Abstract:
Fast ice flow and unstable ice sheet behavior were characteristic features of
the Lake Michigan Lobe of the southern Laurentide Ice Sheet. Such behavior may
result from some combination of subglacial-sediment deformation and decoupled
sliding at the ice-bed interface. Both mechanisms depend on high water pressure
relative to ice pressure. Using the finite-difference groundwater modeling package
MODFLOW we simulate groundwater flow along a 1,040 km flowline, extending
from the south shore of Lake Superior to the Mississippi River near Carbondale,
Illinois. Model simulations indicate that subglacial aquifers were not capable of
evacuating the estimated basal meltwater. A basal drainage system consisting of a
distributed film or canal system, similar to systems hypothesized as underlying Ice
Stream B, West Antarctic Ice Sheet, would transmit sufficient water to prevent
basal water pressure from exceeding the ice overburden pressure. The buried
Mahomet bedrock valley system may have drained enough subglacial meltwater to
stop the advance of the Lake Michigan Lobe. Simulations also suggest that
groundwater flow directions and velocities were substantially different than modern
conditions.
