International Geologiical Congress - Oslo 2008


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CGG-02 Subglacial environments: Processes, sediments, landforms, modelling and experiments


Glacial landsystems of the Saginaw lobe, Michigan, USA


Alan E. Kehew, Western Michigan University (United States)
John M. Esch, Michigan Dept. of Environmental Quality (United States)
Andrew L. Kozlowski, New York State Museum/Geological Survey (United States)


The Saginaw Lobe of the Laurentide Ice Sheet flowed southwesterly and upslope out of the Lake Huron basin on to a rolling bedrock surface that makes up the Lower Peninsula of Michigan. Bedrock lithologies on this paleosurface include Late Paleozoic to Mesozoic carbonate, clastic, and evaporite units that dip gently toward the center of the Michigan structural basin. A series of low cuestas occur where more resistant beds subcrop beneath the glacial drift. Glacial landsystems formed by the Saginaw lobe during a re-advance several thousand years after the LGM overprint landsystems produced by earlier and more extensive LGM advances, which extended about 200 km south of the southern border of Michigan. The readvance terminated at a high-relief, hummocky marginal landsystem (Sturgis moraine) characterized by extensive glaciofluvial fans on its distal margins. Behind this margin is a drumlinized subglacial landsystem that is limited to the subcrop area of the Mississipian Coldwater Shale, where the drift is relatively thin, and where high subglacial pore pressures may have facilitated faster flow. The drumlins extend up-ice to a bedrock cuesta formed by the subcrop of the north-dipping Mississippian Marshall Sandstone. A supraglacial landsystem composed mainly of hummocky stagnation topography lies on the crest and dip slope of the cuesta, although a network of NE-SW trending tunnel channels forms a series of linear valleys across the upland. Eskers are common in the tunnel channels; a rotasonic boring in one reveals a fining-upward sequence from gravel to clayey silt, suggesting that the tunnels remained active for long periods of time during downwasting and/or stagnation of a broad margin. To the northeast the tunnel channels, some also containing eskers, become more distinct and form a radiating array parallel to flow lines of the lobe. The most proximal landsystem consists of low relief till plains with thin concentric marginal moraines. The distribution of landsystems suggests an active margin at the Sturgis moraine, active retreat from this terminal position, stagnation within a broad marginal zone on and near the Marshall cuesta, and finally, active retreat punctuated by intermittent moraine-building pauses or readvances, as the lobe retreated into the Lake Huron basin. The lateral margins of the Saginaw lobe were subsequently overridden by younger advances of the Lake Michigan and Huron-Erie lobes on the west and east, respectively. These advances terminated at the base of a broad, NE-SW trending bedrock upland that is parallel to the Marshall cuesta on the southeast margin of the Saginaw lobe.


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