CONSTRAINTS ON AQUIFER OCCURRENCE IN TUNNEL CHANNELS, SAGINAW LOBE, MICHIGAN

Saginaw Lobe tunnel channels in Michigan are recognized by their linear orientation and NE-SW trend in the central part of the lobe. Previous work has shown that they have a complex origin involving collapse and burial of stagnant ice after subglacial formation. Burial to varying degrees occurred by later advances of both the Saginaw and Lake Michigan Lobes. The fortuitous siting of several large municipal well fields in tunnel channels, as well as test borings in several tunnel channel led to a hypothesis that tunnel channels contain thick, coarse-grained fills and are attractive targets for groundwater development. Two new rotasonic borings in tunnel channels in 2009 require reassessment of the potential of all of these buried valleys for production of groundwater. One boring was located in a broad zone of marginal stagnation traditionally known as the Kalamazoo Moraine. Kames, eskers, and ice-walled lake plains characterize the terrain with eskers common in tunnel channels. The stratigraphy of the boring consisted of ~ 120 ft of well sorted sand, fining upward to a few feet of bedded silt and clay, and overlain by a sequence of 86 ft of very coarse gravel. Bedrock was not reached by the boring. The upper unit is consistent with very high energy subglacial flow and favorable aquifer properties. The second boring, however, revealed a totally different stratigraphy. It was located up-glacier from the marginal stagnation zone, where the topography is more subdued and till is the dominant surficial material. This boring contained only about 15 ft of gravel near the top of the section, underlain by alternating basal till and lacustrine units presumably not related to the tunnel to bedrock at a depth of ~188 ft. Incision of the subglacial tunnel was minor, even though it has a very striking topographic expression. Deep domestic wells in the area to bedrock confirm the lack of coarse deposits associated with the tunnel channel. These borings suggest a different origin for tunnel channels in marginal vs. non-marginal areas and further suggest that aquifers may be restricted to channels formed in areas that may have had abundant subglacial meltwater activity.