Brecciated lithofacies in methane-derived carbonates in Middle-Late Miocene deposits (Northern Apennines, Italy).

More than one hundread occurrences of methane-derived carbonate rocks cropping out in Middle-Late Miocene satellite and foredeep basins of the northern Apennines (pelitic intervals of the Mt. Cervarola and Marnoso-arenacea Formations and slope/closure hemipelagites) have been investigated. These authigenic carbonate blocks are interpreted as fossil equivalents of modern carbonate crusts, slabs or chimneys precipitated at cold seeps.We analyzed stratigraphical, compositional and geochemical aspects of brecciated lithofacies that characterize large portions of carbonate bodies, in order to reconstruct different phases of fluid venting and precipitation of authigenic phases.Apennine seep carbonates include two main types of brecciation: monogenic breccias, made up of micritic angular clasts cemented by sparry calcite or aragonite, and polygenic breccias, made up of extrabasinal and intrabasinal clasts chaotically floating in the authigenic micritic matrix.Detailed petrographic study, including UV fluorescence microscopy and cathodoluminoscope, show different typologies and distribution of mineral components, microfacies and microstructures.Mineral components of carbonates blocks are made of authigenic phases and detrital components. Authigenic minerals include micro- to cryptocrystalline micrite and sparry calcite, aragonite, pyrite, quartz and feldspars. Detrital components consist of siliciclastic, silt-sand sized terrigenous particles, fragmented macrofossils and planktonic foraminifera. Ultra-violet fluorescence microscopy shows the presence of several hydrocarbon inclusions in sparry calcite.Various microfacies are frequently intertwined and chaotically mixed. Biomicrite, pelmicrite, clotted micrite and hybrid arenites are the most common. Clotted micrite consists of dark-brown to black, cloudy aggregates rimmed by calcitic spar. Common microstructures are: veins, occluded by multiple generations of carbonate cements and/or sediment grains, corrosion surfaces, burrows, geopetal fabrics and cavities, and preferential orientation of clastic and bioclastic particles.All carbonate phases reveal strongly negative carbon isotope values (ranging from -56,25 ‰ to -10,72 ‰ vs PDB) and reflect different rate of methane oxidation, as well as variability of bacterial communities which induce carbonate precipitation. Oxygen isotope values range from -9,71 3 to +3,55 ‰ vs PDB; the positive values are concentrated in polygenic breccias.On the base of isotopic and thermometric data, the genesis of polygenic breccia can be probably related to gas hydrates dissociation. In present-day continental margins, this dissociation can trigger a widespread and intense sediment instability. The frequent association of polygenic breccias in methane-derived carbonates and chaotic deposits, could be a further evidence of gas-hydrate dissociation and fluid expulsion, and may represent the fossil activity of pseudodiapirs and/or mud volcanoes.