THE IMPACT OF GLACIATION ON NATURAL TRACER PROFILES IN THE STRATIGRAPHY OF THE EASTERN MICHIGAN BASIN

In this study we use numerical simulations of the δ¹⁸O and δ²H profiles collected during a previous study at a site located at the eastern edge of the Michigan Basin, to unravel the paleo-hydrogeology of the setting and the effects of glacial loading on the solute transport processes. The sedimentary sequence of dolostones, limestones and shales has a thickness of 860 m at this location. The sequence is bounded by a Cambrian aquifer at the bottom, 150 m of hydraulically conductive layers at the top, with two hydraulically conductive horizontal layers at depths of 180 and 320 m. The setting is characterized by anomalously low and high pressures in various formations as measured in several boreholes constructed to the Precambrian. To explore the origin and evolution of the resident fluids, we performed a series of pure-diffusion and advection-diffusion model simulations using a variety of boundary conditions for solute transport. The measured environmental heads were used as the driving force for advection, where substantial underpressure in the middle of the domain caused an upward and downward flow from the lower and upper boundaries respectively. The results show that under the flow gradients observed at the present time, advection cannot be neglected as a solute transport process. The advection term improves the fit of the model results to the measured natural isotope compositions primarily adjacent to the highly-permeable Cambrian aquifer and the discrete horizons in the Silurian formations.The simulations also substantiate the hypotheses that younger fluids likely driven by glacial pressures have invaded all of the permeable horizons over the last 2.5 Ma and in particular the upper formations during the last 100 ka. In the Cambrian aquifer the invasion (or residency) of this fluid has reversed and older fluid began migrating back towards the basin margin at 50 ka. Finally, the parameters measured on a small or local scale when used in large-scale models provide evolution times for natural isotopes profiles that are plausible in a paleo-hydrogeological context.