THE LIMITATIONS OF DRILL CORE ANALYSIS FOR GLACIAL STRATIGRAPHIC INTERPRETATION: AN EXAMPLE FROM EASTERN WISCONSIN

While drill core analysis can be used to differentiate till units when the units have unique till properties, this tool cannot reliably distinguish between till units that have similar properties. Such a problem has arisen in eastern Wisconsin. In this area, the older, pre-Two Creeks, reddish-brown till units of the Kewaunee Formation have similar color, carbonate content and clay mineralogy. Also, there are no known exposures that contain at least two of these till units. Their stratigraphic relationships were interpreted based on small differences in grain size and the presence of sorted sediment layers in drill cores. Recently, mapping completed along western Lake Michigan indicates that these till units are indistinguishable based on grain size. With this information, only the presence of sorted sediment layers between these till units in drill cores distinguish one unit from another. However, these sorted layers can have a genesis not related to the retreat and readvance of ice. In this area, sorted sediment inclusions - some thicker than those recorded in drill cores - are present within homogeneous Kewaunee Formation till. I interpret these inclusions as the result of basal freeze-on. In addition, subglacial water can deposit sorted sediment within a single till unit. Observed in a drill core, these deposits could be misinterpreted to represent an ice retreat followed by a readvance and deposition of another till unit. Therefore, till units with similar properties cannot be differentiated using the presence of sorted sediment layers in drill cores. An exposure containing at least two of these till units with extensive sorted sediment between is needed to properly interpret their relationship.