PRELIMINARY RESULTS FROM GEOLOGIC MAPPING AND SEDIMENTOLOGIC ANALYSIS OF LATE PLEISTOCENE GLACIAL AND POST-GLACIAL DEPOSITS, SOUTHERN FLATHEAD AND NORTHERN MISSION VALLEYS, WESTERN MONTANA

We present results from geologic mapping and sedimentologic analysis of glacial and post-glacial deposits in the southern Flathead and northern Mission Valleys, western Montana. Within the study area, the Flathead Lobe of the Cordilleran Ice Sheet terminated into glacial Lake Missoula as an ice shelf with several divergent “distributary lobes”. Each lobe deposited a geomorphically distinct terminal moraine characterized by hummocky topography and crudely stratified till. Associated highly aggradational subaqueous current ripples suggest that the moraines are largely sublacustrine. Flat-topped landforms downstream (west) of the Big Arm moraine contain clast-supported, well-rounded conglomerate and downstream-dipping meter-scale foresets, suggesting delta progradation into glacial Lake Missoula. A well-preserved fluvial outwash plain is preserved further downstream, reflecting sediment dispersal following lake level lowering.

The main body of the Flathead Lobe terminated at the Polson moraine, immediately south of modern Flathead Lake. The southern (downstream) side of the Polson moraine is cut by glacial Lake Missoula shorelines. The moraine itself consists of crudely stratified conglomerate fining upward into well-sorted sand with highly aggradational subaqueous current ripples, capped by a ~2m section of loess with local ventifacts and paleo rodent(?) burrows. Downstream of the main body of the Flathead Lobe, the Mission Valley contains a >100 m thick section of poorly organized diamictite and laminated silt with abundant drop stones. We observe no evidence of ice contact in these deposits and interpret them to reflect sublacustrine sedimentation in glacial Lake Missoula.

Following terminal draining of glacial Lake Missoula and exposure of sublacustrine deposits in the Mission Valley, the Polson moraine provided the main spill point for melt water from the retreating Flathead Lobe. Incision of the spill point resulted in lowering of the ancestral Flathead Lake level and establishment of a series of fluvial terraces along the lower Flathead River valley. The youngest fluvial terrace, located immediately above the modern Flathead River, contains the Glacier Peak tephra (13,180 cal yr. BP), providing an upper age constraint on the timing of terrace development.