Geochemistry and palaeo-hydrogeology of the weathered zone in the Opalinus Clay

The effects of weathering and decompaction on the Opalinus Clay in northern Switzerland were studied on the basis of a 146 m long drillcore profile. Below 6 m of Quaternary sediments, the rock down to 16 m depth is affected by partial or complete dissolution of siderite, calcite, pyrite and organic matter, with goethite as the only identified product. The reduction level of Fe (Fe2+/Fetot) in the rock increases with depth from <20% to >80% in unweathered rock below 24 m. Porosity reaches 0.39 in the shallowest samples before dropping to normal values of around 0.13 below 30 m. Pore-water composition obtained by rock squeezing evolves with depth from fresh water of the Ca-SO4-HCO3 type to a 10-12 g/L Na-SO4-Cl type in unweathered rock below 40 m. Ground-water samples taken in the weathered zone indicate reducing conditions at present and therefore reflect a remaining redox buffering capacity even in the strongly oxidised rock. The interpretation of the rock properties, in conjunction with the chemical and isotopic signatures of ground and pore waters, suggests that substantial weathering occurred primarily during stages with a lower ground-water table in the Pleistocene, providing access to oxygen via gas diffusion across the partially unsaturated zone. Three evolutionary stages can be distinguished: 1) Long-term pyrite oxidation during Pleistocene surface exposure under dry climate and diffusive transport of SO(4)(2-)into the formation. This explains the higher SO(4)(2-)pore-water concentrations even in the unweathered rock at depth when compared to regional data from deep boreholes. 2) Build-up of the curved Cl- and water-isotope diffusion profiles since similar to 50-60 ka with a depth penetration of 40-80 m. 3) Holocene rise of the water table, leading to self sealing of fractures and the establishment of a diffusive regime in the weathered zone. Under saturated conditions, the depth penetration of rock oxidation is quite limited, which illustrates the substantial buffering capacity of the Opalinus Clay against external geochemical disturbances.

Automated summary

The effects of weathering and decompaction were studied on the Opalinus Clay in northern Switzerland using a drillcore profile. The rock below 16 m depth showed dissolution of certain minerals and organic matter and an increase in Fe reduction level. Porosity decreased with depth and pore-water composition evolved from fresh water to a Na-SO4-Cl type. The interpretation of rock properties and water analysis suggest substantial weathering occurred during Pleistocene stages with lower ground-water table.