Fast flow of the Lake Michigan Lobe: evidence from sediment-landform assemblages in southwestern Michigan, USA

The dynamics of the late Wisconsin Lake Michigan Lobe of the Laurentide Ice Sheet, as interpreted by sediment–landform assemblages along its southeastern margin, are consistent with fast flow driven by high subglacial pore pressures. A major advance of the Lake Michigan Lobe terminated at the Kalamazoo Moraine, where proglacial glaciotectonic activity, marginal stagnation, and release of subglacial meltwater marked the extent of the advance. A large portion of the Valparaiso “Moraine”, west of the Kalamazoo Moraine, is actually a drumlinized, till-capped upland plain rather than an ice–marginal landform assemblage. Lacustrine sediment, underlying the surficial diamicton in the Valparaiso upland, is glaciotectonically deformed to a depth of 10 m. The deformed interval consists of folded sandy and silty lacustrine sediment, which is in places sharply truncated by the overlying diamicton. Decoupling of the glacier from its bed is suggested by a discontinous thin bed of sand at the base of the diamicton, and by the sharp truncation of deformed lacustrine sediment. The association of drumlins, proglacial and subglacial deformation, marginal stagnation, and major ice-marginal outwash-fan deposition are indicative of fast flow, perhaps in a surge. These characteristics are consistent with a terrestrial ice-stream model for the Lake Michigan Lobe.