Past and Present Permeability, Fluid Flow and Self Sealing in a Marl-limestone Sequence in Northern Switzerland

The Effingen Member in northern Switzerland, a potential host formation for the disposal of radioactive waste, has been studied in boreholes and outcrops, with the objective to describe its hydraulic, geochemical and fluid-flow characteristics. The hydraulic conductivity within the sequence of Oxfordian marls and limestones is generally very low, with the exception of a fractured limestone sequence that constitutes a potential lateral flow path. Calcite-celestite veins occur frequently and, in most cases, efficiently seal pre-existing fractures. Geochemical data, in particular 87Sr/86Sr ratios and δ18O values of carbonate, yield contrasting characteristics in veins and rock-matrix carbonate, leading to the conclusion that the veins are externally sourced and document open-system behaviour. Given the fact that even the micrite of the rock matrix was partially recrystallised during this stage fluid infiltration, it is concluded that at some time in the past the formation had a substantially higher permeability than today over an extended period of time and that since then, vein formation, in addition to clay swelling, resulted in an efficient self sealing of the formation, except in the most carbonate-rich beds.