Silica diagenesis in Eocene shallow-water platform carbonates, southern Pyrenees

Spectacularly developed lower Eocene chert in the Corones platform carbonat es of the Spanish Pyrenees is concentrated within a restricted, brackish-wa ter, laminated ostracod-rich facies, which also contains abundant sponge sp icules. The chert occurs as nodular, bedded and mottled varieties, and four petrographic types of quartz are developed: microquartz; length-fast (LF) chalcedony; megaquartz; and microspheres. delta(18)O values of chert range from 29.6 parts per thousand, to 30.9 parts per thousand, (SMOW), which cor respond to a broad isotope rank common for biogenic and diagenetic replacem ent cherts. Calcian dolomite crystals with high Fe and Na are disseminated within the microquartz and LF-chalcedony, but are absent from the megaquart z and host carbonate. The chert is closely associated with desiccation crac ks and with interstratal dewatering structures. Load casts are silicified, and laminae rich in sponge spicules are convoluted. Early cracks related to dewatering are filled by microquartz and quartz cements. Ostracod shells w ithin chert are locally fractured; those in the host carbonate are commonly flattened. Late fractures are filled by LF-chalcedony and megaquartz. Ther e is much evidence for the dissolution of sponge spicules and their calciti zation in the carbonate host rock. Silica for the Corones cherts was derive d from sponges during early diagenesis and shallow burial. Early mechanical compaction and sediment dewatering played a major role in sponge spicule d issolution, migration of silica-rich fluids and the consequent precipitatio n of chert. Quartz cements continued to be precipitated into the burial env ironment.