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 upper Miocene continental sediments of the Madrid basin, central Spain The dedolomites are associated with other carbonates that show abundant diagenetic features. The diagenetic carbonates (Diagenetic Carbonate Zone, DCZ) overlies and grades laterally into lacustrine dolomite (Lower Dolomite Unit, LDU) and evaporites (Evaporitic Unit, EU), and in turn is capped by a paleokarst surface. The main dedolomite fabrics include sutured calcites and radial-fibrous calcites, the latter consisting of pseudospherulite mosaics and fibrous crusts. Other subordinate dedolomite fabrics consist of micro to mesocrystalline mosaics of rhombic, occasionally zoned calcites as well as reworked pseudospherulite crystals. These fabrics are interpreted to have been formed from extensive calcitization of dolostones and associated evaporite facies, the process being triggered and fostered by input of meteoric waters throughout a karstic system. The origin of the radial-fibrous calcites is discussed more specifically in view of their similarities to other calcite fabrics (e.g. Microcodium) often described in meteoric diagenetic environments. Geochemical evidence indicates that dedolomitization occurred in the shallow subsurface (< 40 m depth) and was achieved by oxidizing meteoric-derived ground waters. The diagenetic system behaved as an open system for nearly all trace elements analyzed; but in the shallower zone (vadose zone) the system is partially close respect to strontium. Vertical trends of the stable isotopes also support the proposed paleokarst model characterized by an irregular shallow water table and a narrow vadose zone. The results obtained from the geochemical simulation support that dedolomitization could develop by two combined hydrographical patterns: authigenic recharge through limestones and allogenic recharge. The dissolution/precipitation rates calculated for the main minerals involved in the process are consistent with petrographic- and geochemical evidence.