COSMOGENIC AGE CONSTRAINTS OF MORAINE FORMATION AND OUTBURST FLOODS ASSOCIATED WITH THE OKANOGAN LOBE OF THE CORDILLERAN ICE SHEET, CHELAN, WASHINGTON

New Be-10 absolute ages obtained from twelve granitic erratics within the Columbia River and Lake Chelan valleys of central Washington State place important constraints on local late Pleistocene glaciogenic events. Ages from eight erratics found on terminal, end and lateral moraines of the Okanogan Lobe of the Cordilleran Ice Sheet within the Columbia River valley and lower Lake Chelan valley suggest the lobe was at or near a maximum extent between 14 and 15.5 ka bp. These new Be-10 ages are younger than previous estimates, including Cl-36 ages by Swanson and Caffee (2001) from the neighboring Waterville Plateau. Four Be-10 ages obtained from outburst flood erratics within the Columbia River valley suggest the last flood to pass down the valley did so at roughly 11 ka bp. Whether these erratics represent the Okanogan outburst flood or a younger, smaller glacial outburst is unclear. It does, however, place the Okanogan Lobe north of the Columbia River valley by that time.