POSSIBLE REVISED CHRONOLOGY OF LATE PLEISTOCENE MEGAFLOODING, NORTHWESTERN U.S

OSL dating of flood gravel underlying the Lake Missoula silts in western Montana indicates that the high-energy lake draining(s) occurred at about 21 cal ka, thereby confirming J.T. Pardee’s (1942, GSA Bulletin, v. 53, p. 1569-1600) original interpretation and subsequent regional mapping by Smith (2006, Quat. Res., v. 66, p. 311-322). Hydraulic modeling of Lake Missoula outflow events (Alho et al., this meeting) shows that lake silt sections, e.g., Ninemile, dated by OSL at 15 cal ka (Hanson et al., 2009, CANQUA meeting abstracts), would not survive the erosive effects of high-energy outflows. This newly discovered timing for the largest late Pleistocene megafloods is consistent with OSL dating of coarse-grained rhythmites at White Bluffs in the central Pasco Basin (Bjornstad et al., this meeting), and with the age of the older of two major peaks of freshwater influx to the northeast Pacific Ocean from the Columbia River system (Lopes and Mix, 2009, Geology, v. 37, p. 79-82). In contrast, the association of fine-grained rhythmites (Touchet beds), with the Mount St. Helens set S tephra (~ 16 ka), e.g., the Burlingame Canyon Section, plus our terrestrial cosmogenic nuclide (TCN) dating (using Cl-36) of associated ice-rafted erratics, indicate that the later (and probably smaller) flood events responsible for these deposits, correspond to the later peak of freshwater influx to the northeast Pacific, as identified by Lopes and Mix (2009). A TCN (Cl-36) date from a granite inselberg that was exhumed from its Miocene basalt cover by cataclysmic flood erosion, shows that the later flood episode also probably coincided with the recession of the upper Grand Coulee cataract into Glacial Lake Columbia near modern Grand Coulee Dam. These new results suggest more complexity to the timing of late Pleistocene megaflooding than is implied by the commonly prevailing view that the megaflooding all occurred (with 40 to 100 events) over a 2000-3000-yr interval from about 18.5 to 15 cal ka B.P. (15,500 to 12,700 C-14 ka B.P.), involving exact bed-to-bed correlations from the fine-grained rhythmically bedded sequences in the basins of glacial Lake Missoula (e.g., Ninemile) and Columbia (e.g., Sanpoil), to the Touchet Beds of south-central Washington (e.g., Burlingame), and to the Willamette Silt of northwestern Oregon.