Discriminating texturally similar tills in central Minnesota by graphical and multivariate techniques

Abstract

The sequence of glaciation in the midwestern United States has been defined mainly by till stratigraphy. Correlation of tills over long distances, however, is complicated by spatial variation of sediment characteristics. In east-central Minnesota three till units were previously defined. Two were considered to be lithologically identical and were correlated with the late Wisconsin advance of the Rainy lobe, whereas a third till was assigned to the contemporaneous advance of the Superior lobe. This interpretation was inconsistent with subsequent glaciological reconstructions of the Rainy and Superior lobes. In an effort to define the parent lobe of the deposits in central Minnesota, the tills were sampled, analyzed, and compared by graphical and multivariate techniques. Graphical comparisons of sedimentological parameters were inadequate to distinguish important differences among the groups. However, principal component analysis of the analytical data indicates that the original interpretation is untenable and that one of the tills formerly assigned to the Rainlylobe should now be attributed to the Superior lobe. Problems encountered with correlation of tills exposed at the surface illustrate the complexity and problems associated with subsurface stratigraphic correlation.