Characterization of surficial weathering on calcarenite rocks in the rupestrian system of "San Michele delle Grotte" in 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 the “Calcare di Altamura”. The artificial cavity develops exactly at the contact between the two lithologies, with the first making the walls and vault, and the second being the pavement. Both formations are characterized by discontinuities and show evident instability features and surficial rock weathering. In particular, patinas and crusts extensively cover the walls, ceilings, and fractures inside the cavity. In the present study, a complete mineralogical and morphologic characterization of selected rock samples was carried out in order to assess the link between petrographic and mineralogical features of calcarenite and the weathering mechanisms and, specifically, to define the intensity of both the biotic and abiotic processes. Unaltered and weathered samples of Calcarenite di Gravina were analyzed through optical microscopy, scanning electron microscopy (SEM), and X-ray powder diffraction (XRPD). In thin section, we observed bioclastic packstone to grainstone fabrics, the samples containing microfossils as foraminifera, bivalve shells fragments, and bryozoans. Overall, the unaltered rock is composed of calcite, aragonite, ankerite, and subordinate quartz. In its surficial portion it is possible to find different types of alterations which can be divided into two main groups: group 1 encompasses rock samples with black to green biological coatings; group 2 includes salt-weathered rocks. Group 1 is dominated by algae constructive and destructive features, and from the mineralogical standpoint it contains calcite and minor quartz and gypsum; group 2 evidences efflorescence crusts (crusty white mineral salts) which are dominated by sulfates such as syngenite (K2Ca(SO4)2•2H2O), gypsum (CaSO4•2H2O) and arcanite (K2SO4), and other salts including niter (KNO3) and sylvine (KCl). Scanning electron microscope images allowed to observe a distinctive enhance of microporosity on the rock surface immediately at the contact with the biological structures and efflorescence crusts. The results of our investigation highlight a higher abundance of biological coating where the host rock is well exposed to light, as well as the occurrence of efflorescence crusts which can 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 fertilizing.