QUATERNARY HISTORY OF THE MISSION VALLEY, NW-MONTANA: RESULTS OF GEOLOGIC MAPPING

This study results from EDMAP-supported geologic mapping of the northern Mission Valley and parts of the southern Mission and northern Jocko Valleys, northwest Montana. The main goals of this work were to provide a more complete understanding of the glacial and post-glacial history of the study area and to document the seismic history along the Mission Fault system.

In the northern Mission Valley glacial tills form the most pronounced geomorphologic features. At least two generations of alpine glacial advances, related to Bull lake(?) and Pinedale glaciation, were recognized. During the latest Pleistocene, the Flathead lobe of the Cordilleran Ice Sheet advanced probably once as far south as the town of Polson and formed the Polson terminal moraine. Sedimentologic analyses indicate that the Polson terminal moraine was deposited in a sub-lacustrine environment. Other Quaternary sediments in the northern Mission Valley include Pleistocene lake sediments, colluvium, stream alluvium and outwash sediments. Holocene sediments are mostly alluvial deposits, but also include lake sediments and colluvium. Well pronounced fault scarps, up to 12 m high and 300 m long, are associated with the Mission Fault system and offset Quaternary lateral moraine sediments.

Unlike in the northern Mission Valley, glacial deposits are less common in the southern part of the Mission Valley and in the Jocko Valley. Most of this area is covered by Pleistocene glacial outwash. Well pronounced Pleistocene alpine moraines are only developed around Mission Reservoir. Other Quaternary sediments include Pleistocene sediments from glacial Lake Missoula, Holocene stream alluvium, and colluvium. Metasedimentary rocks of the Belt Supergroup are considerably more common and form all of the local hills and mountains.

Structural analyses of the Mission and St. Mary’s fault system indicate that the northern Mission Valley is seismically less active. Fault scarp analyses suggest that the last significant rupture occurred about 15,000 years ago in the north, while in the southern valley the last tectonic movement occurred about 7,700 years ago.