GEOPHYSICAL INVESTIGATION OF TUNNEL CHANNELS

Tunnel channels were common subglacial drainage features along the southern margin of the Laurentide Ice Sheet during the Marine Isotope Stage 2. In Wisconsin, their locations are marked by a series of sinuous collapse surface depressions that extend ~15 km perpendicular to the margin. Geologic evidence indicates tunnel channels were carved into the surrounding geology through sudden subglacial lake drainage events. It is difficult to estimate the size of the tunnel channels using their modern surface expression because ice likely filled the subglacial channel and they were subsequently buried by ~100s meters of glacial sediment. Their collapsed surface is often the site of kettle lakes that may be hydraulically connected.

We conducted reflection/refraction seismic profile surveys across three tunnel channels located on the western margin of Green Bay Lobe (GBL), located ~6-7 km east of the former ice margin, to determine their size, depth of incision and the local subsurface stratigraphy. Each seismic profile was about 1 km long with 2.5 m common midpoint spacing at nominally 12 folds. Hammer blow and assisted weight drop sources were used and impacts were stacked ca. 5-fold to improve the signal to noise ratio. Passive seismic and geoprobe data were also collected to better constrain the dimensions of the tunnel channels and the seismic velocity model. We will present results from the most studied tunnel channel in central Wisconsin.

The geophysical survey result indicate that the studied tunnel channel incised approximately 60 m into the surrounding material and was approximately 450 m wide. The sediment surrounding the tunnel channel had a velocity of 1400-1500 m/s while the tunnel channel was filled with ca. 60 m of sediment with a slightly lower velocity, 1200-1300 m/s, indicating a different genesis for the sediments inside and outside of the tunnel channel. Atop the region was a 30-m layer of sand and gravel with a low velocity, 400-600 m/s. The average channel relief was six times greater than the corresponding surface relief. The high porosity sediments in the tunnel channels likely provide a pathway of high hydraulic conductivity, which warrants further study.