Palaeo-mixing zone karst features from Palaeocene carbonates of north Spain: criteria for recognizing a potentially widespread but rarely documented diagenetic system

Marine-meteoric mixing zone dissolution effects are a major feature of present day karst systems in carbonate platforms, yet are rarely reported in the geological record. An example is described from the upper Danian platform limestones of the Alava province, in the western Pyrenees, north Spain. This consists of several narrow zones with sponge-like porosity analogous to the “Swiss-cheese” features found in present day mixing zones. These zones are stained by Fe-oxides and overlie limestones which are irregularly dolomitized and contain disseminated pyrite. These high-porosity zones are interpreted as having developed in marine mixing zones where mixing corrosion and microbially mediated processes increased dissolution. If collapsed, ancient mixing zones could be misinterpreted as “terra-rossa” palaeosols. The main criteria to identify them as mixing zone products are their occurrence below a palaeo-meteoric phreatic zone, their association with stratified oxic and anoxic redox zones and petrographic evidence for highly variable calcite saturation states.