GLACIAL OUTBURST FLOOD PATHWAYS AND PROVENANCE OF LATE PLEISTOCENE SEDIMENTARY DEPOSITS IN THE NORTHERN AND CENTRAL CHANNELED SCABLAND, WASHINGTON

LA-ICPMS-based detrital zircon geochronology integrated with standard sedimentary techniques improves understanding of glacial outburst flood pathways and the provenance of sediment deposited in the Channeled Scabland, Washington. Analyses of post-last glacial maximum outburst flood sediment collected from four sites in eastern Washington and northern Idaho reveal the influence that pre-existing and flood-induced drainage patterns as well as Lake Columbia and Lake Lewis had on the distribution and composition of sediment deposited within the Channeled Scabland. Outburst flood sediment collected from near the outlet of glacial Lake Missoula and the confluence of the Spokane and Columbia Rivers in the northern scabland yield prominent Mesoproterozoic and Mesozoic to Early Cenozoic detrital zircon populations. These populations suggest derivation from metasedimentary and plutonic source rocks in western Montana, northern Idaho, northeastern Washington, and southern British Columbia. The basalt-poor, quartzo-feldspathic compositions, paleocurrent orientations, and stratigraphy of the flood sediment are compelling evidence that Lake Missoula flood waters deposited a significant portion of their coarse-grained sediment load (sand-sized and larger) upon reaching Lake Columbia. Outburst flood sediment collected near Moses Lake and the Hanford Reach National Monument, Washington contains a prominent Mesoproterozoic detrital zircon population as well as a higher percentage and broader range of Mesozoic and Cenozoic zircons than in the northern site samples; we attribute this difference to the influx of sediment from central and western Okanogan Highland (north-central Washington) sources. The relatively basalt-rich quartzo-feldspathic nature of the sediment, paleocurrents, and stratigraphy of these central scabland flood deposits indicate that associated flood waters followed primarily western and central scabland pathways. Lake Lewis, generated when flood waters were temporarily impounded behind Wallula Gap, played a key role in promoting the deposition and mixing of outburst flood sediment at the Hanford Reach and likely other downstream sites in the south-central scabland.