GSA Connects 2021 in Portland, Oregon

Paper No. 151-4
Presentation Time: 9:00 AM-1:00 PM


STEINKE, Jessi1, WARD, Brent1 and BOND, Jeffrey D.2, (1)Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada, (2)Yukon Geological Survey, Energy Mines and Resources, P.O. Box 2703, Whitehorse, YT Y1A 0C2, Canada

Yukon has been affected by the Cordilleran Ice Sheet (CIS) multiple times during the Quaternary. Most recent work has focused on mapping the glacial limits and dating the glaciations of the CIS, but little work has focused on alpine glaciations. Thus, the climatic significance of alpine glaciation and their relationship to the CIS are not well understood. Here we report on placer mine exposures that provide information on the extent, timing and relationship of cirque glaciers with the Selwyn Lobe of the CIS.

The study area is located in upper Granite Creek, 1 km outside of the Marine Isotope Stage (MIS) 2 limit of the Selwyn Lobe. The area is at the confluence of two cirque valleys that supported glaciers during at least the last two glacial cycles, previously identified by morphology. Placer mining operations provide ongoing exposures which allow detailed, sequential study of sedimentary units. Samples were taken for radiocarbon and optically stimulated luminescence dating. Initial interpretations, in the absence of dating control, are that the oldest unit is a weathered, gold-bearing alpine till from MIS 4 or 6. A thick, coarse, alpine gravel outwash unit has eroded into the till and interfingers with glaciolacustrine sands above. This glacial lake was formed by the Selwyn Lobe blocking Granite Creek, constraining the outwash and glaciolacustrine deposits to MIS 2. These glaciolacustrine sediments were overridden and deformed by alpine tills that extended a considerable distance beyond the well-defined moraines that were thought to represent the maximum MIS 2 cirque advances.

If the lowest alpine till is MIS 4, this provides important paleoclimatic information as no MIS 4 deposits have been identified from alpine glaciers in Yukon. Also, the Selwyn Lobe has no record of a MIS 4 advance. Alpine cirque glaciers were either limited in extent or out of phase with CIS maximum extent due to aridity during MIS 2. Here the interaction between a CIS-formed glacial lake and maximum extent of alpine glaciers suggest synchronicity. Placer mining excavations can provide a unique opportunity to study the sedimentology and unravel the glacial history of eastern Beringia.