EASTERN EXTENT OF GLACIAL LAKE COLUMBIA DURING GLACIAL OUTBURST FLOODS: NEW EVIDENCE FROM THE SPOKANE VALLEY, WASHINGTON

Recently discovered exposures of rhythmically-bedded lacustrine deposits near Newman Lake, Washington provide new constraints for the eastern extent of glacial Lake Columbia in eastern Washington State. Elevations of the two sites are 681 m and 689 m, and are consistent with glaciolacustrine deposits and strandlines from the western side of Lake Columbia near Grand Coulee. The two exposures total 5 meters of rhythmically interbedded fine sand and silt, with one interval containing rhythmic clay and silt interbeds interpreted as varves. Two coarse sand intervals containing rounded to subrounded, pebble to cobble-sized clasts have been identified within the fine-grained sequence. The coarse clasts are metamorphic to granitic in composition and appear to be derived from the Belt Supergroup and Mesozoic plutons that are common as bedrock to the east. The rhythmically-bedded and varved sediments are interpreted as lake bed deposits of Pleistocene glacial Lake Columbia. The Newman Lake occurrences represent a northern embayment or “arm” from the main portion of Lake Columbia, which occupied the Spokane Valley. The coarse interbeds are outburst flood deposits from glacial Lake Missoula, which periodically drained into Lake Columbia. These deposits are some of the easternmost yet reported for glacial Lake Columbia and help establish that the lake extended to the east, possibly as far as the northern Rathdrum Prairie.