GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION J. HOOVER MACKIN AWARD: ENIGMATIC MOSES COULEE MEGAFLOODS

The Missoula Floods were glacial lake outburst floods that inundated the Columbia River and Channeled Scabland canyon system. Moses Coulee is the second largest Channeled Scabland canyon, but the age and source of Moses Coulee floods is not well established. Lake Missoula megafloods down the Columbia River may have spilled into Moses Coulee when the Okanogan Lobe of the Cordilleran Ice Sheet blocked the Columbia River and before Grand Coulee was fully eroded. Alternatively, subglacial floods from the Okanogan Lobe may have drained directly into Moses Coulee. In the course of mapping in Moses Coulee, we have identified exotic iceberg-rafted boulders, calcrete-cemented gravels and sands, and a pumiceous sand with upvalley-dipping clinoforms found in a side tributary. Using these materials, we hope to constrain the age of Moses Coulee floods with surface-exposure dating and tephrachronology. We will also compare the provenance of Moses Coulee flood sediments to those in other Channeled Scabland settings using petrography and geochemistry. Additionally, we will calculate subglacial hydropotential and drainage networks beneath the Okanogan Lobe upstream of Moses Coulee. Using this data, we hope to place Moses Coulee megafloods into the context of Okanogan Lobe glaciation and the Missoula Floods.