CONSTRAINING THE PROVENANCE OF GLACIAL OUTBURST FLOOD SEDIMENT PRESERVED IN THE LOWER SANPOIL RIVER VALLEY, NORTHERN CHANNELED SCABLAND, EASTERN WASHINGTON

Thick exposures of glacial outburst flood and lake sediment exposed in the Sanpoil River valley of north-central Washington preserve one of the most complete records of megaflood deposits in the Channeled Scabland. Recent suggestions that some of the floodwaters that generated the Channeled Scablands may have emanated from subglacial sources in adjacent British Columbia, Canada in addition to floodwaters from Glacial Lake Missoula, prompted this study to track the provenance of the sediment deposited by the megafloods. Detrital zircon geochronology evaluated within the sedimentologic and stratigraphic framework of the Manila Creek deposits reveals the possibility that this sediment was derived from multiple sources including outburst flood sediment from Lake Missoula, re-worked glacial drift, and alluvium from the upper Sanpoil River valley.

Previously reported detrital zircon analyses of fine sand-sized grains from Glacial Lake Missoula (Ninemile Creek, MT) reveal age spectra dominated by 1600-2000 Ma grains with minor populations of grains at 40-80 Ma, and 2400-2800 Ma. Fine sand-sized detrital zircon grains collected from the coarser, lower beds of rhythmites exposed in the lower Sanpoil River valley and along the upper Manilla Creek valley yield U-Pb ages ranging from ~50-200 Ma, ~1600-2000 Ma, and ~2450 Ma, with the majority of the grains from the upper Manilla Creek valley ranging from ~50-200 Ma. Detrital zircon ages from the uppermost part of the Manilla Creek succession also are predominantly 50-200 Ma, with secondary populations at 1600-2000 Ma and ~2450 Ma. The abundance of 50-200 Ma grains in the upper Manilla Creek deposits favors a sediment source, or sources, west of Glacial Lake Missoula; possible sources of sediment include reworked glacial drift between the Purcell Trench and Sanpoil River valley and (possibly) sediment transported subglacially. Continuing detrital zircon studies of the glacial drift sediments and additional samples in the Sanpoil River valley may further delineate the sources of floodwaters that sculpted that northern Channeled Scablands.