DEDOLOMITES ASSOCIATED WITH KARSTIFICATION - AN EXAMPLE OF EARLY DEDOLOMITIZATION IN LACUSTRINE SEQUENCES FROM THE TERTIARY MADRID BASIN, CENTRAL SPAIN

A variety of calcite fabrics formed by dedolomitization of middle to u pper Miocene continental sediments of the Madrid basin, central Spain The dedolomites are associated with other carbonates that show abundan t diagenetic features. The diagenetic carbonates (Diagenetic Carbonate Zone, DCZ) overlies and grades laterally into lacustrine dolomite (Lo wer Dolomite Unit, LDU) and evaporites (Evaporitic Unit, EU), and in t urn is capped by a paleokarst surface. The main dedolomite fabrics inc lude sutured calcites and radial-fibrous calcites, the latter consisti ng of pseudospherulite mosaics and fibrous crusts. Other subordinate d edolomite fabrics consist of micro to mesocrystalline mosaics of rhomb ic, occasionally zoned calcites as well as reworked pseudospherulite c rystals. These fabrics are interpreted to have been formed from extens ive calcitization of dolostones and associated evaporite facies, the p rocess being triggered and fostered by input of meteoric waters throug hout a karstic system. The origin of the radial-fibrous calcites is di scussed more specifically in view of their similarities to other calci te fabrics (e.g. Microcodium) often described in meteoric diagenetic e nvironments. Geochemical evidence indicates that dedolomitization occu rred in the shallow subsurface (< 40 m depth) and was achieved by oxid izing meteoric-derived ground waters. The diagenetic system behaved as an open system for nearly all trace elements analyzed; but in the sha llower zone (vadose zone) the system is partially close respect to str ontium. Vertical trends of the stable isotopes also support the propos ed paleokarst model characterized by an irregular shallow water table and a narrow vadose zone. The results obtained from the geochemical si mulation support that dedolomitization could develop by two combined h ydrographical patterns: authigenic recharge through limestones and all ogenic recharge. The dissolution/precipitation rates calculated for th e main minerals involved in the process are consistent with petrograph ic- and geochemical evidence.