The Deglaciation History of the Cordilleran Ice Sheet as Recorded in Sediments of Flathead Lake, Montana

Flathead Lake is located in northwest Montana along the former terminus of the Flathead Lobe of the Cordilleran Ice Sheet. It contains a well preserved record of syn- and post- last glacial maximum strata. We have studied this record through a combination of geologic mapping around the lake margins, 3.5 kHz and lower frequency seismic reflection profiling of lake sediments, and piston coring of the lake floor.

The sediment cores in particular contain an important sedimentologic record of the deglaciation history for the Flathead Lobe. Correlation of individual rhythmites and their relationship in grain-size changes and thickness across the lake basin combined with onshore records of recessional moraines enabled us to reconstruct the paleo-position of the retreating ice lobe. A continuous record of generally upward-thinning and fining glacial rhythmites reflects the gradual retreat of the Flathead Lobe up-valley to the north, away from our core sites. Short lived thickening and coarsening trends within the rhythmite succession, especially in the upper part of the cores are attributed to fluctuations in discharge into the lake as a result of high frequency regional climate changes. Sedimentary structures in the cores also reflect the transition from an ice margin proximal setting, dominated by traction transport processes, to an ice margin distal setting, dominated by suspension settle out processes, throughout this time.

The varved sequence is abruptly overlain by a series of coarse upward-fining silt beds. We interpret these beds to reflect either high discharge pulses of glacial meltwater resulting from catastrophic release of glacially dammed lakes on the margins of Flathead Lobe or sub-glacial meltwater releases. Depositional age of these beds is constrained as between 14,150±150 cal. Yr BP (¹⁴C date on a pine needle below the beds) and 13,180±120 cal. Yr BP (Glacier Peak tephra above the beds).