THE GLACIATED LANDSCAPE IN THE GRAND TRAVERSE BAY REGION OF MICHIGAN AND THE NATURE OF THE GREATLAKEAN ADVANCE OF THE LAURENTIDE ICE SHEET IN MICHIGAN, USA

Major landscape features and the youngest Pleistocene stratigraphic units in the Grand Traverse Bay (GTB) region were formed by subglacial and proglacial processes during the Greatlakean phase advance of the Laurentide ice sheet onto the Lower Peninsula of Michigan in post-Twocreekan time (after ca. 11,700 14C yr BP). Inland from GTB, a markedly drumlinized area records radiating ice flow in a sublobe of the Lake Michigan lobe. The drumlin fields are associated with thin sandy till over stratified sand and gravel. The drumlins and substrate are deeply incised by an anastamozed network of tunnel valleys that also radiate toward the Greatlakean readvance limit. That limit is defined by an elevated and pitted outwash plain - the Mancelona sandur - that slopes and increases in width southwestward in the direction of proglacial meltwater flow. The sandur was confined between the ice margin of the readvance limit and a higher head-of-outwash associated with the outer Port Huron moraine. The Mancelona sandur previously was interpreted to be associated with an inner Port Huron moraine, formed before the Twocreekan interstade. However, it must have formed during the post-Twocreekan Greatlakean advance because there are no younger Laurentide-scale outwash systems or moraines that can be related to the Greatlakean limit. Geomorphic and stratigraphic relationships support the following interpretation. Rapid glacial flow and advance into the GTB region was associated with deformation and streamlining of preexisting stratified sand and gravel. Drumlin formation was terminated by large subglacial meltwater discharge that cut or exhumed the large subglacial tunnel channels. Proglacial discharge from the tunnel valleys was associated with the formation of the Mancelona sandur. The landscape and stratigraphy are interpreted to record a glacial advance linked to subglacial hydrology somewhat similar to that of the 1983 surge of Variegated Glacier, Alaska, but on a much larger scale. The Mancelona sandur grades to a lake level at least as high as the Calumet level (post-Twocreekan age; Schneider and Hansel, 1990) of proglacial Lake Chicago, indicating its temporal association with a relatively high proglacial lake level and southward proglacial discharge through the Chicago outlet to the Gulf of Mexico.