SEDIMENTARY RECORD OF DEGLACIATION AS OBSERVED IN LAKES OF NORTHWESTERN MONTANA

Sediment cores recovered from Mud Lake, Horseshoe Lake, Lake Mary Ronan, and Flathead Lake, NW Montana provide an excellent sedimentary and chronostratigraphic record of deglaciation near the former southern terminus of the Cordilleran Ice Sheet. Rhythmites suggestive of a glacially-influenced environment characterize all four lakes at stratigraphic levels below the Glacier Peak Tephra (hereafter GPT; 11,600 14C yr BP). In Flathead Lake, up to six discrete fining-upward beds of coarse silt, interpreted to reflect large discharge hydrologic events related to the wasting of the Flathead Lobe of the Cordilleran Ice Sheet upstream, occur directly below the tephra. Below these ‘event beds', the Flathead Lake record consists of a series of sediment rhythmites, interpreted as annual varves. These rhythmites thin- and fine-upward over several meters, likely reflecting increased distance to the glacial front through time. The pre-GPT sediment record from Lake Mary Ronan consists of an overall upward-thinning series of rhythmites but appears to lack the distinct event beds observed in Flathead Lake. Outsized pebbles are common and inferred to be ice-rafted. In Horseshoe Lake, a sharp unconformity several cm below the GPT separates cm-scale silt- and clay-dominate rhythmites from more homogeneous, organic-rich mud above. In Mud Lake, a sharp unconformity separates a series of ~0.5 cm-thick, sand-bearing silt- and clay-dominated rhythmites that are inferred to be glacially-influenced from an overlying upward-fining layer of organic-rich mud that marks the beginning of post-glacial organic-rich sedimentation. The GPT was not observed in Mud Lake, but plant tissue collected from the fining upward sequence immediately above the sharp contact yielded an un-calibrated AMS 14C date of 29,940±280 14C yr BP. Dispersed organic carbon located 10 cm above the contact yielded an AMS carbon date of 15,210±80 14C yr BP and fossil plant matter 15 cm further up-core yielded an AMS 14C date of 14,870 14C yr BP, together suggesting significant carbon reworking in the basal post-glacial record from Mud Lake. Collectively, these results suggest an asynchronous response to deglaciation among the four lakes and the preservation of potentially high fidelity information during this time of significant environmental change.