DETRITAL ZIRCON GEOCHRONOLOGY AS A TOOL TO ASSESS THE PROVENANCE OF POST-LAST GLACIAL MAXIMUM GLACIAL OUTBURST FLOODING ON THE NORTHERN AND CENTRAL CHANNELED SCABLAND, WA

LA-ICPMS-based detrital zircon geochronology of 21 samples (1854 grains) collected from 11 sites within the Columbia River Basin of eastern Washington, northern Idaho, and western Montana is being used to test hypotheses about the origins and distributions of sediment deposited by glacial outburst megafloods. Results illustrate the influence of both regional and local sources on the geochronological character of sedimentary deposits in the Channeled Scabland, WA.

Catastrophic failures of post-last glacial maximum (LGM) Purcell Trench lobe ice dams periodically released water from glacial Lake Missoula into the Channeled Scabland. Detrital zircons of sediment collected downstream from the ice dam near Clark Fork, ID are predominantly Paleoproterozoic, with lesser populations of Cretaceous and Eocene grains. Detrital zircons collected 200 km downstream from the ice dam near Hawk Creek are similar to the Clark Fork sample, with a prominent population of Paleoproterozoic grains. However, unlike the Clark Fork sample, Cretaceous and Eocene grains also are a co-dominant detrital zircon component. The increased number of the Eocene to Cretaceous detrital zircons is attributed to local, Columbia ice lobe-derived outwash sediment. Similar source influences occur in Sanpoil River valley sediment, where Paleoproterozoic detrital zircon grains from older flood deposits become secondary to locally derived Cretaceous and Eocene grains up-section.

Three detrital zircon samples from the Hanford Reach National Monument (HRNM) in the central scabland highlight the complexity of floodwater pathways within the scabland as well as the increased relative importance of Okanogan ice lobe-derived sediment to the detrital zircon spectra. HRNM sedimentary deposits contain a dominant, northeastern Washington-derived Eocene to Cretaceous population and secondary, north-central Washington-derived Jurassic and western Montana- and northern Idaho-derived Paleoproterozoic populations. In sum, the Columbia Basin detrital zircon data set emphasizes the importance of local and regional sources on megaflood deposit sedimentation and may eventually provide a means to decipher its depositional history.