DETRITAL ZIRCON GEOCHRONOLOGY APPLIED TO LATE QUATERNARY MEGAFLOOD DEPOSITS FROM THE CHANNELED SCABLAND, WA

Detrital zircon geochronology was used along with standard field stratigraphic and sedimentary techniques to evaluate the provenance of sand-rich, glacial-outburst megaflood deposits of the northern and central Channeled Scabland, WA. Samples were collected from slackwater and flood channel deposits in an effort to evaluate flood source pathways from postulated glacial lake and subglacial sources, and to provide preliminary data necessary to test the use of detrital zircon geochronology as a method of correlating discrete event beds in megaflood deposits. This research effort represents one of the first systematic efforts to apply detrital zircon geochronology to glacial outburst flood provenance. Approximately 1,500 grains from six sample locations indicate populations of detrital zircon were derived from two primary regional sources: 1) catastrophic megafloods released from glacial Lake Missoula following post-last glacial maximum (LGM) Purcell Trench lobe ice dam failures that produced predominantly Paleoproterozoic detrital zircon populations, with lesser populations of Cretaceous and Eocene grains, and 2) post-LGM glacial outwash and potential catastrophic sub-glacial megafloods transported via the Okanogan, Sanpoil, and Columbia river valleys that contain primarily Jurassic, Cretaceous, and Eocene detrital zircon populations. The results illustrate the inherent complexity of sediment mixing of both regional and local sources on the detrital zircon geochronologic signatures of the Scabland megaflood deposits. The wide variety and differing concentrations of the detrital zircon populations illustrates the high degree of provenance uncertainty, even in thoroughly mixed glacial outburst flood event beds. While it is considered possible the provenance of central scabland event beds may be determined through detrital zircon geochronology, the data gathered to date does not support this possibility. This data does not clearly identify an outburst flood sediment source other than glacial Lake Missoula, but it does indicate the possibility of a significant input of sediment from the ice sheet west of the ice dam that impounded glacial Lake Missoula.