Onset of temperate carbonate sedimentation during transgression in a low-energy siliciclastic embayment (Pliocene of the Val d’Orcia Basin, Tuscany, Italy)

A Mid-Pliocene sequence in the sedimentary fill of the Val d’Orcia Basin (Tuscany, Italy) records coeval accumulation of temperate carbonate and siliciclastic deposits in close proximity. This study investigates the role played by local parameters in influencing the spatially discontinuous onset of carbonate sedimentation. Carbonate facies developed during transgression of an irregular coastline and were restricted to the more enclosed portion of an embayment. Coralline red algae were the main carbonate producers and are preserved in the skeletal association as isolated branches, rhodoliths, and incipient bindstone. When comparing the sedimentological attributes of the siliciclastic- and carbonate-dominated deposits, no significant differences are observed. They both appear to have accumulated in shallow-marine areas, subjected to general low-energy conditions punctuated by sporadic higher-energy events. Nevertheless, coastal morphology and bedrock composition at the basin margin created local heterogeneities that were sufficient to induce restricted spatial distribution of the carbonate-producing factory.