Sedimentary record of glacial dynamics, lake level fluctuations, and tectonics: Late Pleistocene-Holocene structural and stratigraphic analysis of the Flathead Lake basin and the Mission Valley, Montana, United States America

Integrated analysis of high-resolution seismic reflection profiles, sediment cores and onshore geologic mapping from Flathead Lake and adjacent areas in northwest Montana has revealed considerable information about late Pleistocene and Holocene geologic and tectonic history of the region. During the last glacial maximum, the Flathead Lobe of the Cordilleran Ice sheet advanced twice to its terminal position just south of present day Flathead Lake. During retreat of the lobe, a pro-glacial lake developed with a surface elevation higher than that of modern Flathead Lake. Fault scarps offsetting glacial deposits but not younger stratigraphic units indicate that a tectonic event along the Mission Fault system also occurred during this early post-glacial time. Just after the deposition of the Glacier Peak tephra (13,180+/-120 cal yr BP) the surface level of Flathead Lake dropped to an initial Holocene lowstand at ∼10,000 cal yr BP. This lowstand coincided with another period of tectonic activity along the Mission Fault system. After 10,000 cal yr BP, the lake level stabilized for about 2,000 years before dropping to its lowest Holocene level at ∼7,600 cal yr BP. Seismic and core data from shallow parts of the lake indicate the presence of a fundamental erosional unconformity at this time. This unconformity coincides with the deposition of the Mount Mazama tephra (7,630+/-80 cal yr BP) and provides information about a post-glacial lake-level lowstand during which the surface of the lake dropped about 15m below the modern lake level and Flathead Lake lost about 25% of its present volume. Although no increase in CaCO3 content was observed along this unconformity, simple hydrologic and geochemical models suggest that the lake level lowering was related to a significant decrease in precipitation, likely triggered by the Mount Mazama volcanic eruption. After this short extreme drought, the lake refilled roughly to its modern day level and fluctuated slightly throughout the reminder of the Holocene. Tectonic activity in the study area was low throughout the Holocene, although several vertically offset seismic reflections in Holocene seismic stratigraphic units that may be seismically-related were recognized.