SURFACE GEOLOGIC MAPPING OF THE CONFLUENCE OF THE SWAN AND FLATHEAD VALLEYS, WESTERN MONTANA: PRELIMINARY RESULTS

Located in the northern basin and range physiographic province, Montana's Flathead and Swan Valleys are north-south trending active extensional half-grabens. During the most recent glacial maximum the Flathead lobe of the Cordilleran ice sheet flowed southward down the Flathead Valley, filling the topographic basin currently occupied by Flathead Lake. The Swan Valley hosted a major valley glacier that flowed northward and joined the Flathead lobe at the confluence of the two valleys near Bigfork, Montana. This study investigates the sedimentary record of glaciation from this glacial confluence and the structural geology associated with the northern end of the seismically active Mission Fault. Here we report results from field geologic mapping and analysis of Quaternary and older deposits.

We conducted field geologic mapping at 1:24,000 scale during the summer of 2008 along the eastern shore of Flathead Lake and in the lower Swan Valley. All field observations were located via GPS, filed in a digital database, and displayed in a GIS for evaluation in the context of topographic data and aerial photography. Map units were defined on the basis of sediment textures, compositions, and architectures.

Area highlands are composed of Belt Supergroup rocks mantled by till. Glacial striation azimuths indicate that the Swan Valley glacier flowed northwest down the Swan Valley and turned south around the northern end of the Mission Mountains to merge with the Flathead Lobe. Although glacial influence on local topography is highlighted by the orientation of prominent canyons in the northernmost Mission Range roughly parallel to measured striations, the large scale of these canyons permits interpretation of some structural control on their morphology, possibly by east-west trending strike-slip faults transferring strain between range-bounding normal faults in the Mission and Swan valleys. Ice-contact and ice-margin deposits characterize Quaternary basin fill exposed in area lowlands. The Flathead and Swan valleys are separated by the Echo Lake Moraine complex, which contains clast lithologies originating at least 40 miles north of the field area. Because these lithologies are absent from the Swan Valley, we interpret the Echo lake moraine complex as a medial moraine formed between the Flathead Lobe and the Swan Valley glacier.