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 pres ent day karst systems in carbonate platforms, yet are rarely reported in th e geological record. An example is described from the upper Danian platform limestones of the Alava province, in the western Pyrenees, north Spain. Th is 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 dolom itized and contain disseminated pyrite. These high-porosity zones are inter preted as having developed in marine mixing zones where mixing corrosion an d microbially mediated processes increased dissolution. If collapsed, ancie nt mixing zones could be misinterpreted as "terra-rossa" palaeosols. The ma in criteria to identify them as mixing zone products are their occurrence b elow a palaeo-meteoric phreatic zone, their association with stratified oxi c and anoxic redox zones and petrographic evidence for highly variable calc ite saturation states. (C) 2001 Elsevier Science B.V. All rights reserved.