVENANCE OF GLACIAL OUTBURST FLOOD DEPOSITS IN THE CHANNELED SCABLAND, WA: INFLUENCE OF GLACIAL LAKE MISSOULA, MELT WATER, SNAKE RIVER, AND BONNEVILLE FLOOD SOURCES

Late Pleistocene outburst flood deposits from the northern and southeastern margins of the Columbia Plateau, Washington were derived from glacial Lake Missoula, the Snake River, and melt waters from the retreating Cordilleran Ice Sheet. LA-ICPMS-based detrital zircon geochronology conducted in concert with detailed sedimentologic and stratigraphic analyses of post-last glacial maximum (LGM) outburst flood deposits reveal the complexities of sedimentary influx into the Channeled Scabland. Stratigraphically low slackwater sediment deposited in the southernmost Sanpoil River valley, Washington records repeated influxes of flood water into Lake Columbia from glacial Lake Missoula. These flood waters delivered quartzo-feldspathic sediment that contains prominent populations of Archean, Paleoproterozic and Late Mesozoic-Early Cenozoic detrital zircons likely derived from source rocks in western Montana, northern Idaho, and northeastern Washington. Stratigraphically higher slackwater sedimentary deposits from Manila Creek (a tributary of the Sanpoil River) are attributed primarily to melt waters derived from the retreating Cordilleran Ice Sheet. These melt waters transported and deposited quartzo-feldspathic sediment that contains a prominent Late Mesozoic to Early Cenozoic detrital zircon population derived from northern Washington and possibly southern British Columbia sources. Post-LGM, basalt-bearing, quartzo-feldspathic outburst flood sediment collected from glacial Lake Missoula slackwater deposits at Tammany Creek in northwestern Idaho contains a diverse detrital zircon population with a strong Snake River and Bonneville floodwater signature. This sediment contains Archean- to Cenozoic-aged detrital zircons that include Triassic-Jurassic and Neoproterozoic (ca. 750-600 Ma) grains. The composition of the sediment at Tammany Creek suggests that the Bonneville Flood (ca. 14.5 ka) and remobilized Snake River sediment may have strongly influenced the lithologic character of Missoula Flood deposits in the south-central scabland. If so, we surmise that the detrital zircon signature of the Tammany Creek slackwater deposits may provide a temporally distinctive lithologic marker within slackwater flood deposits downstream.