Seep carbonate deposits with very negative δ13C values (chemoherms) are documented from various tectonic settings such as convergent plate boundaries, active transform and passive continental margins. In the middle Miocene foredeep basin of the northern Apennines, several pelitic intervals intercalated within turbidite sediments include carbonate bodies interpreted as chemoherms on the basis of isotopic data (δ13C as low as – 41.33 ‰ PDB) and paleontological evidences. These bodies are associated to features of intense sediment instability such as slumps, intraformational breccias and olistostromes. Sedimentological, mineralogical, chemical and isotopic studies where carried out on in-situ carbonate bodies from two of these pelitic intervals in order to reconstruct fluid sources and chemoherm genetical mechanisms and to document the connection between fluid seepage and syndepositional tectonics. Massive (micritic) carbonate authigenic phases of chemoherms consist of low Mg- and Sr-calcite and/or aragonite. Non stoichiometric dolomite and minor amounts of sulphur phases (sulphides and Ca sulphate) are concentrated in the brecciated lower portion of the bodies. Main carbonate phases from the muddy sediments which host the chemoherms consist of high Mg- and Sr-calcite with minor amounts of non-stoichiometric dolomite. The turbidite sediments (Te interval) differ from the pelites by containing significant amounts of stoichiometric (detrital) dolomite. Mineralogical analysis of the carbonate phases reveals that chemoherm bodies are richer in detrital quartz and in chlorite- smectite and lower in illite than the surrounding pelites. Differences are also observed in the distribution of some paleoenviromental indicator elements (Ni, Co, Cu, Zn), which indicate unstable oxidative conditions during or after chemoherm precipitation. Isotopic data (δ13C values as low as –20.31 ‰ PDB) of carbonates from pelites that surround the chemoherms indicate that the carbon is at least in part of biogenic origin and probably related to diffusion of CO2 and methane-rich fluids through the sea floor around the main vents where chemoherms grow. Mineralogical (enrichment in smectite and dolomite, Ca sulphate precipitation) and chemical (high chlorine content) features of some chemoherm bodies suggest that the aqueous fluids associated to the seeping gas were highly saline brines of problematic origin. Some features suggest genetical analogies between the chemoherm deposition in the Miocene foredeep basin and present sedimentary conditions where gas hydrates are generated. It is hypothesized that the pelitic intervals represent sedimentary episodes on top of ephemeral structural highs related to blind faults linked to the advancement of Northern Apennine deformational front. It is possible that the growth of structural highs was an effective mechanism for concentrating gas hydrates by gradually raising the base of their stability zone. Gas hydrates or similar fluids could have played an important role in triggering sediment instability and slumping along the margins of structural highs. The relation between methane seepage, sediment slumping and tectonics suggests a reinterpretation of many chaotic deposits and soft-sediment deformation features as a product of the episodic release of overpressured shallow methane accumulations in slope sediments.
Methane-related carbonates in the Miocene Apennine foredeep (Italy). Sedimentological, mineralogical and geochemical considerations / Conti, Stefano; Fontana, Daniela; Gubertini, A; Sighinolfi, Gp; Tateo, F.. - STAMPA. - (2003), pp. 60-60. (Intervento presentato al convegno 3rd Latinamerican Congress of Sedimentology tenutosi a Belèm-Parà. nel June 8-11).