Abstract

Close scrutiny of the till deposited during the Late Wisconsin advance of the Grantsburg sublobe across east-central Minnesota enables us to reconstruct the glaciological nature of a mid-latitude Pleistocene ice lobe. The sublobe, an offshoot of the Des Moines lobe, advanced to the northeast about 16,000 yr B. P., overriding drift of the Superior lobe, an ice mass that had advanced from the Lake Superior basin approximately 20,000 yr B.P.

Because the tills are compositionally distinct, variations in Grantsburg tills can be discerned that resulted from incorporation of Superior lobe drift. Patterns of Grantsburg till dilution can in turn be interpreted to reconstruct some basic physical characteristics of the sublobe. To quantify compositional variability, analytical methods were selected to characterize the tills. Texture, carbonate content, clay mineralogy, and magnetic susceptibility, when considered jointly, adequately distinguish the tills and constitute the means of detecting dilution of Grantsburg tills.

Results of analysis indicate that the composition of Grantsburg till changes progressively downglacier as a consequence of prolonged contact with Superior lobe drift terrane. Marked variations across the sublobe are also apparent with greater dilution at the margins and relatively lower levels of Superior lobe drift incorporation at axial positions. This pattern suggests that the Grantsburg sublobe was a semipolar ice mass, frozen to the substrate at its margin and at its pressure-melting point along its axis.

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