THE IMPORTANCE OF GLACIOFLUVIAL FEATURES WITHIN MICHIGAN PORT HURON MORAINE

Evidence based on topographic quadrangles, aerial photographs, soils maps, and extensive fieldwork indicates that the Inner and Outer Port Huron moraines in the northwestern part of Michigan's Southern Peninsula represent a mega-assemblage of related glaciofluvial formations marking successive marginally stagnant glacial termini. Surficial sediments consist of ice-contact stratified drift and proglacial sand and gravel. Diamictons are very limited in extent and usually occur in the form of flow till. Uppermost sediments within the intervening Mancelona Plain show a transition from coarse, poorly sorted proximal deposits dominated by longitudinal bars to distal, fine-textured, well-sorted braided-stream deposits displaying sandy bedforms. Paleocurrent indicators show that meltwater streams first flowed directly away from the ice margin and then turned 90 degrees to the right (southwest) to flow parallel to the sandur's trend. Based on these data, we propose that what were previously mapped as the Inner and Outer Port Huron moraines are actually the collapsed upstream portions of the Mancelona and Outer Port Huron outwash aprons, respectively. Although till is present in the subsurface, well-log data and exposures reveal that waterlain deposits predominate in the upper 50 m of the stratigraphic column. The large volumes of superglacial fluvial drift and apparent absence of well-integrated subglacial drainage features are consistent with a glacier exhibiting a frozen-bed margin. Small disintegration ridges associated with major heads of outwash probably record the transition to warmer conditions with final deglaciation. Such relationships agree with recent interpretations of glacial dynamics in the northern Midwest and provide insights into glacial and sedimentological processes associated with a conspicuous and chronologically important moraine centrally located along the southern margin of the Laurentide Ice Sheet.