ORIGIN OF ESKER AND TUNNEL VALLEY ASSEMBLAGES IN THE SAGINAW LOBE, BARRY COUNTY, MI

A recent surficial mapping project funded through the USGS EDMAP program investigated northeast-southwest trending esker and tunnel channel pairs in the Saginaw Lobe of the Laurentide Ice Sheet, Barry County, MI. The length of the eskers in the area varies, with the most continuous reaching a length of up to 5 km. Eskers can have a significant effect on ice flow dynamics by providing conduits capable of transporting and discharging large volumes of subglacial meltwater. The relationship between eskers and tunnel valleys is not fully understood; however, the effect this relationship has on subglacial hydraulics makes them important in characterizing aquifers in parts of southwestern Michigan.

The mapping project served to locate important geomorphic features while analysis of a borehole and ground penetrating radar surveys provided subsurface data. A rotosonic borehole drilled at the crest of a esker and tunnel valley pair yielded coarse sand and gravel in tunnel valley deposits fining upward to interbedded fine sand and silt near the top of the esker. The stratigraphy suggests that the tunnel existed for long periods of time with flows decreasing until slowly moving or standing water remained. Ground penetrating radar revealed near surface bedding conformed to esker topography. The surveys contain small normal faults in addition to horst and graben features suggesting slumping on esker flanks or melt out of buried ice blocks.

Sandy diamictons interpreted as meltout till occur discontinuously in areas of hummocky topography. Landforms such as kames and kettles in close proximity to eskers situated within tunnel valleys indicate significant downwasting of the Saginaw Lobe in this area. The model developed for deglaciation of the Saginaw Lobe includes persisent but decreasing flow in subglacial tunnels during retreat of the ice margin. Tunnel channels, including those in this area, were probably formed time-transgressively in the marginal zone of the glacier. This corresponds to models which postulate the Saginaw Lobe was in state of stagnation and retreat as ice movement to the north began shifting to the Huron-Erie and Lake Michigan Lobes.