Morphological and mineralogical characterization of surficial weathering on calcarenite rocks in the rupestrian system of ?San Michele delle Grotte? at Gravina in Puglia (Bari, Apulia)

San Michele delle Grotte is a rupestrian cave excavated into the "Calcarenite di Gravina" formation, resting over the Cretaceous formation of "Calcare di Altamura". Both rock types show evident instability features and intense surficial rock weathering. A detailed mineralogical and morphologic characterization of Calcarenite di Gravina Fm. samples has been carried out by means of optical and petrographic microscope (OM and PM), scanning electron microscopy (SEM) and X-ray powder diffraction (XRPD). In thin section, bioclastic packstone to grainstone fabrics were observed. In the surficial portion of the rock it is possible to find biological colonizations and efflorescences: the first are dominated by algae covering the calcite substrate, whilst the efflorescences contain mainly sulfates such as syngenite, gypsum and arcanite, and other salts including niter and sylvine. A distinctive enhance of microporosity on the rock surface immediately at the contact with the biological colonizations and efflorescences was observed. A high abundance of biological colonizations was found where the hostrock is well exposed to light, whereas efflorescences prevail in the innermost parts of the cavity. Efflorescences may originate from both anthropogenic air pollution (wet and dry airborne deposition that provides sulfate and nitrate anions) and circulation of soil water whose composition can be modified, for instance, by fertilizers. Analysis of the weathering conditions of calcarenite rocks is a crucial factor in determining the likely possibility of failures and collapses in underground environments.