DOLOMITIZATION CONTROLLED BY MICROBIALITES IN THE CARNIAN PATCH REEFS (DOLOMITES, ITALY)

The role of microbialites as causing factor of early dolomitization processes was underestimated. The organodolomitization, induced by the organic matter preserved in the autochthonous micrites, helps to explain the dolomitization processes of large carbonate volumes. It also suggests that particular attention is necessary in interpreting dolomitization as primary in the fossil record. Actually, organodiagenetic processes can induce dolomitization of Ca-microbialite with microfabric miming the primary texture. The recrystallized and dolomitized Carnian samples from Rifugio Vallandro and the well-preserved isochronous erratic boulders of Alpe di Specie (Dolomites, South Tyrol, Italy) offer a unique case study for investigating the relationships between microbialites and dolomitization processes (Guido et al., 2018). Rifugio Vallandro section contains at least three frame-builder horizons, in part constituted by deeply dolomitized boundstone, with corals in life position, sponges, chaetetids and stromatoporoids. Microbialites look like peloidal crusts with aphanitic fine texture, showing organic matter relics, similar to those of Alpe di Specie. Geomicrobiological characterization of Alpe di Specie microbialites indicates the presence of heterotrophic bacterial mediation in the deposition of autochthonous micrite inside the cryptic microcavities of the skeletal framework. The occurrence of Sulfate Reducing Bacteria biomarkers, the lack of specific molecules typical of cyanobacteria, and REE pattern are consistent with a biomineralization in suboxic conditions (Tosti et al., 2014). Similar processes can be hypothesized for the deposition of the primary carbonate matrix of Rifugio Vallandro patch reefs. Microbial extracellular polymeric substance (EPS) and other organic compounds, trapped inside the fine crystals of the peloidal and aphanitic micrite, may have been the cause of the dolomitization processes in the Rifugio Vallandro microbialites. Actually, high pH and high alkalinity, derived by the degradation of the organic matter, may have promoted the dolomitization processes because the high pH increases the concentration and activity of the dissolved CO32-, thereby increasing the dolomite supersaturation and reaction rates (Slaughter and Hill, 1991; Mazzullo, 2000). The degradation of organic matter causes early diagenesis of the microbialite, producing very finely crystalline dolomite (aphanodolomite). The study on the control of dolomitization processes from different primary micrites will facilitate understanding also the dolomitization of large bioconstructions.