IMPACT OF PLEISTOCENE GLACIATION ON THE HYDROGEOCHEMISTRY OF MID CONTINENT SEDIMENTARY BASINS

Basinal fluids and hydrocarbons have migrated significant distances in sedimentary basins over millennial timescales as a result of topography, tectonics, and density-driven flow. There is also an accumulating body of evidence for similar fluid migration events on much shorter timescales driven by the advance and retreat of kilometer-thick ice sheets across the mid continent of North America. Continental glaciation scoured and exposed bedrock along basin margins, and deposited glacial sediments of varying thicknesses and permeabilities. This intermittent exposure of bedrock aquifers also permitted the selective invasion of meltwaters to great depths in sedimentary basins, significantly altering basin-scale flow systems.

Integration of geochemical datasets with finite element modeling of variable density groundwater flow has enabled us to better constrain the impact of glaciation on fluid migration and salinity structure. Investigation of Devonian shale gas plays and adjacent regional aquifers has resulted in an areally-extensive database on fluid and gas geochemistry that makes a compelling case for reorganization of basin-scale flow systems in the Illinois and Michigan basins by Pleistocene glaciation. A 2D hydrologic model of the northern margin of the Michigan Basin has been constructed to simulate modern topographically-driven meteoric recharge and Pleistocene glacial meltwater recharge. Preliminary results show that under current hydrogeologic conditions freshwater preferentially penetrates Devonian carbonate aquifers and locally suppresses salinity. Increased hydraulic heads during ice sheet advance should significantly increase recharge, groundwater flow, and depth of penetration of freshwater.

Modeling the impact of glaciation on the hydrogeochemistry of basinal brines has important implications for understanding the forces involved in generating large changes in salinity profiles at basin margins. For example, deep meltwater penetration into fractured, organic-rich Devonian shales has produced economic deposits of microbial gas along the margins of the Michigan and Illinois basins. Large salinity differences between the two basins likely controlled the infiltration depth of meltwater, which was significantly deeper in the Illinois than the Michigan basin.