NEW LIDAR DATA REVEALS LATE QUATERNARY GEOLOGY DETAILS IN THE BITTERROOT VALLEY, WESTERN MONTANA

Because much of the Bitterroot Valley of western Montana is forested, high-resolution, airborne LiDAR data have enabled a better understanding of the Cenozoic geology, especially late Quaternary features. The Bitterroot Fault, an Eocene, mylonitic, north-trending, gently east-dipping normal fault, separates the Bitterroot Range to the west from the Bitterroot Valley. Although the area of LiDAR coverage does not image the range front completely, available data reveal younger, high-angle normal faults, some sections of which lie well east of the prominent range front. A 2.6-m-high scarp near the mouth of Big Creek cuts a glacial outwash terrace of probable late Wisconsin age, providing evidence for a surface-rupturing earthquake during the latest Quaternary or Holocene. Southwest of Hamilton, scarps up to 13 m high offset older alluvial fan and glacial till deposits, indicating recurrent surface-rupturing earthquakes during the Late Quaternary. This scarp also appears to mark the easternmost exposures of the mylonitic Bitterroot Fault, which lies up to 2 km east of the range front. Thus, despite an extremely low historic seismicity rate, the Bitterroot Valley may have a greater seismic hazard than currently recognized.

The Bitterroot River has a broad flood plain comprised of a network of braided channels along much of its length. A series of alluvial terraces distributed along the river and principle tributaries indicate the river system is actively downcutting. Along the southern reaches of the Bitterroot River, interbedded fine-grained and fluvial Oligocene(?) deposits of the Ancestral Bitterroot River have failed in a series of landslides, some of which are historically active, along terrace risers. Near the range front west of Victor, landslides have occurred in these Ancestral Bitterroot River deposits—consisting almost entirely of exotic Belt quartzite cobbles—that lie up to 390 m above the modern river channel.

Well-imaged shorelines of Glacial Lake Missoula are preserved on moderatly steep slopes underlain by Tertiary alluvial fans and bedrock-derived colluvium along the eastern valley margin and on Quaternary alluvial fans, debris flow deposits, and glacial till deposits along the western valley margin, dominantly from elevations of 1075 m up to a maximum stillstand of 1995 m above mean sea level.