SURFACE EXPOSURE DATING OF LATE PLEISTOCENE MISSOULA FLOOD DEPOSITS IN THE WESTERN CHANNELED SCABLAND REGION, WASHINGTON

During the late Pleistocene, western North America’s glacial Lake Missoula repeatedly released outburst megafloods over a period of several thousand years. During part of this epoch, a near-maximally advanced Okanogan Lobe dammed the Columbia River and diverted floods into Grand Coulee. In an effort to refine the flood chronology in this region, we measured ²⁶Al and ¹⁰Be exposure ages on flood and ice-transported boulders. We sampled high-relief (> 1 m), rounded, granitic boulders with well-preserved surfaces.

To reconstruct the timing of the last Grand Coulee floods, we dated boulders on the lowest surface of the Ephrata Fan, a large flood deposit at the outlet of Grand Coulee. Surface exposure ages indicate boulder deposition on this surface at 15.3 ± 0.4 ka (n=6), implying that the Columbia River drainage remained blocked by Okanogan Lobe ice at this time. To date the retreat history of the Okanogan Lobe, we measured glacial erratics from the Waterville and Omak plateaus. These ages indicate that the Okanogan Lobe had retreated to the north of the Withrow Moraine by 14.5 ± 0.3 ka (n=2) but continued to block the Columbia River. The ice front was located on the Omak Plateau by 14.1 ± 0.3 ka (n=2), opening the Columbia River drainage pathway. Following opening of the Columbia River, a large flood deposited boulders on Priest Rapids bar at 14.0 ± 0.5 ka (n=5).