GLACIAL LAKE MISSOULA BEDS – FLUCTUATING LAKE LEVELS?

Multiple sections of the lake-bottom sediments in Glacial Lake Missoula (GLM) have not been previously described. Stratigraphy and lake-drainage modeling show that peak discharges from GLM predate the silty deposits. However, the widely cited work on the Ninemile Section, begun in 1970, dominates the interpretations of cyclic deposition in the lake. Most previous studies interpreted the glacial-lake silt/rhythmites as evidence for a few to dozens of fillings and drainings of GLM.

A well exposed, 0.5 km-long, ~15 m-thick section of silts, along the Clark Fork River near Garden Gulch at 46.7087˚N, -113.2583˚W, recorded subaqueous sedimentation and erosion. The Lake Missoula beds overlie a bedrock ridge at the upstream end, but mostly clast-supported, fluvial, boulder-gravel and sands which are capped by a paleosol. The lacustrine section fines-upwards from gravel to rippled sand to rhythmically bedded very fine-grained rippled sand, silt, and minor clay. A paleosol caps the section. Lateral tracing of beds shows at least 2 unconformities in the sequence. Locally, 1.7 m above fluvial gravels, a weak soil developed on poorly sorted medium-grained sand in the silt sequence. A more extensive erosional unconformity in the middle of the section is mappable across the exposure; this contact is highlighted by soft-sediment loading of eroded silts by gravelly sand.

The Garden Gulch section is interpreted to represent transgression of Lake Missoula beds over a stable late Pleistocene landscape. These sections in peripheral portions of the basin are a proximal density flow facies to those at the more distal Ninemile Section. Two lake transgressions may be recorded based on the lower unconformity. The upper unconformity indicates that lake-bottom scour or lake-level lowering interrupted sedimentation. Lake-bottom exposure is not suggested, as the silt units were not dewatered before sedimentation resumed. The basal paleosol may represent time between the early, large-magnitude floods and Lake Missoula bed deposition. The top of the Garden Gulch section, at an altitude of 1190 m, was deposited in water shallower than at Ninemile, where the altitude is 950 m. If the sections are correlative, they support the hypothesis of cyclic lacustrine deposition due to lake-level fluctuation rather than by draining and filling cycles.