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Systematic variation in the clay-mineral composition of till sheets; Evidence for the Erie Interstade in the Lake Michigan basin

G. William Monaghan
G. William Monaghan
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January 01, 1990

X-ray diffraction analyses of till samples collected from multi-till exposures along the Lake Michigan shoreline in Michigan and Wisconsin show a systematic variation in the clay-mineral composition of successive till sheets. A general increase in the relative amount of 10Â clay occurs within a succession of till sheets, beginning with early or middle Wisconsinan Glenn Shores till and continuing through late Wisconsinan (Woodfordian) Ganges–New Berlin till and Saugatuck–Oak Creek till. A significant decrease in the relative amount of 10Â clay, however, occurs within the post–Mackinaw Interstade (late Woodfordian) Ozaukee-Haven and Two Rivers tills. These changes in clay-mineral composition are apparently related to major ice-margin fluctuations since a significant retreat of the Lake Michigan Lobe has been recognized between deposition of each of the above till sheets.

Morphostratigraphic correlation of the Powell Moraine of the Erie Lobe with moraines of the Saginaw and Lake Michigan Lobes, as well as correlation between till units of the Saginaw and Lake Michigan Lobes, indicates that the retreat recorded between deposition of Ganges–New Berlin and Saugatuck–Oak Creek tills of the Lake Michigan Lobe is correlative with the Erie Interstade.

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