Abstract

A Late Pleistocene volcanic ash couplet consisting of a Glacier Peak ash layer and an underlying Mount Saint Helens J ash layer has been identified at three sites in the Colville Valley area of northeastern Washington. This ash couplet has been reported as far east as northwestern Montana and therefore appears to have widespread distribution south of the International Boundary. Because areas covered by the Cordilleran Ice Sheet, as well as by local mountain glaciers and icefields, were undergoing extensive deglaciation when these ash layers were deposited, about 11 200 BP, the ash couplet is an important time-stratigraphic marker, and its identification at a site provides information about the extent of deglaciation at that time.The ash couplet is easily recognized in the study area. Distinguishing characteristics include (i) the medium-sand-size (0.2–0.4 mm) rounded glass fragments and abundant mafic crystals in the fine-sand fraction of the Glacier Peak ash, a white layer 5–10 mm thick; (ii) the fine sandy silt and mafic-crystal-poor Mount Saint Helens J ash, also a white layer 5–10 mm thick, below the Glacier Peak ash; and (iii) the stratigraphic position of the couplet beneath the much younger Mazama ash.The presence of the Glacier Peak and Mount Saint Helens J ash couplet in the Colville Valley, about 50 km north (upglacier) from the Late Wisconsin terminal moraine near the town of Springdale, indicates that the active margin of the Colville sublobe of the Cordilleran Ice Sheet had retreated at least that distance by 11 200 BP.

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