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.

ABSTRACT Glaciotectonic deformation of glacigenic deposits in southwestern Michigan is described and analyzed to determine the source of stress of these strained sediments, which manifests as overturned folds and other deformation similar to shallow crustal décollements. The succession is exposed in 11 aggregate mining operations along the Valparaiso Upland, in portions of Berrien, Van Buren, and Allegan Counties in southwest Michigan. Observed deformation includes a complex array of folds, faults, and thrust features as much as 5 m below the surface exposure of the pit face, consistent with horizontal compressional stresses that were generally aligned with ice flow. Fabric measurement of elongated clasts in the surficial till indicates ice flow from northwest to southeast across the area and parallel to drumlins in the area. Stratigraphically, the area is dominated by fine, lacustrine deposits with coarse sand and gravel capped by the Saugatuck Till during the last glaciation. Sediment grain size, pore-water pressure fluctuations, and topographic relief are interpreted to be responsible for the deformation observed as the Lake Michigan Lobe overrode a proglacial lake basin, including fans and deltas, as it advanced eastward to the Kalamazoo moraine. The fine texture and fabric of the lacustrine sediment package restricted the flow of subglacial water and caused abrupt local increases of pore-water pressure and concomitant coupling and decoupling of the bed-substrate interface. Advancing ice deformed sediments in two stages: (1) proglacially along a décollement at the ice margin, and then (2) subglacially as ice overrode the sediments.

ABSTRACT Recent mapping in southwestern Michigan conducted through U.S. Geological Survey STATEMAP, EDMAP, and Great Lakes Geologic Mapping Coalition projects has produced new interpretations of the origin of the landforms and sediments of the Lake Michigan and Saginaw lobes of the Laurentide Ice Sheet and the dynamics of these lobes. The Lake Michigan lobe advanced southeastward into a proglacial lake at least as far east as the Kalamazoo moraine. During its advance, the lobe extensively deformed the lacustrine sediments it overrode. These structures will be discussed in several pits. When ice backed away from the Kalamazoo moraine, it formed a series of proglacial lakes, several of which were described for the first time in the studies upon which this guidebook is based. As the ice retreated, lowland areas between morainal uplands were utilized by meltwater drainage events, some of them probably catastrophic in nature. The Saginaw lobe stagnated over a broad marginal area as it retreated northeastward toward Saginaw Bay. The resulting stagnant marginal zone is coincident with the subcrop of the Marshall Sandstone. Enhanced basal drainage into the underlying sandstone may have played a role in the dynamics of the lobe. High-relief, supraglacial landforms such as hummocky topography and ice-walled lake plains overprint subglacial landforms in this region, which include large tunnel valleys with inset eskers. Better understanding of the glacial geology of this region is critical to economic development, management of water resources, and exploration for aggregates and other resources.