Glacial landsystems and dynamics of the Saginaw Lobe of the Laurentide Ice Sheet, Michigan, USA

摘要

The surficial terrain of the Saginaw Lobe in southern Michigan is divided into 4 landsystems, numbered sequentially from southwest to northeast, containing related assemblages of sediments and landforms, two of which appear to have a genetic relationship with the bedrock units over which they lie. Land-system 1 consists of the Sturgis Moraine, a terminal/recessional moraine, and an adjacent drumlin field. The moraine is a high-relief ridge with hummocky topography and ice-walled lake plains at its crest, and thick, coarse outwash fans on its distal margin. The drumlin field extends up-glacier from the Sturgis Moraine across the subcrop of the Mississippian Coldwater Shale to the subcrop of the overlying Marshall Sandstone. The low permeability of the shale may have increased basal pore pressures into a range in which deformation of basal sediment into drumlins occurred. Landsystem 2 overlies the subcrop of the Marshall Sandstone. Stagnation of the lobe behind a discontinuous moraine in this area is indicated by high relief, collapse topography, composed of kames, eskers, and ice-walled lake plains. A system of tunnel valleys terminates and merges in this landsystem with large outwash fans and plains sloping away from the margin. High basal drainage into the sandstone may have facilitated stagnation across a broad area to form this landsystem. Landsystem 3 may have also developed during stagnation of the lobe, but it differs from landsystem 2 by its more subdued relief. It contains a network of evenly spaced, southwest-trending open tunnel valleys that contain discontinuous esker segments. Landsystem 4 consists of till plains and low recessional moraines formed as the Saginaw Lobe retreated downslope into Saginaw Bay of the Lake Huron Basin. Tunnel valleys are absent in this area.