Microbially-induced Fe and Mn oxides in condensed pelagic sediments (Middle-Upper Jurassic, Western Sicily)

Abstract

This article presents a petrographic comparison of the Rosso Ammonitico facies of Western Sicily and the original Rosso Ammonitico Veronese of Northern Italy based on a total of 27 sections. The Rosso Ammonitico has been the subject of numerous controversies that range from bathyal to shallow-water platform sedimentation. Therefore it seemed interesting to verify if the term Rosso Ammonitico has the same geologic connotation from region to region. The Middle-Upper Jurassic Rosso Ammonitico of Western Sicily is a condensed succession formed during a period of extensional synsedimentary tectonics related to the spreading of the Ionian Ocean. Slope-to-basin or pelagic carbonate deposits characterize the sedimentation which consists of reddish mudstones and wackestones. The abundant fauna is composed of radiolarians, protoglobigerinids, Saccocoma, Bositra associated with ammonites. A few ferruginous hardgrounds, Fe–Mn oxide crusts and Mn-coated condensation horizons are also present. The red matrices contain abundant Fe–Mn encrusted, microbored and bioeroded bioclasts. Sporadic Fe–Mn oncolites composed of amorphous Mn-minerals and goethite are also conspicuous. The matrix, as well as the shells and the fillings of the complex associated veinlets, are frequently altered into calcite microsparite. Submicronic iron bacterial and fungal filaments associated with mineralized extracellular polymeric substances (EPS) are observed in the matrix. They record dysaerobic microenvironments at or near the sediment–water interfaces. Early mineralized discontinuities enhanced by subsequent pressure dissolution are reported in the succession. Mn–(Ni) bacterial filaments are exceptionally observed in the cortex of the Fe–Mn oncolites. As a consequence of an early lithification, the Mn filaments are poorly preserved. The pigmentation of the rock is due to the dispersion of submicronic oxyhydroxides (now goethite and amorphous iron) formed by bacterial mediation during early diagenesis in microaerophilitic environments. As in the case of the original ‘Rosso Ammonitico Veronese’, Fe and Mn bacteria/fungi were able to produce bioconstructions which have no bathymetric significance. The limiting factor must have been the oxygen content which was low in these very quiet and relatively deep environments. Thus the Rosso Ammonitico of northern and southern Italy have a number of points in common, but some obvious dissimilarities are observed that explain some of the various sedimentological interpretations. Among them, the chemical composition is of particular importance, more Fe is present in Veneto (Northern Italy) while Mn is associated with the Sicilian Rosso Ammonitico. The Sicilian localities were more proximal to oceanic magmatic sources which were related to the activity of the oceanic crust.