DESPERATE AND (SEMI-) DATELESS: TOWARD A COSMOGENIC EXPOSURE AGE RETREAT CHRONOLOGY OF THE DES MOINES LOBE IN MINNESOTA

Retreat of the Des Moines Lobe from its Last Glacial Maximum (LGM) extent in Iowa was punctuated by several stillstands and/or readvances. In Minnesota, prominent moraines, include portions of the Algona moraine straddling the Iowa-Minnesota border, and the Big Stone Moraine. In its retreat, the Des Moines Lobe also partially overrode the now palimpsest Alexandria Moraine that was constructed by an earlier advance of the Rainy-Wadena Lobe. The nature of Des Moines Lobe retreat has implications for ice sheet dynamics after the LGM, and perhaps more significantly the history of Glacial Lake Agassiz, in particular its drainage through a southern outlet spillway, Glacial River Warren (the ancestral Minnesota River). However, the pacing of ice retreat is constrained by a few ages obtained through radiocarbon or optically stimulated luminescence dating, that are in most cases only indirectly associated with moraine deposition.

Toward the goal of establishing a better deglaciation chronology of the Des Moines Lobe in Minnesota we have thus far sampled 29 moraine boulders for cosmogenic ¹⁰Be exposure dating in a region in which undisturbed boulders are sparse owing to agriculture. Pending additional ¹⁰Be/⁹Be measurements, preliminary ages (ignoring outliers) at one location on the Alexandria Moraine have a mean is 14.2 ± 1.1 ka (n= 4), and at another location (n=3) are 10.2, 12.0, and 25.5 ka. At the former location, ages are reasonably consistent with previously established timing of Des Moines Lobe retreat and thus presumably represent deposition on the preexisting core of the Alexandria Moraine. The spread at the second location is more problematic, but potentially the oldest reflects initial construction of the moraine, whereas the two younger ages could be due to exhumation, or possibly later meltout from a persistent ice core.

In addition to moraine boulders, we sampled boulders and bedrock surfaces in the Minnesota River Valley ~ 10 km south of the Big Stone Moraine. Mean boulder age (n=3) is 12.3 ± 0.8 ka while a bedrock surface was dated at 14.2 ka. The latter possibly represents deglaciation, however, the boulders being younger were likely transported by Glacial River Warren during its waning role as the southern outlet for Glacial Lake Agassiz.