Impacts of Pleistocene glacial loading on abnormal pore‐water pressure in the eastern Michigan Basin

The hydromechanical effects of Pleistocene glacial loading on the Michigan Basin are assessed using numerical analysis based on coupled stress and pore‐water pressure. The two‐dimensional model domain included the Basin cross section and extended 10 km into the Precambrian. In the analysis, we considered the effects of the number of glacial loading cycles, the presence and connectedness of a deep Cambrian aquifer, the direction of glacial advance, the effect of a wet versus dry glacier/soil interface, topographic effects, density‐driven flow effects, and lithosphere flexure on the development of anomalous pressures. The modeling results were compared with data collected from highly instrumented wells completed in the eastern margin of the Basin. The present‐day results define regions of significant underpressure in the upper Ordovician and lower Silurian formations characterized by very low hydraulic conductivity and regions of overpressure where hydraulic conductivity is higher. To achieve the degree of underpressure observed in the instrumented wells using the model, a specific loading cycle applied over 100 000 years was repeated four times. As the number of loading cycles increased, the Paleozoic formations reached a state where the underpressures remain constant. Our results also illustrate the difference in poroelastic modeling between the application of mechanical loads on the land surface and the application of an equivalent hydraulic head, where the latter developed overpressures rather than the observed underpressures. The modeling also shows that the overpressures observed in the Cambrian formations are most likely to be the result of density‐driven flow. Finally, the simulations show that the effects of lithosphere flexure in the hydromechanical models results in the development of lateral stresses that generate overpressures rather than underpressures in the southern half of the domain. As there are no suitable observation points, these results remain unconfirmed, and further study is warranted.