Mid-latitude microbial-mediated modern beachrock formation (Santa Maria di Ricadi- Southern Italy)

The modern beachrock of Santa Maria di Ricadi bay is composed of coarse sandsized quartz and feldspar grains colonised by a living complex microbial biofilm. Grains are cemented by high-Mg Ca-carbonate characterised by microbial aphanitic and peloidal micritic fabrics. Aphanitic micrite commonly forms isopachous rims, up to 30 μm in thickness, enveloping all the surface of the clasts, whereas peloidal micrite fills either partially or totally voids among clasts. At the nanoscale both micrites are characterised by a tight aggregation of nano-spheres, 80-200 nm in diameter, forming irregular to globular major-size structures typical of microbial-induced bio-mineralization processes. However, in some cases, micritic rims show a gradual transition to a clear isopachous micro-sparitic calcite cement, implying a possible switch to abiotic precipitation in a marine phreatic environment.