SEDIMENTARY RECORD OF PALEOSEISMICITY ALONG THE MISSION FAULT IN SEDIMENTS OF FLATHEAD LAKE, NW-MONTANA

The Mission fault is a major, active, basin-bounding normal fault in northwestern Montana and is responsible for the impressive topographic relief of the Mission Mountain front. Trenching studies of the fault demonstrate that it underwent an estimated 7.5 magnitude prehistoric rupture about 7,700 cal. yr BP and has a Quaternary recurrence interval of 4000-8000 years (Ostenaa et al., 1990). Closely associated with the Mission fault and underpinning much of the Mission and Flathead Valleys is a complex of Quaternary deposits associated with Glacial Lake Missoula (Pleistocene) and Pleistocene through Holocene sediment associated with Flathead Lake -- a remnant of Glacial Lake Missoula. The surface expression of the Mission Fault currently projects underneath the eastern edge of Flathead Lake. The occurrence of Flathead Lake (550 km² surface area) creates a unique opportunity to examine the detailed record of Late Pleistocene to recent seismicity associated with the Mission Fault. We studied over 270 km of 3.5 kHz seismic data from Flathead Lake and a set of eight piston cores that we recovered from the lake. The longest core contains both the Mount Mazama volcanic ash (7,630 cal. yr. BP) and the Glacier Peak volcanic ash (13,750 cal. yr. BP), demonstrating that the lake has received sediment continuously since the last glacial period. Our data suggest that the sediments of Flathead Lake contain a record of past seismic events in the form of large slump blocks, listric normal faults, and areas of liquifaction, that we infer to be related to seismicity along the Mission fault system. Significant normal faults are overlapped by younger sediment of varying ages, demonstrating multiple episodes of significant seismicity, consistent with multiple events revealed by trenching studies of the Mission Fault immediately south of Flathead Lake (Ostenaa et al., 1990). Onlap stratal relationships within the lake indicate a complicated history of lake level fluctuations, likely related to a combination of climate change and tectonic history of the fault system.