SAGINAW LOBE TUNNEL VALLEYS, SOUTH-CENTRAL MICHIGAN: DEPTHS OF SUBGLACIAL INCISION AND SEDIMENT FILL

Saginaw Lobe terrain in south-central Michigan includes of a set of open to partially buried NE-SW trending tunnel valleys that terminate within a broad tract of high relief hummocky topography interpreted as a stagnant marginal zone. The valleys have undulatory long profiles, contain eskers, and trend parallel to the inferred hydraulic gradient of subglacial meltwater.

Over the past several years, 6 deep, continuously cored, rotasonic borings have been drilled within and near these valleys in conjunction with mapping projects funded by the Great Lakes Geologic Mapping Coalition. Subsurface data of this type are necessary to constrain hypotheses used to determine the origin of the valleys, as well as to investigate their aquifer potential.

Based upon the drilling conducted to date, the depth of incision of the valleys and the thickness of sediment fill appear to increase in the down-ice direction. Tunnel valleys north of the E-W trending Thornapple River valley are shallow, open, and contain only several meters of coarse sediment. By contrast, valleys south of the Thornapple are incised to a depth of 60 m or more, and are filled to a greater degree with extremely coarse gravels. A comprehensive genetic model for the valleys remains elusive, but their origin could include erosive, subglacial bursts of meltwater followed by deepening and filling in the stagnant marginal area by meltwater derived from surface melting. The presence of eskers within the valleys requires ice creep into the subglacial conduits, decreasing their size, so that final sediment deposition to form the eskers occurred within smaller conduits during ice stagnation.