FROM ICE CONTACT TO OUTBURST FLOODS: NEW 1:24,000 SCALE GEOLOGIC MAP OF WESTERN MONTANA FROM THE NORTHERN MISSION RANGE TO CAMAS PRAIRIE

We present a new 1:24,000 scale surficial geologic map of >2000 km² from western Montana. The map area is located within the northern Basin and Range province and includes parts of the northern Mission and Salish Ranges and portions of the Swan, Flathead, Mission, and Little Bitterroot Valleys.

The map area includes the former southern terminus of the Flathead Lobe of the Cordilleran Ice Sheet. The geometry of the lobe’s terminal moraine has strongly influenced the shape of the southern shoreline of modern Flathead Lake. Each major bay occurs within an arcuate segment of the terminal moraine that is interpreted to reflect one of several ‘distributary lobes’ formed by divergence of glacial ice around bedrock topography.

Two major spillways transported melt water below the retreating Flathead Lobe during deglaciation. The eastern of these spillways is the lower Flathead River valley, where melt water incised the Polson Moraine and down cut through ~300m of Pleistocene sublacustrine diamict and laminated siltstone deposited in glacial Lake Missoula. Outburst flood deposits that include armored flood bars and boulder fields occur within the lower Flathead River Valley and are interpreted to reflect the final surges of melt water through ancestral Flathead Lake. The western spillway includes the Big Draw Valley west of Flathead Lake, the Little Bitterroot River Valley, and Camas Prairie. Melt water transported down the western spillway deposited cobble- to boulder-bearing gravel outwash down the length of the Big Arm Valley and may have in part provided input to a late-glacial ephemeral lake that occupied much of the Little Bitterroot Valley. The famous 'giant current ripples' of Camas Prairie, located in the southwestern portion of the map area, consist of weakly stratified, moderately sorted gravel with abundant boulder-sized erratics. At least three suites of bedforms were mapped; each suite was formed from a different topographic saddle over Markle Pass, located between Camas Prairie and the Little Bitterroot River.

Pediment surfaces cut onto alluvial deposits characterize several of the tributary drainages of the Little Bitterroot and lower Flathead Rivers. We interpret these alluvial deposits as Paleogene, probably Eocene, and related to initiation of Basin and Range extension in the study area.