Bitterroot peatlands may help date latest geologic fault activity
Peatlands are not common in the Northwestern Rocky Mountains but they do exist. A 1998 survey of peatlands on the national forests in the northwest lists two from Ravalli County, the Lost Trail Pass Fen, near Sula, and the Rock Creek Wetland near Darby.
Some people may think of the Swan Valley when they think of peatlands. The survey found the peatlands along the valley bottom in the Swan Valley to be “the most extensive, floristically diverse concentration of peatlands in the state.” The Bitterroot, in comparison, is a pretty arid valley where the water seems to want to pass right through, without lingering long enough to start the self-sustaining process that really defines a bog or fen and the formation of peatland.
Oddly enough, however, the Bitterroot Valley does hold a very large peatland located off the national forest land and thus not part of the survey, spreading over hundreds of acres of private land on the west side of the river south of Hamilton. Peatlands are generally defined as wetlands with waterlogged substrates and approximately 30 centimeters or more of peat accumulation. These peatlands near Victor were close to five feet thick before a few feet was removed.
Once peat has developed up to about one foot in depth, the availability of oxygen and nutrients essential to plant growth drops sharply, and plant roots must obtain their mineral nutrients from the saturated, oxygen- poor peat. Lack of oxygen and cool temperatures limit plant growth, microbial decomposition, and subsequent nutrient cycling. Peatlands are, therefore, dependent on external supplies of nutrients from either the atmosphere or inflowing, mineral-enriched water.
These peatlands drew the interest of local geology instructor George Furniss at the Bitterroot College. What he found out when he made his inquiries was that these peatlands did not go unnoticed by the local settlers.
“The Smith family stripped two to two and a half feet of peat off the top of about three hundred acres,” said Furniss. A man named Colby Smith told him how he and his dad and his brothers had devised a method to strip about two to three feet of peat off the top of the ground and put it through a shredding machine. Then they bagged it and it was trucked to K-Marts across the nation for sale to gardeners. They mined the peatland in the 1960s and early 1970s and reclaimed the area simply by spreading grass seed on it and creating pasture.
Recognizing its geological interest, Furniss got permission from Smith and took his students out to view the site, always mentioning that somebody should get a carbon date on the peat and see when the peatland was formed.
This year a student of his, Saundra Amsden, said, “Hey, it’s time to do it.”
So they did. The core samples were gathered this past February. Using a hand soil auger they removed and saved three samples at 1.8 feet, 2.5 feet and 3.5 feet depth. At 3.5 feet they hit sand and gravel, marking the deepest level of the peat.
The samples were dried and the sample from the 2.5 feet depth was sent to Beta Analytic, Inc. in Miami, FL, for radiocarbon dating. The goal was to determine whether the peat deposit is old enough to be considered Ice Age or whether it dates to the post-Ice Age Holocene.
“Either way this information was going to improve our knowledge of the geological history of the Bitterroot Valley,” said Furniss.
Due to the fact that peat is so high in organic matter and full of carbon, they got a very accurate reading from the sample once it was vaporized and examined under a mass spectrometer. By using the half-life of the carbon 14 in the sample, the age of the peat was determined to be 7,940 years old plus or minus 30 years. That places it in the post Ice-Age Holocene Era but prior to the Little Ice Age.
Then the question arose as to what would have transpired at that spot 8,000 years ago to create such a peatland. It was hard to account for the peatland by the usual methods of formation, like an old oxbow in the river.
Then some other very unusual geologic information came to light when the County used the modern technique of Lidar mapping to update its map of the designated Floodplain area in Ravalli County. The old floodplain maps were only accurate to a couple of feet. The new Lidar mapping was accurate to within a few inches.
When Jeff Lonn at the Montana Bureau of Mines and Geology saw the maps, something very special caught his eye.
The existence of a fault running diagonally from the southwest to the northeast across the Bitterroot Valley has been recognized for a long time. It has been studied and determined to be inactive. This left the Bitterroot Valley near the bottom of the list in terms of earthquake hazard, something of concern to certain government agencies and insurance companies.
But what Lonn saw in the Lidar mapping was evidence of another fault line, this one also running the length of the valley but holding closer to the Westside mountains. This fault showed signs of being active in more recent times and potentially still active, but its last activity was not precisely dated.
Furniss said that Lonn was looking for help at the time to determine the date of the fault’s active formation, so he took a look at the Lidar images himself.
“I looked at Victor and Bear Creek and saw quite an expression of the fault there, in the early part of the Interglacial period still getting lots of mudslides but it did not offset the floodplain of Bear Creek in a long time. It’s got the look of an 8,000-year-old fault,” he said.
Then he and his students took another look at the peatlands in relation to the newly discovered fault line and saw that the western edge of the peatland runs along the eastern edge of the fault.
“We believe this allowed a down-drop tilting of the area… creating an extensive shallow lake or pond that was kept active for long enough to form a peatland,” said Furniss. “Visual examination along the fault line shows several natural springs that would have kept the area wet, even during the drier seasons.” He said an offset that had sunken down would allow springs to express and form a “fen” when water emerges in a fault and it makes peat.
In short, by dating the beginning of the formation of the peatland at 7,940 years ago, Amsden and Furniss may have also roughly dated the fault slippage that could have formed them.
The report, by Saundra Amsden and Furniss, was published at the University of Montana on April 27.
This is just the beginning of a more complex verification process, however. Plans call for more sampling and dating along east/west lines in the peatland area to get more depth readings that could help determine if the area does have a downward tilt on the west side. Some core sampling and dating along a north/south line is also planned to determine the area of the down-drop tilt, which could be as extensive as 1,200 square acres or about 1.84 square miles. This will require drilling equipment and multiple radiocarbon dating samples from numerous sites and depths in the proposed area.
“This is an exciting thing for the Bitterroot College to be doing,” said Furniss.
Furniss said he was also excited about what his Environmental Science classes had been doing along the Bitterroot River.
“We know there is a nitrate problem along the river and have been looking at it for a number of years,” he said. He said his students had been checking the Corvallis Canal at publicly accessible spots where it flows along the west edge of Hamilton and picks up groundwater in the winter.
The class examined the groundwater collecting in the canal back in February and early March when there was no irrigation and the results showed heavy loads of nitrates when the river showed none. They examined the length that runs from Main Street bridge to Veterans Bridge and found a small tributary at an oxbow just past the hospital which their test results showed was delivering about 4 pounds of nitrates per day to the Bitterroot River.
Furniss said that the tributary runs along the city’s surge pond which is part of the City’s sewage treatment plant. They followed the canal even further north to where groundwater was emerging near the Daly Mansion area and found an additional 4 pounds of nitrate-nitrite.
“They have a permit to discharge 94 pounds,” said Furniss, “but I don’t think this 8 pounds is being counted.”
He said he had been told that some of his students thought the information was so important that one of them had called the mayor about it. He said he doesn’t know what came of the call but he thinks the mayor may be looking into it.
Furniss said that one class tested for nitrites and nitrates at spots along the canal three years ago and that the recent tests show an increase. He said the class advocated continued testing to see if it continues to increase and at what rate.
“I think the students felt like they were doing something that could really help the community because none of these amounts are dangerous, but they are showing an increase above background that may not be currently accounted for,” said Furniss.
He said the students did a good job of quality control using field blanks and duplicates, and used a trustworthy laboratory.
Furniss said the students seemed to really enjoy doing a “real world type thing.” The results were also published as well and oral presentations were made at the recent Conference for Undergraduate Research in Missoula. He said presenting to peers, not just to their teacher, “really perks them up.”