Silica diagenesis in Eocene shallow-water platform carbonates, southern Pyrenees JORDI GIME  NEZ-MONTSANT*, FRANCESC CALVET* and MAURICE E. TUCKER   *Departament de GeoquõÂmica, Petrologia i Prospeccio  Geolo Ágica, Facultat de Geologia, Universitat de Barcelona, 08071 Barcelona, Spain (E-mail: jordi@antartida.geo.ub.es)  Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK ABSTRACT Spectacularly developed lower Eocene chert in the Corones platform carbonates of the Spanish Pyrenees is concentrated within a restricted, brackish-water, laminated ostracod-rich facies, which also contains abundant sponge spicules. 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. d 18 O values of chert range from 29á6& to 30á9& (SMOW), which correspond to a broad isotope rank common for biogenic and diagenetic replacement cherts. Calcian dolomite crystals with high Fe and Na are disseminated within the microquartz and LF-chalcedony, but are absent from the megaquartz and host carbonate. The chert is closely associated with desiccation cracks and with interstratal dewatering structures. Load casts are silici®ed, and laminae rich in sponge spicules are convoluted. Early cracks related to dewatering are ®lled by microquartz and quartz cements. Ostracod shells within chert are locally fractured; those in the host carbonate are commonly ¯attened. Late fractures are ®lled by LF-chalcedony and megaquartz. There is much evidence for the dissolution of sponge spicules and their calcitization in the carbonate host rock. Silica for the Corones cherts was derived from sponges during early diagenesis and shallow burial. Early mechanical compaction and sediment dewatering played a major role in sponge spicule dissolution, migration of silica-rich ¯uids and the consequent precipitation of chert. Quartz cements continued to be precipitated into the burial environment. Keywords Chert, Eocene, limestone, Pyrenees, silica diagenesis, sponge spicules. INTRODUCTION Carbonate rocks commonly contain silici®ed fossils, burrow structures, evaporites, chert nod- ules and persistent beds composed of chert. The origin of chert in limestone has been much debated, and there are two major problems: the source of the silica and the timing of silici®cation. Cherts are exceptionally well developed in the carbonate rocks of an Eocene shallow-water platform in the Spanish Pyrenees. Of particular interest is the clear association of silici®cation with desiccation cracks, early interstratal dewa- tering cracks and structures and late fractures, which enables the timing of the silici®cation to be considered within the diagenetic history of the sediment. Dissolution and mobilization of silica in an early and shallow burial diagenetic stage resulted in the silici®cation of interstratal dewa- tering structures induced by shrinkage and pro- gressive physical compaction of the sediment (upward-injected dykes, cracks, convoluted spic- ule-rich laminae, etc.) similar to those described by Tanner (1998) in sandstones. In view of this, a model of interaction between silici®cation and other early diagenetic processes requires the Sedimentology (1999) 46, 969±984 Ó 1999 International Association of Sedimentologists 969