LITHOLOGIC SUBDIVISION OF DES MOINES LOBE TILLS IN MINNESOTA: IMPLICATIONS FOR THE TIME-VARYING DOMINANCE OF ASSOCIATED ICE-STREAMS

The late Wisconsin Des Moines lobe flowed through eastern North Dakota, Minnesota, and into central Iowa. It advanced repeatedly followed by widespread stagnation. Matsch (1972) attributed the glacial deposits of southern Minnesota to the northwest-sourced Des Moines lobe and called them the New Ulm Till. Since then, all shale-bearing till in southern Minnesota has been referred to as New Ulm Till.

Efforts to formalize lithostratigraphy in southern Minnesota (Lusardi, in prep.) have identified at least eight distinct northwest-provenance tills distinguished using the texture and lithology of the very coarse-grained sand fraction. For most of these tills, two variables—percentage of sand and shale—result in statistically defensible clusters that identify a lithostratigraphic unit (Harris, 1998). These interpretations are based on analysis of over 12,000 till samples in Minnesota and nearby states. Lithostratigraphic units have been correlated from northeastern North Dakota to Iowa, a distance of nearly 1,000 kilometers. At the surface, units are continuous, mappable features with boundaries associated with ice-marginal landforms such as moraines and tunnel valley mouths. Cross-cutting and stratigraphic relationships of units, interpreted from drill cores, support a consistent sequence of events that deposited the tills.

The distribution of lithostratigraphic units in the study area supports the hypothesis that the Des Moines lobe was fed by multiple ice streams that were sourced in lithologically distinct areas. It is the time-varying dominance and interaction of the ice streams that result in tills of varying provenance within the Des Moines lobe.

Harris, K.L., 1998, Computer-assisted lithostratigraphy, in Patterson, C.J., and Wright, H.E., Jr., eds., Contributions to Quaternary studies in Minnesota: Minnesota Geological Survey Report of Investigations 49, p. 179-192.

Matsch, C.L., 1972, Quaternary geology of southwestern Minnesota, in Sims, P.K., and Morey, G.B., eds., The geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 548-560.