THE IMPACT OF ICE-SHEET MECHANICAL LOADING AND PERMAFROST DURING GLACIAL CYCLES ON FLUID FLOW PATTERNS IN INTER-CRATONIC SEDIMENTARY BASINS

Cycles of glaciation have occurred several times over the last millions of years. During these cycles inter-cratonic sedimentary basins in North America were repeatedly over-run by up to 4 kilometers of ice. Presently, the aquifers in these basins are an important source of fresh groundwater. Oxygen-isotope studies suggest that large volumes of glacial meltwater were emplaced around the time of the last glacial maximum. The hydrodynamics in sedimentary basins must have been dramatically different during glaciation. We set-up a suite of heuristic numerical models to describe the hydrodynamics in sedimentary basins during ice-sheet advance and retreat. Our models are transient and describe fluid and coupled heat flow. We considered the impact of polar (dry base) and temperate (wet base) ice-sheets on ground water hydrodynamics when compared to Holocene conditions. In this approach we could consider the effects of thawing and development of permafrost during ice-sheet advance and retreat on the rates of recharge below the ice sheet. Moreover, the model describes mechanical loading by the ice as a mechanism to generate excess-head and under-pressure in easily compressible units (e.g. shale) during ice-sheet advance and retreat respectively. The effects of mechanical loading are especially well illustrated in the model scenario of a dry-based ice sheet. We show that steady-state hydrodynamic conditions are probably never reached during one glacial cycle. Considering permafrost as a mechanism that decreases permeability near the surface, does not seem to significantly alter fluid flow patterns on a regional scale. The temperate ice sheet model shows how during glaciation, the direction of fluid flow in the basin becomes inverted and fluid flow velocities deep in the basin are up to an order of magnitude higher than during interglacial periods. The rates of sediment loading and un-loading due to advance and retreat of the ice are several orders of magnitude higher than those resulting from processes like sediment deposition and erosion. Because the unloading by the ice sheet is faster than the loading during advance, sub-hydrostatic hydraulic heads of several tens of meters in low-permeability units potentially could still be present thousands of years after the ice-sheet retreated from the basin.