Hydrocarbon seepage during the Messinian salinity crisis in the Tertiary Piedmont Basin (NW Italy)

Seep carbonate deposits of Messinian age have been recently found in the Tertiary Piedmont Basin (NW Italy). These carbonates are preserved as blocks within a chaotic unit emplaced during the Messinian salinity crisis (MSC). They show negative δ13C values (from −27 to −15‰ VPDB) that indicate the involvement of hydrocarbon-rich fluids in their genesis. Three types of carbonates are recognised: (i) vuggy carbonates; (ii) Lucina carbonates; and (iii) tubeworm carbonates. Vuggy carbonates are characterised by carbonate pseudomorphs after gypsum and probably formed during the first stage of the MSC. They are the product of a complex diagenesis, influenced by both hypersalinity and seepage of hydrocarbon rich fluids. These rocks lack chemosymbiotic assemblages, reflecting their formation under extreme environmental conditions, inhospitable for most metazoans. In contrast, Lucina and tubeworm carbonates are characterised by chemosymbiotic macrofauna, represented respectively by Lucina bivalves and putative vestimentiferan tubeworms. The latter have not commonly been documented in ancient seep carbonates and have never been reported from the Messinian sediments of the Piedmont Basin. Both Lucina and tubeworm carbonates are interpreted as the product of hydrocarbon seepage during the second MSC stage. These two types of carbonates formed under less severe conditions than the vuggy carbonates, allowing the survival of seep-dwelling metazoans. During the second MSC stage, the seafloor was probably characterised by an irregular topography and a thin bottom layer of dense anoxic brines, produced by the dissolution of gypsum. It is suggested that vestimentiferan worms were able to thrive on morphologic highs with the posterior part of tubes just below the oxic– anoxic interface, but the anterior part projecting into oxic water. The infaunal Lucina bivalves were only able to live at seeps with an overlying oxic water column. The studied carbonate deposits show features reflecting the uncommon interaction of hydrocarbon-rich seep fluids and sulphate-enriched waters – the latter resulting from both evaporation and dissolution of gypsum – and allow to reconstruct the evolution of a seepage system during the MSC.