POST LGM DYNAMICS AND BASAL DRAINAGE REGIME OF THE SAGINAW LOBE, SOUTH-CENTRAL MICHIGAN

The Saginaw lobe of the Saginaw-Huron Ice Stream re-advanced after the Late Glacial Maximum in southern Michigan, building a large terminal moraine, the Sturgis moraine, at its maximum extent. Retreat from this margin was active across a landscape that is characterized by drumlinized upland blocks bounded by south-southwest trending tunnel valleys and late-stage sluiceways that carried meltwater westward from the Huron-Erie lobe. Several discontinuous ice-marginal positions have been mapped in this area based on the presence of large, subaerial outwash fans heading at the ends of short, shallow tunnel valleys. Boulder gravels indicate high discharges in the tunnel-valley flows. Ice-marginal scarps are also present. Ice-marginal positions and uplands in general are associated with bedrock highs. About 75 rotosonic and geoprobe borings drilled while mapping Calhoun County indicate that the generalized stratigraphy consists of a basal diamicton overlying bedrock, an interval of sorted sediments ranging from sand and gravel to silt and clay, an upper, sandy diamicton that forms the drumlin cores, and discontinuous outwash at the surface and in tunnel valleys. A previous model for this area proposed that the drumlins and tunnel valleys were formed during a single, catastrophic subglacial sheetflood. Detailed mapping for this study suggests instead, that the tunnel valleys were formed in several episodes as the ice margin backwasted through the area. The basal drainage regime of the lobe would have been composed of a discrete conduit network, with abundant meltwater from surface and marginal melting and low overall basal pore water pressure. We suggest that the drumlins were formed during the re-advance, when basal drainage was distributed and basal pore pressure was high, thus facilitating till deposition and deformation to form drumlins.