EVIDENCE FOR TWO OUTBURST FLOODS OF NON-MISSOULA ORIGIN IN NE WASHINGTON STATE

Previously unreported and large scale (55 m high and at least 1 km long) foreset boulder-rich gravel beds of Pleistocene age are exposed in the upper Columbia River valley. The main exposure is south of Kettle Falls WA along the shores of Lake Roosevelt (the large reservoir behind Grand Coulee Dam). The foresets dip about 28 degrees toward the south, in keeping with outburst flooding from British Columbia. Most of the gravels are matrix supported. Clasts in the beds include intermediate porphyritic volcanics, granitics, quartzite, gneiss, slate, and conglomerate. Outburst flooding from North Idaho, down the Pend Oreille River, into the Columbia above the international border is possible, as is outburst flooding directly from glacial meltwater in B.C. itself.

Large scale foreset beds of boulder-rich gravels also lie in the lower Spokane Valley along the "Spokane Arm" of Lake Roosevelt. Their outcrop is 4 km long and at least 15 m high. Boulders range up to 1.5 meters in diameter, suggesting ice-drop deposition. Clasts are dominated by granitics but also include quartzites, intermediate porphyritic volcanics, and basalts. Belt Series rocks, suggestive of Glacial Lake Missoula, are notably rare. The boulder-rich gravel foresets dip down-valley toward the west. Source of the outburst flood responsible for the beds may be from the northeast, down the Purcell Trench in Northern Idaho, and across into the Lake Coeur d'Alene area. Stratigraphically above the foresets beds are multiple horizontal but strongly cross-bedded beds of dark, basalt-rich sands. The basalt sands are interbedded with light beds of lacustrian fines. These latter two bed types are consistant with outburst flooding from Glacial Lake Missoula repeatedly entering Glacial Lake Columbia in the late Wisconsin. In coluvium above the dark sand beds lies a layer of volcanic ash which is an excellent chemical match to the climactic eruption of Mt. Mazama 6850 B.P.