CHRONOLOGIES AND DYNAMICS OF INTERLOBATE INTERACTIONS OF THE SOUTHERN MARGIN OF THE LAKE MICHIGAN LOBE (Invited Presentation)

The sedimentology and morphology of the interlobate Kettle Moraine strongly contrasts with the Gilman Moraine. These features are products of the interaction of the Lake Michigan lobe with the Green Bay lobe on the north, and the Erie lobe on the east, respectively. Initial interactions occurred during the local LGM (ca. 24.2 cal. kyr B.P.) with subsequent modification during deglaciation. For the Kettle Moraine, late stage sedimentation of eskers, ice-walled channels and lakes likely occurred just prior to 20.6 cal. kyr B.P. during the Woodstock Phase, and for the Gilman Moraine, prior to about 22.3 cal. kyr B.P. during the Livingston Phase. In Wisconsin, mapping of till lithologies indicate the first interaction was during the Shelby Phase (Lake Michigan lobe) as diamicton of the Tiskilwa (= Zenda) Fm is confidently traced as far north as Geneva Lake, Wisconsin. If similar to the characteristics observed in northern Illinois, much of the sediment comprising the Kettle Moraines bulk was deposited during the Shelby Phase, with notable decorative landform enhancement (eskers, ice-walled channels, ice-walled lakes) during the Woodstock Phase. Maximum ages of the Tiskilwa Fm in northern Illinois are 28.2 cal. kyr B.P., 4 thousand years older than in central Illinois. Is this due to an older advance of the LML (attributed to the Marengo Phase) or does it reflect little organic carbon production or preservation in sediment dating from ca. 24 – 28 cal. kyr B.P.?

In the case of interaction of the Erie and Lake Michigan lobes, the Gilman Moraine is an uncommon example (for Illinois) of a glaciotectonic feature. The ca. 10 m of topographic relief across the moraine is manifest primarily by ice-walled lake plains containing fossiliferous laminated sediment. Tundra plant fossils indicate a minimum age of 22.2 cal. kyr B.P. The origin of the Gilman Moraine is inferred from the semi-parallel crests of the latter and local LGM margin of the Iroquois Moraine (Erie lobe). Johnson et al. (1986) determined the lobe provenance from relative abundance of pyrite and epidote (higher in LML sediment) and garnet (higher in EL sediment).

I suggest that the decidedly different styles of sedimentation and morphology of these interlobate areas reflect not only differences in matrix texture, but the behavior of the glacier beds due to changes in ambient moisture that immediately affected dominant basal glacial processes, as well as glacier rheology. Of course, myriad other factors came into play such as albedo and buried bedrock topography.