THE EFFECT OF TUNNEL CHANNELS ON LAURENTIDE ICE SHEET DYNAMICS

Tunnel channels were common subglacial drainage features along the southern margin of the Laurentide Ice Sheet. 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 resulted from sudden subglacial lake drainage eroding into subglacial material and that only one or two drainage events occurred for any particular channel. 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.

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 and depth of incision. Each seismic profile was about 1 km long with 2.5 m common midpoint spacing at nominally 12 folds. Passive seismic data were also collected to better constrain the dimensions of the tunnel channels. Numeric models were used to investigate their effect on subglacial mechanics.

Preliminary results indicate the studied Wisconsin tunnel channels incised approximately 60 m into subglacial material and were approximately 300 m wide. The average channel relief is six times greater than the corresponding surface relief. Estimated erosion rates from a Meyer-Peter and Müller sediment transport model suggest that a 2-m bank full channel flowing at 2.6 m/s (estimated from geologic data) could erode the GBL channels in 30 days. Diffusion modeling indicates the channel would have reduced the pore water pressure over a region ~7 times the width of the tunnel channel by an average ~410 kPa for an area of ~30 km² along the length of the tunnel channel. The corresponding effective pressure increase in the surrounding till would have provided a mean additional shear strength of 250 kPa over the affected area. The additional shear strength would have reduced till deformation in the vicinity of the tunnel channel and could have led to a restabilization of the region. The formation and rapid drainage of large subglacial lakes at the base of the Laurentide could be an analog for the modern-day subglacial lakes drainage observed beneath the West Antarctic Ice Sheet.