Abstract

Marine gas hydrates (clathrates) are ice-like solids composed predominantly of methane and water derived from methanogenic and thermogenic sources. Clathrates are readily destabilized by changes in pressure and/or temperature, which, in turn, are mainly due to tectonics, sea level change, sea-floor mass wasting and shallow geotherm raise (Trehu et alii, 1999; Kennett & Fackler-Adams, 2000). During clathrate dissociation, the resultant methane and water become trapped by impermeable stratigraphic intervals (marlstones, mudstones and siltstones), producing overpressurization and associated low shear strength in pelitic sediments, making them highly susceptible to soft sediment deformations, and triggering slumps, slope failures and sediment gravity flows (sediment instability). Methane-based chemosynthetic communities and authigenic methane-related carbonates (chemoherms) are frequently associated with pelitic sediments bearing gas hydrates, confirming a relationship between carbonate formation and gas hydrate decomposition (Paull et alii, 1995; Suess et alii, 1999). These carbonates show peculiar brecciated structures, irregular vuggy fabrics and dense networks of non-systematic fractures, induced by a process similar to hydro-fracturing of semi-consolidated sediment, due to ascending gas bubbles and the deposition of irregular gas hydrate layers (Bohrmann et alii, 1998). The occurrence of pelitic intervals enclosing chemoherms, and intraformational and extraformational slides within the middle Miocene foredeep turbidites (Marnoso-arenacea Formation) of the northern Apennines rises many questions about relationships between tectonics and sedimentation. In this paper, the classical gravitational hypothesis is integrated with a more complex syndepositional model. Pelitic intervals are probably linked to thrust activity; structural highs affected the middle Miocene sedimentation, with pelitic sediments overlapping structural reliefs, which could be related to blind thrusts. Thrust faults may have interrupted the gas hydrates stability zone, promoting hydrate destabilization, upward flow and entrapment of methane and water in impermeable pelitic sediments. Local inhomogeneity related to fractures contributed to seeping of fluids, random precipitation of authigenic carbonate, and chemosynthetic faunas. Development of fluid overpressures beneath pelitic structural reliefs may eventually undermine their stability causing deformation processes, sliding and slumping. Gas outburst is not only testified by chemoherm autobrecciated structures but also by the scraping off and mixing of the enclosing sediments (pelites and sandstones) during the rapid fluid rise along conduits or fractures (Conti & Fontana, 1999a, 1999b). Multiple phases of explosive breakage of micritic carbonates, slumping, sliding and reworking processes as debris flows or local turbiditic events are responsible of the chaoticization of wide portions of pelitic intervals. In this way, pelitic intervals with chemoherms reveal relationships between tectonics, cold seeps and sediment instability. In conclusion, pelitic intervals enclosing chemoherms are useful indicators of the advancing deformational front of the northern Apennines, thus heralding tectonic phases, as also demonstrated by the presence of extraformational slides of internal provenance (Berti et alii, 1994).

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