DISTINGUISHING PRE-WISCONSINAN TILLS IN SOUTHEASTERN MINNESOTA

Dodge and Olmsted counties, located in southeastern Minnesota, are composed of pre-Wisconsinan glacial sediments of the Iowan erosion surface. The landscape across these counties has been heavily modified by fluvial, eolian, periglacial, and pedogenic processes during the Sangamonian interglacial, Wisconsinan glaciation, and Holocene. These processes diminished the distinctiveness of glacial landforms that can aid in recognizing the distribution of individual till units. Mapping till units in this region based on landforms is especially challenging, therefore, tills must be distinguished using lithostratigraphy.

In central and northern Minnesota, analysis of till texture (grain size), and composition of the very coarse sand (1-2 mm) fraction has been successfully used to distinguish till units. As Laurentide ice flowed across Minnesota, ice lobes crossed landscapes composed of different rock types. Rock detritus incorporated into the ice was deposited as till with a distinct combination of lithologies indicative of ice provenance.

This method was applied to till samples collected in Dodge and Olmsted counties, previously assigned to multiple till units. Regardless of stratigraphic position, all samples had statistically similar composition. These results imply that the lithology of the 1-2 mm sand fraction may not be an appropriate method for recognizing pre-Wisconsinan till units. While this method is successful elsewhere, tills in Dodge and Olmsted may lack diagnostic lithologies because as ice flowed from the various provenances to southeast Minnesota, they eventually converged flow paths, mixing together the diagnostic combinations of lithologies. Alternatively, post-depositional leaching and weathering altered the lithology of the 1-2 mm sand fraction of the till making them difficult to distinguish.

Mapping till units is important to locating aggregate and understanding groundwater resources. The abundance of older tills in Dodge and Olmsted counties, being mapped as part of our County Geologic Atlas program, provides us with an opportunity to test alternative methods of till analysis, such as geochemical signatures, in an effort to distinguish between individual till units. This may greatly improve mapping of tills in southeastern Minnesota.