SEDIMENTARY ANALYSIS AND DETRITAL ZIRCON GEOCHRONOLOGY OF LATE WISCONSIN GLACIAL OUTBURST FLOOD DEPOSITS, WESTERN MONTANA AND EASTERN WASHINGTON

Detrital zircon geochronology combined with sedimentary analysis of a Late Wisconsin glacial Lake Missoula sediment source area in western Montana and three glacial outburst flood depositional sites within the Columbia Plateau, eastern Washington raise questions about floodwater paths into the Channeled Scabland. Glacial Lake Missoula samples were collected from the lower part of the Ninemile section in Montana and glacial outburst flood samples were collected from slackwater flood deposits that accumulated in the northwestern, north-central and south-central parts of the scabland. Sand-sized zircons from the Ninemile site are dominantly (85%) Archean-Proterozoic (2780 Ma to 1400 Ma) with few Cretaceous-Eocene (82 to 43 Ma) ages. Slackwater flood deposits contain a large population (up to 70 %) of Late Mesozoic-Cenozoic (160 Ma to 30 Ma) zircons and a relatively smaller though still significant percentage of Archean-Proterozoic zircons. A strong Precambrian component in the Ninemile detrital zircons is interpreted as evidence they were derived from the Mesoproterozoic Belt Supergroup and older cratonal sources to the east. The Mesozoic-Cenozoic detrital zircons in the slackwater deposits likely originated from plutons exposed primarily in northern Idaho and Washington, and southern British Columbia. Floodwaters from Lake Bonneville floodwaters that entered the scabland from the south via the Salmon-Snake River system appear to have had only a minor influence on glacial outburst flood detrital zircon populations. The well-rounded nature of Precambrian zircon grains and the more euhedral character of many Mesozoic-Cenozoic zircons provides complementary textural evidence of sedimentary provenance and likely floodwater paths. The relative abundance of Precambrian grains in outburst flood samples examined suggests that waters released from glacial Lake Missoula played a major role in shaping the Channeled Scabland. The relatively high percentage of younger zircons in these sedimentary deposits, though, opens the possibility that Mesozoic-Cenozoic zircons also were delivered via glacially related floodwaters that originated in northern Washington and southern British Columbia.