A contribution to the reconstruction of Miocene seepage from authigenic carbonates of the northern Apennines (Italy)

Authigenic carbonates from outcrops of the northern Apennines consist of small and irregular lenses and exhibit numerous features indicative for cold-seep settings. Detailed petrographic, mineralogical and geochemical studies from two Miocene deposits are presented. The first carbonate outcrop named Fosso Riconi is located in the foredeep basin of the Apenninic chain whereas the second deposit represents a satellite basin called Sarsetta. The stable isotope data from specific carbonate minerals show a wide range of values well known from other paleoseeps of the Apennine Mountains. From all carbonates analyzed during this study the majority of seep carbonates are formed by low-Mg calcite and ankerite. Those minerals have δ13C values between -7 and -23‰ V-PDB suggesting variable amounts of carbonate derived from oxidized methane, from seawater (DIC), and from sedimentary organic matter. Dolomite samples have the lowest δ13C values (-30.8 to -39.0‰ V-PDB) indicating methane as the main carbon source.The results from this study indicate an evolutionary formation of the seeps and development of the authigenic carbonates, influenced by the activity of chemosynthetic organisms from which large lucinid clams are preserved. Bioirrigation of those clams controlled the sediment-water exchange, and is here considered as an explanation for the anomalous Mg content of the calcite. We hypothesize that seep-carbonates are formed during periods of active methane-rich seepage, whereas during periods of slow seepage carbonate formation was reduced. Despite different geological settings, the two examined deposits of Sarsetta and Fosso Riconi show similar features, suggesting that the pattern of fluid circulation at both seep sites played a major role in their carbonate formation.