TOWARDS A LATE-GLACIAL LAKE-LEVEL HISTORY FOR GLACIAL LAKE MISSOULA, MONTANA

Previous workers proposed that fine-grained glaciolacustrine sediments deposited in the last phase of Pleistocene glacial Lake Missoula represent up to 80 filling and draining cycles. Proposing an accurate lake-level history requires correlating multiple stratigraphic sections that record initial transgression to final regression. Although cyclic sedimentation may suggest filling and draining cycles, subaerial exposure surfaces within the glaciolacustrine section is clear evidence for lake-level lowering. Correlation between sections in the basin is required to differentiate between full and partial lake drainage, as deposits range in elevations between the outlet at ~660m and the highstand water elevation of ~1280 m.

Outcrops near Garden Gulch, in the upper reaches of the Clark Fork River valley, allow documentation of a lake-level history near a highstand position, full-pool location. This section is at 1170-1183 masl, thus the lake-level history for this site reflects the number of times the lake reached within ~100 m of full pool. The section contains glaciolacustrine sand, silt, and clay with exposure surfaces delineated by paleosols and by periglacial features, such as sand wedges, ice-wedge casts, and cryogenically disrupted bedding. Thin massive silt (loess?) and fining-upward sequences of glaciolacustrine sediments overlie exposure surfaces. At least 8 such sequences are recognized; 4 additional lake-lowerings have equivocal evidence, such as coarsening-upward sequences in massive silts. Along 5 of the exposure surfaces, currents were capable of carrying angular cobble-sized clasts from nearby bedrock outcrops across the exposed lakebed during or after the lake-lowering events. The gravel was then periglacially modified.

Optical dating of two quartz sand samples, from near the base and top of the section support correlation of the Garden Gulch section to those in the Missoula area, 80–100 km downstream. Correlations of stratigraphic sections suggest 8-12 lake-level lowerings of >200-300 m from late deep-lake phases. Sections near Missoula record 34 lake-level lowering events, suggesting many partial fillings during most lake cycles. Whether the cycles represent complete or only partial lake-drainage requires further correlations between deep-lake and shallow-lake positions.