2008 Joint Annual Meeting (5-9 Oct. 2008): Rock Magnetic Studies of Glacial Lake Missoula Flood Sediments

During the late Wisconsinan, the Purcell Trench lobe of the Cordilleran Ice Sheet advanced south into Idaho where it dammed the Clark Fork River to create glacial Lake Missoula in western Montana. Concurrently, the Okanogan Lobe, farther west, dammed the Columbia River creating glacial Lake Columbia in northeastern Washington. Between approximately 17,200 and 11,300 ¹⁴C years ago, glacial Lake Missoula repeatedly filled and emptied catastrophically through its dam. The floodwaters flowed initially into glacial Lake Columbia before spilling across the Channeled Scablands in eastern Washington. At least 46 separate floods are recorded in exposures of glacial Lake Columbia sediments. Detailed rock magnetic studies were carried out on samples from three flood and intervening lacustrine beds at the Manila Creek site in northeastern Washington and on three lacustrine units from glacial Lake Missoula sediments at the Ninemile Creek site, Montana. The sediments record strong and stable magnetic remanence. Temperature-dependent measurements of the saturation magnetization indicate that glacial Lake Missoula sediments are dominated by hematite and glacial Lake Columbia sediments are dominated by titanium-poor magnetite. The S-ratio, which reflects the presence of high-coercive minerals, varies systematically throughout flood and interbedded lacustrine units at the Manila Creek site. These data indicate that the coarser flood sediments are dominated by local material, whereas the finer grain sizes that settle out at the end of the flood are dominated by sediments of a glacial Lake Missoula provenance, over 200 kilometres away.