Genesis of an extensive interlobate deposit in eastern Wisconsin, USA

Carlson, A.E., and Mickelson, D.M. (03-2)

Understanding the depositional environment of glacial landforms is critical to interpreting past ice sheet dynamics and climate change. Conceptual models of landform genesis exist, but often fail to properly describe individual landforms. Such is the case with the northern Kettle Moraine (nKM) of eastern Wisconsin B an interlobate deposit formed between the Lake Michigan and Green Bay lobes of the Laurentide Ice Sheet during retreat from its Last Glacial Maximum position. We use sedimentology and landform distribution in this interlobate deposit to interpret a unique genesis. The nKM consists of a low, debris poor axial zone between two, long, high-relief hummocky ridges that grade east and west into broad hummocky zones and pitted outwash. In the low axial zone there are moulin kames up to 55 m high. Diamicton and poorly sorted sand and gravel core these moulin kames and have an average clast roundness of 0.55"0.09 on a scale of 0.1 to 0.9. In contrast, the hummocky areas have an average roundness of 0.78"0.03. Cross beds and imbricated clasts indicate that melt water flowed towards the nKM axis. However, little fluvial debris is found in the axis area and the sediment there underwent little fluvial transport relative to the adjacent ridges and hummocky zones. Thus, we interpret the two ridges as subaerial, supraglacial channels that were incised into ice and abutted stagnant ice in the axial area as suggested by their abrupt ice contact face and hummocky nature. These channels drained each lobe adjacent to the stagnant ice in the nKM axis which diverted melt water and protected the axis from glaciofluvial deposition. This axial stagnant ice was preserved by initial debris cover deposited as ice lobes thinned. Later, this debris accumulated in the channels on either side or in moulin kames. Therefore, the high relief hummocky sides of the nKM and the bulk of nKM sediment were deposited mainly in supraglacial channels against stagnant ice along the axis. This suggests that the dominant depositional environment for the nKM was supra-glaciofluvial with little subglacial or proglacial sedimentation. This suggests that the application of such genetic models to other landforms may deserve reevaluation.