SUBGLACIAL DRAINAGE AND AQUIFER POTENTIAL: SAGINAW LOBE, MICHIGAN

The Saginaw Lobe of the Laurentide Ice Sheet contains an extensive network of channelized subglacial drainageways. Targeting of these systems for aquifer exploration should be a priority for water managers in the region.

The Saginaw Lobe advanced southwesterly from Saginaw Bay of Lake Huron on to an upland underlain by Paleozoic formations of the Michigan Basin. A network of relatively straight tunnel channels was cut beneath the ice in a radiating pattern mimicking glacial flow lines of the lobe. These channels, which extend to the distal margin of the lobe in northern Indiana, are typically eroded into bedrock and filled with coarse-grained glaciofluvial sediment. Eskers are common within the channels in some areas. During retreats and readvances of the Saginaw Lobe and lateral overriding by the Lake Michigan and Huron-Erie Lobes, the tunnel channels were filled to a variable degree by stagnant ice and sediment. The channels probably formed close to the margin and in a time-transgressive fashion during ice retreat after each advance.

Five genetic types have been recognized to date: (1) deep, unburied channels; (2) channels partially buried by moraines of Saginaw Lobe readvances; (3) channels partially to completely buried by Saginaw lobe deposits not located near an ice margin; (4) channels partially buried by Lake Michigan Lobe sediments; and (5) channels containing eskers.

Based on scant data from water well logs and test drilling, the channels appear to contain thick sections of sand and gravel. A rotasonic boring in a large esker yielded a fining upward section, capped by fine sand and silt, suggesting gradual formation during ice stagnation and retreat. If other eskers are similar, the tunnel channels would constitute better aquifers than the eskers. Further subsurface characterization is need to fully characterize these channels.