PAIRED TUNNEL CHANNELS: ON THE ORIGIN OF A NEW VARIETY OF TUNNEL CHANNEL IN INDIANA

High resolution, digital elevation models of the southern margin of the Laurentide Ice Sheet (LIS) have revealed previously undescribed landforms throughout the Midwestern United States. A subset of tunnel channels herein termed paired tunnel channels (PTCs) have been identified throughout the Midwest, and this study looks at ones found in NE Indiana, NW Ohio, and SE Michigan between the Huron-Erie (HEL) and Saginaw Lobes of the LIS. PTCs are defined as a series of linear, parallel depressions separated by a continuous central ridge, and are oriented oblique to recessional moraines. Interestingly, PTCs crosscut recessional moraines of the HEL and have a surficial connection to traditional tunnel channels and proglacial spillways. The aim of this study is to describe the stratigraphy associated with PTCs and to determine their method of formation.

PTCs are found in two distinct stratigraphic and geomorphic regions separated by the Wabash-Erie Channel at Fort Wayne, Indiana. The northern region is composed of <10 m clayey till (Lagro Fm.) underlain by outwash of variable thickness (5–15 m) (Huntertown Fm.) and by >10 m loamy till (Trafalgar Fm.). Depth to bedrock (Antrim Shale) in the region is 75–120 m. PTCs in this region were studied with dipole-dipole electrical resistivity arrays, and many vertical structures believed to be blowouts from the Huntertown Fm. were found beneath surficial depressions. The southern region is composed of Lagro Fm. overlying limestone bedrock (Wabash Fm.) with numerous buried valleys including the Teays River Valley. Depth to bedrock is 10–20 m and is much deeper in the buried valleys. Passive seismic tomography was completed over a PTC, indicating it overlies a buried valley.

We propose that PTCs form as a result of over-pressurization of groundwater within a confined aquifer that is dramatically released to the bed of the LIS as blowouts. Initially, the blowouts are eroded and widened by meltwater, but as the meltwater supply drops off and water then stagnates, sediment partially fills the depressions. The position of the LIS and the stratigraphy control where the PTCs will occur and their geometry. This raises questions on the interaction between subglacial processes and the substrate in areas of complex stratigraphy, as well as what the recessional moraines in the region actually represent.