The evolution of the Miocene platform-basin system in the northern Apennines: what can we learn from seep-carbonates?

Cold seep-carbonates documenting the expulsion of fluids enriched in methane have been identified in a variety of basin margins and tectonic-sedimentary settings. A number of geological factors influences methane seep activity, but primarily regional and local tectonics, mainly at the deformation front of accretionary prism and inner foredeep. Tectonics constrains the fluid circulation system, with faults and fractures serving as conduits and channelling water and methane up to the seafloor. Recent studies suggest that seep-carbonates occurrence is also controlled by climatic changes and their formation seems to correlate with cold periods and sea level low-stand. A drop of the hydraulic pressure on the plumbing system during sea level lowering in glacial phase could increase methane flows at seeps, inducing carbonate precipitation. The correlation between methane-derived carbonates and climate has been recently suggested for the Miocene of the northern Apennines by means of a sedimentological and biostratigraphic study of seep-carbonates and the enclosing hemipelagic marls (Vicchio Formation) (Fontana et al., 2013). The study suggests a correlation between the carbonate precipitation and the middle Miocene glacial cooling event (Mi3b). The triggering of the ascent and emission of methane-rich fluids may be related to the eustatic fall and in turn to the water pressure drop. A detailed stratigraphic and biostratigraphic study of seep-carbonates and enclosing marls in foredeep deposits of the Tuscan-Romagna Apennines has allowed a precise dating of seepage in slope and basinal successions. The analysis of the δ18O and δ13C records of carbonates, δ13Corg excursion of organic matter and TOC on the marls enclosing the authigenic carbonates has been performed in order to verify a correlative trend in correspondence of the climatic cooling event. A paleoecological study on benthic foraminifera assemblages has also allowed to detail these peculiar environments. The events defined from this study in basinal deposits have been correlated with depositional changes and discontinuity surfaces in the adjacent temperate-type carbonate platforms, and have contributed to the definition of modes and rates of the demise of carbonate deposition. The identification of cold phases and lowering of sea level in slope-basinal deposits and their detailed timing, may be a useful tool for correlation between deep depositional setting and coeval shallow-water successions. This approach may also provide important constrains in the reconstruction of the evolution of the Miocene platform-basin system in this complex compressive setting of the northern Apennines. Fontana D., Conti S., Grillenzoni C., Mecozzi S., Petrucci F. & Turco E. 2013. Evidence of climatic control on hydrocarbon seepage in the Miocene of the northern Apennines: