Abstract
In the Miocene of the northern Apennines, a widespread carbonate precipitation was induced by the expulsion of methane-rich fluids. Numerous outcrops of carbonate masses share sedimentological, textural and geochemical features with present-day gas hydrate-associated carbonates. We hypothesize the contribution of paleo-gas hydrate destabilization on the base of the heavy oxygen isotope signature, the presence of distinctive sedimentary features (breccias, pervasive nonsystematic fractures, and soft sediment deformation), the close association between seep carbonates and sedimentary instability, and the huge dimensions of seep carbonates bearing brecciated structures.
Highlights
Authigenic 13C depleted carbonates have been described to form within or in proximity to gas hydrates, and gas hydrate-associated carbonates have been sampled from present-day settings in direct contact with the sea-floor or a few meters below it [1,2,3]
Authigenic seep carbonates in the northern Apennines are concentrated in Middle-Late Miocene pelitic successions in two different geological settings: in the foredeep and in Epiligurian wedge-top basins (Figures 1 and 2) [15, 37]
Following present-day analogues, the only means to infer a possible role of gas hydrates in fossil seep carbonates are (1) the heavy oxygen isotope signature, (2) the presence of distinctive sedimentary features linked to clathrites, (3) the close association between seep carbonates and sedimentary instability, and (4) the huge dimensions of seep carbonate deposition bearing brecciated structures
Summary
Authigenic 13C depleted carbonates have been described to form within or in proximity to gas hydrates, and gas hydrate-associated carbonates have been sampled from present-day settings in direct contact with the sea-floor or a few meters below it [1,2,3]. Brecciated structures are usually reported and restricted to large chemoherm complexes [1, 6, 7] It is well known [8] that gas hydrate stability depends on temperature, pressure, and availability of gas and water. Different geologic processes, such as sea-level drop, tectonic uplift, climate changes, and hot fluid rise, can induce the destabilization of gas hydrates.
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