Impacts of Pleistocene glaciation on large-scale groundwater flow and salinity in the Michigan Basin

Pleistocene melting of kilometer-thick continental ice sheets significantly impacted regional-scale groundwater flow in the low-lying stable interiors of the North American and Eurasian cratons. Glacial meltwaters penetrated hundreds of meters into the underlying sedimentary basins and fractured crystalline bedrock, disrupting relatively stagnant saline fluids and creating a strong disequilibrium pattern in fluid salinity. To constrain the impact of continental glaciation on variable density fluid flow, heat and solute transport in the Michigan Basin, we constructed a transient two-dimensional finite-element model of the northern half of the basin and imposed modern versus Pleistocene recharge conditions. The sag-type basin contains up to approximately 5km of Paleozoic strata (carbonates, siliciclastics, and bedded evaporites) overlain by a thick veneer (up to 300m) of glacial deposits. Formation water salinity increases exponentially from <0.5gl^-1 total dissolved solids (TDS) near the surface to >350gl^-1 TDS at over 800m depth. Model simulations show that modern groundwater flow is primarily restricted to shallow glacial drift aquifers with discharge to the Great Lakes. During the Pleistocene, however, high hydraulic heads from melting of the Laurentide Ice Sheet reversed regional flow patterns and focused recharge into Paleozoic carbonate and siliciclastic aquifers. Dilute waters (<20gl^-1 TDS) migrated approximately 110km laterally into the Devonian carbonate aquifers, significantly depressing the freshwater-saline water mixing zones. These results are consistent with ¹⁴C ages and oxygen isotope values of confined groundwaters in Devonian carbonates along the basin margin, which reflect past recharge beneath the Laurentide Ice Sheet (14-50ka). Constraining the paleohydrology of glaciated sedimentary basins, such as the Michigan Basin, is important for determining the source and residence times of groundwater resources, in addition to resolving geologic forces responsible for basinal-scale fluid and solute migration.