Origin of Esker and Tunnel Valley Assemblages in the Saginaw Lobe, Barry County, Michigan
C. J. Woolever1, A. E. Kehew1, W. A. Sauck1, and A. L. Kozlowski2
1Department of Geosciences, Western Michigan University
2New York State Geologic Survey/New York State Museum
A 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. Eskers can have a significant effect on ice flow dynamics by providing conduits capable of transporting and discharging large volumes of subglacial meltwater.
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.
The model developed for deglaciation of the Saginaw Lobe includes persistent 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.
AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas