PhD student: Claudio Argentino. Tutors: Prof.ssa Daniela Fontana, Prof. Stefano Conti. Introduction. Seep-carbonates derive from the anaerobic oxidation of ...
Authigenic carbonates as tracers of methane-rich fluids emission into the seafloor: fossil examples (Miocene, northern Apennines) compared to modern analogues PhD student: Claudio Argentino Tutors: Prof.ssa Daniela Fontana, Prof. Stefano Conti Introduction Seep-carbonates derive from the anaerobic oxidation of methane (AOM) performed by microbial consortium of oxidizing archaea and sulphate-reducing bacteria (Boetius & Wenzhöfer, 2013). These authigenic carbonates are typically 13C depleted and can form within or in proximity to gas hydrates. Gas hydrates occur at shallow depth in the sediment or at the seafloor when there is an adequate supply of water and methane at relatively low temperatures and high pressures (Paull & Dillon, 2001) and their dissociation may trigger submarine slope failures at the continental shelf representing a risk to coastal areas (tsunamis) and to seabed infrastructures and boreholes (Maslin et al., 2010). Miocene seep-carbonates of northern Apennines represent a complete archive of different seepage systems and show peculiar structures indicative of possible relationships with gas hydrates (Conti et al., 2011) so they offer a unique opportunity to investigate the spatial and temporal evolution of these environments. The outcrops of Moggiona, Gattaia and Poggio delle Campane (Tuscan Apennines) were studied through field activity (facies analysis and geological prospection), microfacies, SEM and XRDP. Methods Field work consisted primarily of mapping the distribution and geometry of seep carbonates, coupled with an accurate facies analysis. Petrographic study for microfacies identification was performed using standard optical microscopy. SEM analyses were carried out using a FEI NOVA NANO SEM 450 at Centro Interdipartimentale Grandi Strumenti (C.I.G.S.) of the University of Modena and Reggio Emilia. Semi-quantitative spot analysis where performed with X-EDS Bruker QUANTAX-200 system. The material was observed in secondary electron (SE) and back-scattered electrons (EBSD) at the acceleration voltage of 5-15 kV. Samples were coated with carbon for highresolution SE recording and with EBSD analyses. XRDP was carried out on micritic samples, veins and enclosing marls. Each sample was homogenized in agate mortar by hand grinding and mounted on an Al holder using the side loading technique. Data sets were collected with a Panalytical X'Pert PRO diffractometer in the angular range 3-80° 2θ using a step scan of 0,0167° 2θ and 50 s/step.
Results Moggiona Geological setting and previous isotope data: the Moggiona outcrop is enclosed in the Vicchio Marls near the boundary with the turbidite Falterona Fm. Based on nannofossil associations of the enclosing marls, the age of the deposits is attributable to the Burdigalian. Carbonates are depleted in δ13C ranging from -40.22 to -19.28‰ V-PDB, while δ18O vary between -9,90 ‰ and +0,70 ‰ VPDB (Conti et al., 2010). - Geological prospection: the outcrop consists of 7 carbonate bodies ranging in dimensions from few meters to 80 m and thickness up to 7-8 m. Deposits have lenticular to stratiform morphologies extending along strike (E-W direction) with subvertical attitudes concordant with enclosing marls. They follow the regional structural trend and they are aligned parallel to an overthrust that superimposes the Falterona Fm. above the Vicchio Marls. - Facies analysis: all the carbonate bodies are pervaded by an irregular array of tube-like conduits with diameters up to 3 cm and several cm long mainly filled by mottled micrite or biogenic debris (coquina debris). Conduits are generally concentrated at the base of the bodies but in some cases they reach the top. The macrofossil content is scarce and represented by local concentrations of articulated lucinid clams of relatively small dimensions (3-5 cm) and in life position; rare gastropods were found. - Thin sections: carbonates are composed of dark-brown micrite enclosing quartz, k- feldspars and rare muscovite. Clotted micrite is a common feature, and some beggiatoa-like filaments were observed. Samples are cross-cutted by veins and conduits associated to fragmented foraminifera tests and mollusk shells. Framboidal pyrite is common. Gattaia Geological setting and previous isotope data: seep-carbonates are included in the basal portion of the Marnoso-arenacea Fm., in a pelitic horizon made up of fine-grained turbidites and hemipelagites. Preliminary biostratigraphic data based on foraminifera indicate a Langhian age. Carbonates are depleted in δ13C ranging from -34,28‰ to -27,65‰ V-PDB, while δ18O vary between -0,92‰ and 0,21‰ V-PDB (Tamborrino, 2010). - Geological prospection: the outcrop represent one of the wider type 1 seep-deposits ever reported in literature. Four lenticular decametric to heptometric carbonate bodies were described. The larger
one is 206 m long with mean thickness of 10 m. The general bedding attitude is subvertical striking 330° and follow the regional structural trend. The contact with the enclosing pelite is transitional and pinch-out lateral terminations were observed. Small-scale slump features within the pelites at the top of the main carbonate body indicate interactions between fluid flow and synsedimentary deformation. - Facies analysis: a detailed facies analysis was conducted on the main carbonate deposit. Conduits and veins are common both at the base and at the top with a random orientation; their diameter reaches 2 cm with a length up to few meters. They are associated to monogenic/polygenic intraformational breccias. Lucinid concentrations are observed in the middle part of the body and at the top; they are always disarticulated and parallel to the body surface indicating shells reworking by rising fluids. The southern part of the deposit show a common laminated feature. - Thin sections: Carbonates are constituted by mottled micrite. Two main microfacies are reported: brown micrite rich in planktonic forams (tests are well preserved); dark micrite rich in fine-grained siliciclastic fraction (quartz and K-feldspars). Framboidal pyrite is common and often fills forams internal cavities. Peloidal fabrics are generally observed inside conduits. - SEM and EDS analysis allowed the observation of rosette-like structures of pyrite framboids, oil inclusions and the identification of a C-rich beggiatoa-like filament (0,5 mm long). - Preliminary mineralogical analyses: carbonates are composed of calcite (86-93%) with minor amounts of ankerite (always < 5%), enclosing variable amounts of quartz, k-feldspar with scarce muscovite/Illite and chlorite. Poggio delle Campane Geological setting and previous isotope data: the outcrop is included in the upper part of the Marnoso-arenacea Fm., upper Serravallian-Tortonian in age (further biostratigraphic analysis are required). Preliminary isotope analysis conducted on a bulk rock sample provided negative δ13C 32,22 ‰ and a significantly positive δ18O +2,21 ‰ (Conti & Fontana, 2011). - Geological prospection: the outcrop consists of a large chemoherm 20 m thick and minor metersized bodies enclosed in marly sediments comprising rare laminated silty-arenites. - Facies analysis: a dense pattern of veins dominate the base of the main chemoherm, and it is associated to monogenic breccias with subangular particles (up to 3 cm in size). The upper part of the deposit is constituted by large articulated Lucinid clams (diameter up to 12 cm) and show a
sharp contact with the brecciated facies. The macrofossil content is various and comprehends black shell fragments of mytilids, often orientated and locally concentrated, and vestimentiferan tubeworms. Vuggy fabrics are common and they are irregularly distributed in the deposits. - Thin sections: carbonates are constituted by dark micrite rich in highly fragmented planktonic foraminifera and silt-sized siliciclastic grains (quartz, k-feld and muscovite). Conduits are generally filled by sparitic cement including some peloidal micrite. Non-gravitational fabrics sensu Martire et al. (2010) were often observed as cavity-filling in the vuggy carbonates.
All the studied outcrops are representative of high energy seepage conditions. In some cases (Gattaia, Poggio delle Campane), clathrite-like sedimentary features in addition to heavy δ18O values allowed to infer the occurrence of gas hydrates during carbonate precipitation. Future work Results provided new insights into Miocene seepage systems of the Mugello and Casentino area (Tuscan Apennines) and constitute the basis for further detailed research. Next autumn/winter I will perform a detailed field work focused on sedimentary instabilities associated to the seep-carbonates outcrops. I will describe the inner geometries, typologies of internal soft sediment deformations and investigate the spatial and temporal relationships with seep carbonates. Next winter/spring I will spend a semester at University of New Hampshire under the supervision of Prof. J.Johnson, comparing fossil and modern seepage systems and will attend the Geotectonic course. As modern analogue I will consider the Hydrate Ridge area offshore central Oregon (Cascadia convergent margin), in which several studies have documented mass wasting deposits and gas hydrates occurrence and can provide useful criteria for the interpretation of Apennine deposits. References Boetius, A. & Wenzhöfer, F., 2013. Seafloor oxygen consumption fuelled by methane from cold seeps. Nature Geoscience, 6(9), 725-734. Conti, S., Fontana, D. & Mecozzi, S., 2010. A contribution to the reconstruction of Miocene seepage from authigenic carbonates of the northern Apennines (Italy). Geo-Marine Letters, 30(3-4), 449-460 Conti, S. & Fontana, D., 2011. Possible relationships between seep carbonates and gas hydrates in the Miocene of the Northern Apennines. Journal of Geological Research, 2011. Martire, L., Natalicchio, M., Petrea, C.C., Cavagna, S., Clari, P. & Pierre, F.D., 2010. Petrographic evidence of the past occurrence of gas hydrates in the Tertiary Piedmont Basin (NW Italy). Geo-Marine Letters, 30(34), 461-476.
Maslin, M., Owen, M., Betts, R., Day, S., Jones, T.D. & Ridgwell, A., 2010. Gas hydrates: past and future geohazard?. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 368(1919), 2369-2393. Paull, C.K. & Dillon, W.P., 2001. Natural gas hydrates: occurrence, distribution, and detection. Washington DC American Geophysical Union Geophysical Monograph Series, 124. Tamborrino, L., 2011. Studio sedimentologico e composizionale di calcari metano-derivati legati a dissociazione di gas idrati nell’Appennino settentrionale. MSc Thesis, Università di Modena e Reggio Emilia.
