Microbialite-Vermetid community (Salento Peninsula, southern Italy): a late Miocene example of automicrite deposition

A carbonate facies composed mainly of micrite and vermetid gastropods was recognized in the Late Miocene reef complex of the Salento Peninsula (southern Italy). The main purpose of this study is to give an account of the conditions which led to the deposition of this facies. Field, optical microscope, and epifluorescence observations, together with SEM, EDS, and FT-IR analyses have been carried out. Three types of micrite-rich microfacies have been recognized: (a) low fluorescent detritic micrite with a few fine bioclastic grains; (b) fluorescent mudstones/wackestone rich in fine bioclastic grains; (c) fluorescent thrombolitic peloidal micrite. The first type of micrite is mainly associated to the basal layers of the community, which is characterized by vermetids still in life position. The overlying deposits, which show the occurrence of horizontally leaning vermetid shells (isooriented vermetid facies), were interpreted as the product of redeposition of the basal assemblage, due to the overloading or other destabilizing processes. These sediments are constituted mainly by type (b) and type (c) micrites. The last one represents a typical organomicrite, most probably related to the presence of decaying organic matter. FT-IR analyses permit to distinguish two main types of organic source: 1) one probably deriving from metazoan and planktonic organisms, that characterize the mudstones/wackestones microfacies; 2) the other linked to metabolic activity of microorganisms that induced biomineralization of automicrite in confined spaces.