FOLLOWING THE RELATIVE WEATHERING FOOTSTEPS OF KEN PIERCE THROUGH THE NORTHERN BASIN AND RANGE REGION, USING NUMERICAL DATING

Ken Pierce and coworkers pioneered the regional application of relative weathering criteria to northern Basin and Range Quaternary landforms and sediments. They and others correlated glacial, alluvial, and loess deposits with Pinedale and Bull Lake glaciations, and inferred that other prominent landforms correlated with intervening climatic fluctuations. We summarize luminescence and cosmogenic radionuclide ages to test those correlations.

In the Lost River Range and Big Lost River valley, new ages document Pinedale and pre-Pinedale glacial and alluvial activity. At Rock Creek, outwash terraces and fans indicate an extended period of aggradation 54-33 ka, followed by incision to form younger erosional terraces in the Lost River fault zone. A limited ¹⁰Be chronology indicates moraine ages of 50-40, 25, and 20 ka. These age ranges are broadly correlative with published, nearby alluvial fan and terrace ages.

Moraines in the adjacent Pioneer Mountains, where ice dammed the East Fork Big Lost River and generated outburst floods, yield ¹⁰Be ages of 21 and 19 ka. Additional exposure ages from ³He on boulder bars 100 km downstream, however, indicate two dominant age modes of 21 and 35 ka and suggest that extensive glaciation occurred at the flood source during both time periods.

Alluvial fans and moraines in the Birch Creek valley also provide evidence of both Pinedale and pre-Pinedale climatic fluctuations. ¹⁰Be ages of moraines mantling the Lemhi Range indicate Bull Lake and Pinedale glaciations, and a single age from a series of intermediate moraines is 45 ka. On the Beaverhead Range front bordering the eastern Snake River Plain, 24 OSL ages from alluvial fan gravels indicate aggradation 115-110 ka, 94-84 ka, 74-66 ka, 61-43ka, and 22-16 ka.

To the degree that relative and numerical age records can be directly compared, these results suggest that the general conclusions of Ken Pierce and colleagues were accurate. Eastern Idaho alluvial fans aggraded 22-16 ka and are associated with moraines dated to 25-18 ka, correlative with the Pinedale glaciation at its type locality. Limited Bull Lake-correlative ages are also present, and many landforms, especially alluvial fans and terraces, reflect climatic fluctuations in the long period between the Bull Lake and Pinedale events.