RESULTS OF GEOLOGIC MAPPING OF THE MISSION FAULT SYSTEM AND ASSOCIATED GEOLOGY, NORTHWEST MONTANA

This study results from EDMAP-supported geologic mapping conducted during the 2002 field season. The main goals of this mapping were 1) to document on the geometry and surface expression of the Mission fault, a major normal fault in northwestern Montana and 2) to provide a more complete understanding of the glacial and post-glacial history of the study area. Major rock units include very low grade metasedimentary rocks of the Meso-Proterozoic Belt Supergroup as well as unconformably overlying glacially and non-glacially derived Quaternary rocks. Glacial deposits result from several advances and retreats of the Flathead Lobe of the Cordilleran Ice shield and different alpine glaciers from the Mission Mountains. Non-glacial deposits include minor Holocene fluvial and eolian deposits.

Within the map area, several fault scarps associated with the Mission Fault offset Quaternary glacial sediments. These scarps are up to 10m high and the longest scarp extends to a length of about 300m. Scarps project north into Flathead Lake, where offshore 3.5 kHz seismic reflection data reveal significant stratal offsets associated with major slumping events that we interpret as pre-15ka seismites (Hofmann and Hendrix, 2002a,b). Independent trenching studies of the Mission fault about 10 km south of our map area suggest that the fault has an active history of seismicity that overlaps with the age of sedimentation in Flathead Lake basin. The last major event that has produced a surface rupture along the Mission fault occurred 7,700 +/- 200 y.b.p. and was estimated at magnitude 7.5 (Ostenaa et al., 1990, 1995).

Additional geologic mapping further to the south along tectonic strike provide information about the interaction of two major fault zones, the N-S trending Mission Valley fault and the SE-NW trending Lewis and Clark fault zones. In this area basement rocks of the Precambrian Belt Supergroup are dominant. Quaternary sediments occur only as recent alluvium along modern rivers or as outwash fans in the northwestern part of the mapping area. Glacially derived sediments are rare. Only at Mission Reservoir are lateral and terminal moraines of an alpine glacier recognizable as map units. In this area fault scarps that offset Quaternary sediments are uncommon and an age of movements along the different fault systems difficult to determine.