LATE PLEISTOCENE MEGAFLOODING FROM THE CORDILLERAN ICE SHEET: NEW DATA AND NEW QUESTIONS

OSL (optical stimulated luminescence) and TCN (terrestrial cosmogenic nuclide) geochronology, plus 2-d hydraulic flow modeling (2dHFM), indicate complex patterns in time and space for late Pleistocene megaflooding along the southern margins of the Cordilleran Ice Sheet (CIS). Consistent with regional mapping and stratigraphic studies (Smith, 2006) and with 2dHFM (Alho et al., 2010), as well as with the original interpretation by Pardee (1942), the largest outbust(s) from Glacial Lake Missoula (GLM) occurred prior to the emplacement of rhythmically bedded silt sequences on the paleolake floor that were described by Chambers (1971, 1984). OSL dating reveals the latter to be younger than ~15 ka (Hanson et al., 2012), and thus not correlated, as previously inferred (Waitt, 1985), to other multi-bedded sequences in the Channeled Scabland (CS) region that contain the 16-ka Mount St. Helens set-S tephra. 2dHFM results show that blockage of the Columbia River by the CIS Okanogan Lobe (OL), ~20-15 ka, was critical to megaflood routings for the CS region. Pre-OL blockage flooding probably emplaced coarse-grained rhythmite sequences in the northern Pasco Basin, possibly correlated to a ~23-ka melt-water pulse in the eastern Pacific Ocean (Lopes and Mix, 2009). Post-OL blockage flooding from/through Glacial Lake Columbia (GLC) probably emplaced gravel-dune-mantled bars along the Columbia River valley near Wenatchee and Trinidad, WA. Flood pathways diverted by the OL over the loess-mantled eastern CS region probably emplaced the silty rhythmite sequences in the Yakima and Walla Walla valleys, dated between ~17-15 ka (Waitt, 1980, 1985; Clague et al., 2003). Retreat of the Upper Grand Coulee cataract around the same time period resulted in a 250-m drop in the level of GLC, generating a major megaflooding pulse through the western CS, possibly correlated to the ~17-ka melt-water pulse in the eastern Pacific (Lopes and Mix, 2009) and to ~16 ka ice-rafted erratics in the Pasco Basin and above Wallula Gap (TCN dates). The whole late Pleistocene history of flooding extended over a prolonged time period, perhaps from ~27-14 ka, as indicated by both marine melt-water pulses (Lopes and Mix, 2009) and ground-water recharge records (Brown et al., 2010). Megaflooding from non-GLM sources may explain some this newly documented complexity.