Northern Apennines (Italy) host several outcrops of methane derived seep carbonates often associated with sedimentary instabilities such as intraformational slumps, extraformational slides and diapiric processes. Despite the vast body of literature on seep-deposits from the Monferrato to the Umbro-Tuscan sectors of the mountain chain, only a few studies focus on the interplay between paleomethane seepage and sedimentary instabilities (Conti and Fontana, 2002) because of the lack of good exposures. In this study we investigate an outcrop located in the Mugello area of the Tuscan Apennines (Prati Piani di Palazzuolo) characterized by the co-occurrence of methane derived authigenic carbonates, chemosynthetic fauna and slump-like structures. The examined outcrop consists of several carbonate blocks with max length of 3.5 m enclosed in a 100 m thick pelitic interval, interpreted as sedimentation above a structural high in the inner foredeep at the front of the Middle Miocene accretionary wedge. The blocks are mainly located at the base of the pelitic interval and have the same strike as the enclosing sediments. At various levels, pelitic sediments show soft sediment deformation structures (folds, slumps). Within the slumps, we sampled some concretions (silty-carbonate in composition). The δ13C value in the carbonates ranges between -18.2‰ and -33.22‰, confirming they are methane derived (AOM-related in origin). Pelites surrounding the carbonate blocks (in situ or transported for a short distance. Isotopic δ18O values are comprised between -4.31‰ and 0.87‰. The concretions are slightly depleted in 13C (-5.47‰; -10.95‰). Preliminary data suggest the presence of a seepage system active before the onset of the instability and able to sustain chemosynthetic communities at the seafloor. The interstitial overpressures might have reduced the shear strength of the fine-grained sediments and constituted an important preconditioning factor for diapiric or mass-transport structures. However, this conclusion is still speculative and needs further investigation.
MIOCENE SEDIMENTARY INSTABILITIES ASSOCIATED WITH METHANE DERIVED AUTHIGENIC CARBONATES: A NEW CASE STUDY FROM THE NORTHERN APENNINES (PALAZZUOLO OUTCROPS, ITALY) / Argentino, Claudio; Johnson, Joel; Conti, Stefano; Fioroni, Chiara; Fontana, Daniela. - (2017), pp. 48-48. (Intervento presentato al convegno International Meeting of Sedimentology 2017 tenutosi a Toulouse nel 10-12 October 2017).
MIOCENE SEDIMENTARY INSTABILITIES ASSOCIATED WITH METHANE DERIVED AUTHIGENIC CARBONATES: A NEW CASE STUDY FROM THE NORTHERN APENNINES (PALAZZUOLO OUTCROPS, ITALY)
CONTI, Stefano;FIORONI, Chiara;FONTANA, Daniela
2017
Abstract
Northern Apennines (Italy) host several outcrops of methane derived seep carbonates often associated with sedimentary instabilities such as intraformational slumps, extraformational slides and diapiric processes. Despite the vast body of literature on seep-deposits from the Monferrato to the Umbro-Tuscan sectors of the mountain chain, only a few studies focus on the interplay between paleomethane seepage and sedimentary instabilities (Conti and Fontana, 2002) because of the lack of good exposures. In this study we investigate an outcrop located in the Mugello area of the Tuscan Apennines (Prati Piani di Palazzuolo) characterized by the co-occurrence of methane derived authigenic carbonates, chemosynthetic fauna and slump-like structures. The examined outcrop consists of several carbonate blocks with max length of 3.5 m enclosed in a 100 m thick pelitic interval, interpreted as sedimentation above a structural high in the inner foredeep at the front of the Middle Miocene accretionary wedge. The blocks are mainly located at the base of the pelitic interval and have the same strike as the enclosing sediments. At various levels, pelitic sediments show soft sediment deformation structures (folds, slumps). Within the slumps, we sampled some concretions (silty-carbonate in composition). The δ13C value in the carbonates ranges between -18.2‰ and -33.22‰, confirming they are methane derived (AOM-related in origin). Pelites surrounding the carbonate blocks (in situ or transported for a short distance. Isotopic δ18O values are comprised between -4.31‰ and 0.87‰. The concretions are slightly depleted in 13C (-5.47‰; -10.95‰). Preliminary data suggest the presence of a seepage system active before the onset of the instability and able to sustain chemosynthetic communities at the seafloor. The interstitial overpressures might have reduced the shear strength of the fine-grained sediments and constituted an important preconditioning factor for diapiric or mass-transport structures. However, this conclusion is still speculative and needs further investigation.
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