EVOLUTION OF THE TWIN CITIES MEMBER TILL: IMPLICATIONS FOR LITHOLOGIC VARIATION IN THE NEW ULM FORMATION

Till of the Twin Cities Member of the New Ulm Formation contains lithologic assemblages associated with two distinct provenances. These interbedded sediments resulted from a branch of the Des Moines lobe (DML), the Grantsburg sublobe, advancing over red sediments of Superior provenance within the St. Croix moraine, and eroding and incorporating that material into its yellow (2.5Y), loam-textured debris load of Riding Mountain provenance.

Recent field investigations in Hennepin and Isanti Counties along the axis of the Grantsburg Sublobe characterized down glacier changes in the lithologic composition and mixing characteristics of Twin Cities Member till. With increasing distance, occurrences of distinct mixing zones decrease and the Twin Cities Member till becomes homogenized. The homogenized till is higher in Superior provenance clasts and intermediate in color (brown, 10YR) between the two end member lithological compositions.

Along its central axis in the Minnesota River Valley, the DML advanced over yellow (2.5Y) clay loam till deposited during previous glacial advances. Along its margins, however, the DML overrode coarser-textured sediments of contrasting provenance. Particularly along its left lateral margin, the DML advanced over a topographic high of the Alexandria moraine composed of yellow (2.5Y), sandy till, outwash, and ice-contact sediment. Mixing of sediments of contrasting provenance resulted in systematic variability in the amount of Cretaceous shale in the 1-2mm very coarse-grained sand fraction with no change in color. Higher shale concentrations (Riding Mountain provenance) along the central axis of the lobe and progressively lower concentrations toward the eastern margin, indicates increased erosion and incorporation, allowing for more comminution of soft, Cretaceous shale into smaller (<1mm) clast sizes.