Anatomy of seep-carbonates: ancient examples from the Miocene of the northern Apennines (Italy)

Ancient hydrocarbon seep-carbonates of Miocene age have been reported from marine sedimentary successions of the northern Apennines in the form of irregularly shaped micritic masses containing abundant fossil remains of oligotypic assemblages. They are recognized by their peculiar biological, sedimentological and geochemical features, and represent an excellent on-land example of carbonate bodies interpreted as the remains of ancient cold seeps. In the Apenninic chain, the abundance and the extent of the outcrops provide a rare opportunity to study the geometry, facies distribution and internal structures of fossil methane-derived carbonates. The present study reports an integrated view of the sedimentology, mineral paragenesis and geochemistry of these seep-carbonates in two different tectono-sedimentary settings: foredeep and satellite basins. The main result of this study is the recognition of 17 facies in the investigated seep-carbonates. The detailed facies and microfacies analysis led to the identification of different structures, which occur in well-defined parts of the carbonate masses. The recurring and broadly similar stratigraphy and facies association from the two considered geological environments is diagnostic of the different stages of seep-carbonate formation and evolution.