IMPACT OF PLEISTOCENE GLACIATION ON FLUID AND SOLUTE TRANSPORT IN INTRACRATONIC SEDIMENTARY BASINS: INTEGRATIVE STUDY OF FORMATION WATER GEOCHEMISTRY AND NUMERICAL MODELING

Pleistocene melting of km-thick continental ice sheets significantly impacted regional-scale groundwater flow in the low-lying stable interior of the North American craton. Glacial meltwaters penetrated hundreds of meters into the underlying sedimentary basins, disrupting relatively stagnant saline fluids and creating a strong disequilibrium pattern in fluid salinity. Migration of meteoric waters into fractured organic-rich Devonian shales enhanced generation of economic reservoirs of microbial gas (methane). Major differences in the salinity structure of the Illinois, Michigan, and Appalachian Basins likely controlled the extent of glacial meltwater circulation and biogeochemical processes along the shallow basin margins. To better constrain the timing of meteoric recharge into the Michigan Basin and the impact of glaciation on fluid flow and solute transport, we integrated hydrologic modeling with isotope geochemistry of groundwater in the shallow and deep aquifer systems.

A transient 2-D finite element model of variable-density groundwater flow, heat and solute transport was constructed for the northern half of the Michigan Basin. Here, prolific drilling for hydrocarbons has provided important constraints on structure, fluid and rock properties. Salinity increases exponentially from less than 0.5 g/L TDS near the surface to greater than 350 g/L at depths ~800 m. The modeling results show that modern groundwater flow is primarily restricted to shallow glacial drift aquifers. During the Pleistocene, however, hydraulic loading of ice sheets reversed regional flow patterns, and focused recharge into Paleozoic aquifers. Dilute waters (salinity < 100 g/L) migrated ~200 km laterally into the Devonian carbonate aquifers, significantly depressing the freshwater-salinewater mixing zone. Radiocarbon ages and δ¹⁸O values of groundwater in Devonian carbonates and overlying shales are consistent with recharge beneath the Laurentide Ice Sheet (14 to 50 ka bp). These paleowaters are isolated from shallow flow systems in overlying drift aquifers. Constraining the paleohydrology of sedimentary basins has implications for residence times of potable water resources, generation and migration of hydrocarbons, and stability of basinal fluids during meteoric water invasion.