Sediment-landform assemblages in southern Michigan: Implications for basal processes of the Saginaw Lobe of the Laurentide ice sheet

Abstract

ABSTRACT Lobes, or ice streams, of the southern Laurentide ice sheet readvanced periodically during their overall retreat after the Last Glacial Maximum in the Great Lakes region. The Saginaw Lobe readvanced around 20 ka to form a prominent moraine, the Sturgis moraine, near the Michigan-Indiana border. Detailed mapping of nineteen 7½ min quadrangles at a scale of 1:24,000 in and adjacent to Calhoun County, Michigan, supports the interpretation that a large drumlin field behind the moraine was formed at this time, when the basal drainage of the glacier was distributed with high basal pore pressure. During retreat, after moraine construction, the drainage mode switched to a conduit-type system, in which meltwater drained to recessional ice-marginal positions through tunnel valleys. We mapped at least three discontinuous ice-marginal positions on the basis of coarse-grained, subaerial fans beginning at the ends of the tunnel valleys. There is a close association of kames with the tunnel valleys at these locations, suggesting that supraglacial meltwater contributed to the subglacial drainage. Our results support a model in which the drumlins were produced by deformation of the basal diamicton during ice advance prior to the formation of the tunnel valleys during ice retreat. This hypothesis rebuts a previously proposed model for this area in which the drumlins and tunnel valleys, along with boulder gravel deposits, were attributed to formation during a single, catastrophic subglacial sheetflood.