COSMOGENIC EXPOSURE DATING OF LATE PLEISTOCENE TERMINAL MORAINES OF THE SOUTHWESTERN LAURENTIDE ICE SHEET

The moraine record of the southwestern Laurentide Ice Sheet is key for reconstructing changes in ice sheet geometry through time. In places, however, terminal moraines of outlet lobes of the southwestern Laurentide have poor age control. Age limits on most moraines are based on a small number of numerical ages (chiefly from radiocarbon) or broad morpho-stratigraphic correlations to dated deposits far afield. Some portions of terminal and recessional moraines of the last (Wisconsin) glaciation in Minnesota, South Dakota, and North Dakota have undisturbed crests with abundant erratic boulders suitable for cosmogenic exposure dating. These include segments of the Alexandria moraine deposited by the Rainy Lobe in Minnesota, a suite of moraines deposited by the western Des Moines Lobe in South Dakota and Minnesota, and the Zeeland and Burnstad moraines deposited by the western James Lobe in North Dakota. Cosmogenic exposure dating of terminal moraines in this region requires some consideration of moraine deposition and post-depositional processes affecting moraine crests. Broad hummocky moraines, such as the Alexandria moraine, may be compound features representing multiple ice advances and likely developed by slow melting of buried, stagnant ice. Ascertaining meaningful exposure ages of such moraines requires sampling numerous boulders over a broad area. Generally, sampling strategies require careful development but are limited by the fragmented nature of undisturbed portions of the moraine crests, land access, and boulder frequency. We report exposure ages from multiple segments of the Alexandria moraine and from segments of moraines in western North Dakota and discuss some considerations for developing cosmogenic chronologies of moraines of the southwestern Laurentide.