Volumetrically significant recharge of Pleistocene glacial meltwaters into epicratonic basins: Constraints imposed by solute mass balances

Abstract

Melting of kilometer-thick continental ice sheets, during Pleistocene glaciation, profoundly altered regional-scale groundwater flow in the low-lying stable interior of the North American craton. In this paper, we show that large volumes of glacial meltwater penetrated to great depths in underlying sedimentary basins through regionally extensive Silurian–Devonian carbonate aquifers, disrupting relatively stagnant saline fluids, and creating a strong disequilibrium pattern in fluid salinity. These dilute, isotopically light, meteoric waters migrated into overlying fractured, organic-rich Upper Devonian shales and significantly enhanced microbial methanogenesis, generating a unique class of natural gas deposits along the shallow basin margins. New data on formation water chemistry of Upper Devonian Antrim Shale gas wells along the northern and western margins of the Michigan Basin were integrated with previously published shale data and brines in subjacent Silurian–Devonian carbonates. This comprehensive database provides important constraints on fluid and solute transport, and makes a compelling case for the reorganization of drinking water resources and salinity structures along the shallow basin margins by Pleistocene glaciation. Na–Ca–Cl–Br relations and mass balance calculations were used to determine the relative volume of meteoric waters and sources of salinity in Antrim Shale fluids. The majority of shale formation waters contain greater than 50% meteoric water, with most containing over 80% meteoric water up to 300 m beneath the shale subcrop, despite the presence of a strong salinity gradient. Meteoric water recharge dissolved variable amounts of halite from evaporite-bearing Silurian–Devonian carbonates along the flow path into the shale and displaced highly saline NaCaCl remnant marine brines. The majority of Antrim Shale fluids owe greater than 60% of their salinity to halite dissolution, while less than 40% of Cl − is from mixing with brines in Silurian–Devonian strata. Oxygen and hydrogen isotope chemistry and Carbon-14 age dating indicate these NaCl brines were likely generated since the Late Pleistocene. Our conceptual model of fluid flow along the Michigan Basin margins and its role in generation of microbial gas were extrapolated to the larger glaciated Midcontinent region, where the Silurian–Devonian carbonate subcrop is continuous along the Illinois, Michigan, and Appalachian basin margins, and is overlain by Upper Devonian black shales. Major differences in the hydrostratigraphy and fluid salinity of the epicratonic basins controlled the extent of meteoric water invasion and generation of microbial methane. Increased hydraulic gradients from melting of the continental ice sheets greatly enhanced recharge of dilute waters into deep Silurian–Devonian aquifers and overlying fractured shales, reversing regional-scale groundwater flow and altering the major ion composition. These fluid migration events occurred over relatively short time scales (thousands of years), compared to well-documented basinal-scale fluid migration events driven by tectonics and sediment compaction (over millions of years). Understanding the hydrogeochemistry of the saline-meteoric water mixing zones and the sources of salinity in sedimentary basins are both important for constraining fluid and solute transport, chemical evolution of basinal fluids, and physical stability of brines during meteoric water invasion.