LATE PLEISTOCENE MISSOULA FLOODS—15,000-20,000 CALENDAR YEARS BEFORE PRESENT FROM RADIOCARBON DATING

Deposits from the late Pleistocene Missoula floods extend from western Montana to the abyssal plain of the northeastern Pacific Ocean and are a regionally important stratigraphic horizon. To more precisely define the age of these deposits, we assessed radiocarbon analyses of materials in direct stratigraphic relation at several sites along the flood routes. Combined calibration (OXCAL, v. 3.5; Ramsey, 2001, Radiocarbon, v. 43) of 13 selected radiocarbon ages from material within or interbedded with flood deposits, as well as 15 ages from deposits stratigraphically above and below Missoula flood deposits, yields a coherent estimate of 20–15 cal ka for the Missoula floods. Additional age limits result from (1) consideration of the Mount St Helens So and Sg tephras from Mount St. Helens, which fell after at least 28 floods (Waitt, 1980, J. Geol., v. 88) and before at least 31 floods (Clague et al., 2003, Geology, v. 31) and for which a similar calibration analysis suggests an age of 16.3 ± 0.3 cal ka (see also Clynne et al., 2008, USGS Professional Paper 1750); and (2) the 17.4 ± 0.2 cal ka age of the Bonneville flood (Marrero, 2009, New Mexico Institute of Mining and Technology M.S. thesis), which flowed down the Snake River valley, leaving deposits overlain by about 21 Missoula flood deposits at Tammany Bar near Lewiston, Idaho (Waitt, 1985, GSA Bulletin, v. 96). This 20–15 cal ka range is consistent with the two possible paleomagnetic secular-variation correlations with flood deposits proposed by Clague et al. (2003, GSA Bulletin, v. 31) and the 2000–3000-year duration and timing proposed by Atwater (1986; USGS Bulletin 1661) for ≥89 Missoula flood deposits interbedded within glacial Lake Columbia sediment in Sanpoil valley. Outstanding questions include (1) the age of Missoula flooding down the upper Columbia valley prior to the Okanogan lobe forming glacial Lake Columbia and (2) whether or not any Missoula floods occurred before the 17.4 cal ka Bonneville flood.