INVESTIGATING ARID ALPINE PLEISTOCENE GLACIATION IN THE PIONEER MOUNTAINS OF MONTANA USING COSMOGENIC 10-BERYLLIUM EXPOSURE DATING

This project uses cosmogenic 10-Beryllium dating to investigate the glacial behavior in the arid Pioneer Mountains. We are testing the hypothesis that the proximity of the Pioneer Mountains to the Laurentide Ice Sheet (LIS) and leeward side of the Continental Divide strongly influenced the timing of maximum glacial expansion due to limits in precipitation as the growth of the LIS altered regional atmospheric circulation patterns.

The focus of this study is on Dingley Creek located on the west side of the range, and Canyon and Birch Creek valleys located on the east side. We collected 29 boulder samples and dated them using cosmogenic 10-Beryllium to date the maximum extent of the 3 valleys. The average ages excluding outliers for Dingley Creek are 19.4 ± 0.5 ka (n = 9), Birch Creek 19.6 ± 0.6 ka (n = 5), and Canyon Creek 19.8 ± 0.4 ka (n = 6) which all overlap within measurement error. This gives an overall Last Glacial Maximum (LGM) average of 19.6 ± 0.5 ka (n = 20).

Of the few outliers, Dingley Creek only had 2, the younger of which (15.5 ka) is odd because it lies on the smaller outermost moraine that joins with the larger right lateral moraine. The older outlier (55.5 ka) is a large flat boulder that likely did not fully erode previous nuclides. Birch Creek had only one outlier which was considerably younger (16.8 ka) and downslope from the crest suggesting it may have rolled. The Canyon Creek terminus area yielded several large outliers (254.5 ka, 100.4 ka, and 161.0 ka) along the left lateral moraine that suggest their deposition was from nearby rockfall of quartzite with prior exposures. Due to the large spread, it's unlikely these represent Bull Lake age deposits.

By comparing our average LGM results with that of other regional ages from the Northern Rockies (Laabs et al., 2020), our TCN values produce relatively older maximum extents. This suggests the LGM extent in the Pioneer Mountains is more similar to the Wind River (22.3 ka) and Beartooth Mountains (19.8 ka). This contrasts with the younger ages of the western and southern Greater Yellowstone Glacial System and Teton Range (17.1 ka). The Pioneer Mountains hold a unique record of the glacial response of these dry, continental, mountain ranges indicating that maximum extents occurred earlier than other higher precipitation ranges at comparable latitudes/elevations in the Northern Rockies.