Late Quaternary History of the Beaverhead River Canyon, Southwestern Montana

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Geography and Geology


Biological, Environmental, and Earth Sciences


The Beaverhead River, part of the headwaters of the Missouri River, flows through a 190 m-deep canyon incised across the Blacktail Range of southwestern Montana. Discontinuous, unpaired terraces occur at three ranges in elevation above the modern floodplain: ~11±3 m, ~34±8 m, and ~65±7 m with some terrace remnants above ~75 m. The three terrace levels have gradients similar to that of the modern floodplain. A minimum age of 90 ka (Sangamon Interglaciation) was obtained for a bench of calcareous tufa at 39 m using the U-Th disequilibrium series. Assuming an average incision rate of ~43 cm/1000 years based on the tufa age, then the approximate ages of the ~11 m and ~65 m terrace levels are ~26 ka and ~150 ka, respectively. The >75 m terraces are more than ~180 ka and the age of the canyon is greater than ~460 ka. These estimates ages for terrace levels are consistent with major terrace development during times of Pinedale and Bull Lake glaciation.

The Blacktail fault offsets Bull Lake outwash `6.5 m about 29 km southeast of the Beaverhead River Canyon, and the fault scarp can be traced to within ~11 km of the canyon. The displacement rate across the fault is ~5.4 cm/1000 years (if the outwash is equivalent to the youngest Bull Lake glaciation at ~120 ka), which is only 13% of the average canyon-incision rate. Based on the large difference between the incision rate and the fault-displacement rate as well as on the parallelism of terrace levels to the floodplain, uplift and rotation of the Blacktail Range, relative to adjacent basins has not been as significant as normal fluvial processes in terrace development. Landslides may also have been an important factor in segregating terraces within different stretches of the canyon at different times.

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Guidebook to the Geology of Eastern Idaho

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