COSMOGENIC-NUCLIDE AGES FROM THE OKANOGAN LOBE OF THE CORDILLERAN ICE SHEET AND ASSOCIATED OUTBURST FLOOD ERRATICS

Cosmogenic-nuclide exposure ages from 12 erratics within the Columbia River and Lake Chelan valleys, central Washington State, place new constraints on local late Pleistocene glaciogenic events. Eight Be-10 ages from end and lateral moraine erratics of the Okanogan Lobe indicate it was at or near maximum by 14,500±200 Be10 yr B.P. While this agrees with Atwater's (1986, 1987) inferred Okanogan Lobe maximum of 14,800±375 yr B.P. based varve counts and a C-14 date in sediments of the Sandpoil Arm of Glacial Lake Columbia, it is several thousand years younger than six Cl-36 ages from glacial erratics of the Withrow Moraine on the neighboring Waterville Plateau (Swanson and Caffee, 2001).

Four Be-10 exposure ages from outburst flood erratics within the Columbia River Valley indicate the last flood to pass between Pateros and Wenatchee occurred 10,800±500 Be10 yr B.P. This is significantly younger than Atwater's (1986) inferred retreat of the Okanogan Lobe north of the Columbia River at 13,050±650 yr B.P. If this flood cannot represent the final Okanogan Outburst Flood, then it must describe a large, late jökulhlaup from a far retreated Okanogan Lobe.