Geological Processes and products related to Miocene cold seeps

Cold seeps are commonly associated with the presence of authigenic carbonates, due to interaction between hydrocarbons-rich fluids, geological processes (tectonics, sediment instability, chaotic bodies) and biological activity.In these environments hydrocarbon-rich fluid emissions support flourishing communities of mussel, clam bivalves, gastropods, tube worms and bacterial mats, which rely on chemosynthetic energy for their metabolism. The formation of authigenic carbonates at cold vents is closely related to the anaerobic oxidation of methane (AOM) which releases sulfide and bicarbonate into the pore water. Seep-carbonates have been reported worldwide both in modern and ancient sedimentary deposits. Fossil seep-carbonates provide unique archives of focused methane seepage in the geological record. by their geochemical, petrographic, mineralogical, isotopic and structural inventory. In the northern Apennines numerous outcrops of seep-carbonates are excellently exposed. 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 hydrocarbon-derived carbonates. They were recognized as products of the microbial oxidation of methane-rich fluids on the basis of their megafaunal assemblages and geological, compositional and isotopic attributes Our investigations have been carried out by means of detailed field observations of the northern Apennine seep-carbonates coupled with geochemical, petrographic, mineralogical and isotopic studies.In particular, this multidisciplinary approach permits an assessment of the pathways and origin of the fluids from which carbonates have precipitated (marine seawater source, marine organic matter source, thermogenic and biogenic methane); the final aim is the creation of a model for fossil seep-carbonate formation and development. The case study of the Salsomaggiore Ridge has been reported. This Ridge, a thrust-related anticline forming inside the middle Miocene foredeep at the outermost front of the Apenninic orogenic wedge, revealed a patchy distribution of mound- to pod-shaped seep-carbonates which, in few cases, preserve the deeper fluids feeding system consisting in a randomly arranged web of cm- to dm-sized tube conduits. The seep-carbonate facies and morphotypes are highly variable; either or not associated to typical macrofauna, they both encrust and cement siliciclastic turbidites or they inter-finger with hemipelagic marls. Seep-carbonates appear also brecciated with mottled and vuggy fabrics; a chaotic body reworks the seep-carbonates and other intra-basinal clasts. Stack of mound-shaped and bivalves-rich seep-carbonates are mainly located on the northern limb of the anticline while cementing and inter-fingering carbonates predominate on the southern limb of the same anticline. Northern and southern limbs differ also in the isotopic signature of the seep-carbonates which, overall attributable to methane oxidation, present markedly negative δ13C and positive δ18O only on the northern anticline limb. The above variability and differences are attributed to two different seepage modes which correspond also to evolutionary steps of the Salsomaggiore Ridge: a slow seepage mode that allows deposition of carbonate crusts, pods, mounds and seep-related fauna; a fast seepage mode that produce brecciated fabrics and chaotic bodies. The latter mode is considered coeval to tectonic pulses that generated the Ridge. By this integrated methodology applied to other cropping out examples in the Northern Apennines, we aim to reconstruct the complex mechanism of methane-rich fluid venting through the geological record, in particular in the Miocene