Carbonate minerals in marine clay formations – a review of five European mudstones

A good knowledge of mineralogy, diagenetic evolution and tectonic, burial and thermal history is essential for the understanding and potential evaluation of a clay formation in the context of the disposal of radioactive waste. To inform the knowledge base, here we present a petrological review of five reduced marine clay formations that have been studied by national radioactive waste programmes in three European countries (Toarcian, Callovian–Oxfordian and Tégulines clays in France, Opalinus Clay in Switzerland and Boom Clay in Belgium), with a focus on carbonate minerals and their role on the clay permeability/porosity and pore water chemistry. Carbonates in marine clay formations include detrital bioclasts and different generations of diagenetic calcite (micrite, euhedral calcite, microsparite), dolomite and siderite that precipitated from early post-deposition to maximum depth of burial across a range of isotopically determined temperatures during diagenesis. Variations of carbonate content, distribution and chemistry from one formation to another and even through a single formation are highly dependent on the depositional and early post-depositional conditions (water depth, redox conditions). The presence of detrital clasts in clay rocks results in a porosity decrease. Precipitation of diagenetic carbonates contributes to block pores and leads to lower permeabilities. Carbonate chemistry during burial provides information about changing trends in pore water chemistry in sediments through geological time.