Bitterroot fault under study

 

By Michael Howell

 

One of the high points in the geologic history of the Bitterroot valley, or one of the lowest depending on which side of the fault line you were on, came when the earth cracked at the edge of the Idaho Batholith pulling the Sapphire Mountains off the top of the Bitterroot Mountains about 40 to 50 million years ago. Signs of this gently sloping fault line can be observed cutting at an angle across the valley floor. The fault was determined to be inactive since that time and the Bitterroot valley was considered the least active area seismically in all of Western Montana. But a recent study being done by Mike Stickney and Jeff Lonn, a couple of geologists from the Montana Bureau of Mines and Geology, may put the Bitterroot valley back on the map of active seismic zones.

 

When the Lincoln earthquake struck this summer, these geologists were already in the midst of a study of the Bitterroot Valley’s own active fault. According to Lonn, indications of the much newer, active fault line were discovered when the scientists examined the latest Flood Plain maps produced by Ravalli County. The laser based mapping technology called LIDAR produces maps with an accuracy of from two to three inches. The old flood plain maps were only accurate to within three feet or so. What Lonn and Stickney were able to identify on the new maps were some indications of a fault line running across the base of the Bitterroot mountains from Darby to Lolo.

 

The county flood plain mapping was focused upon the flood plain located on the valley floor and did not extend up the mountain slopes. The scientists decided it was worth a better look and applied to the U. S. Geological Survey’s Earthquake Hazards program to study the faults history and estimate how many large earthquakes might occur along it.

 

The “preliminary view” that is emerging is that it is this fault that dropped the valley down and lifted the mountains up in much more recent times. Although the Bitterroot Detachment Fault shows no signs of activity for the last 50 million years, this newly discovered fault line was created only 15,000 years ago. This was determined by recognizing that the fault line has cut a relatively straight line across the top of not only the 50-million-year-old fault line, but also across 15-million-year-old Ancestral Bitterroot River gravels, but also the most recent 15,000 year-old glacial deposits at Lake Como, Lost Horse and the Victor area. The scientists are still busy studying the fault through geologic mapping by comparing scarp heights on the different geologic units of different ages. Lonn said they do not know as yet how fast the fault is moving but they hope to figure that s out. Results of this study will be published next April.

 

Last Saturday, Lonn led an all-day walking field trip to examine the Bitterroot fault along the Coyote Coulee Trail south of Hamilton.

 

The Bitterroot fault, which runs along the base of the Bitterroot Mountains from Lolo to Darby, was long thought to be inactive until LIDAR imaging, which produces very precise maps of the earth’s surface, showed that the fault offsets the most recent glacial deposits in the Lake Como, Lost Horse, and Victor areas. The fault dropped the valley down and lifted mountains up, and it probably caused many large earthquakes as it did so. In April, MBMG geologists Mike Stickney and Jeff Lonn received a grant from the U.S. Geological Survey¹s Earthquake Hazards Program to study the fault’s history and estimate how often large earthquakes might occur along it. They hope to accomplish this through geologic mapping and by comparing fault scarp heights on the different geologic units of different ages. Results of this study will be published next April.

 

On the nine-mile hike participants got a chance to examine the fault. This trip will be for people willing to hike about 9 miles and who want to see not only the fault scarp but who also have an interest in learning more about the fascinating geology of this area. Participants will view the Bitterroot fault as it cuts across 15,000-year-old glacial deposits, 15-million-year-old Ancestral Bitterroot River gravels, and the famous 50-million-year-old fault (now inactive) that pulled the Sapphire Mountains off the top of the Bitterroot Mountains. Participation is limited to 25. Email jlonn@mtech.edu or call 363-7753 to reserve a spot. Meet at the Coyote Coulee Trailhead at 9 a.m. on September 30.