Magalhaes V.H.1vhm¤ua.pt,Rocha F.2tavares.rocha¤ua.pt,Pinheiro L.M.1lmp¤ua.pt
1Centre for Environmental and Marine Studies, Geosciences Department, University of Aveiro
2Geobiotec Centre, Geosciences Department, University of Averio
Extensive work was carried out in the study of fluid escape structures in the Gulf of Cadiz (GC) during the last 10 years, that allowed the identification of 43 mud volcanoes (MV), confirmed by coring from 200 to 4000 m water depth(Pinheiro et al., 2003). Gas hydrates(Mazurenko et al., 2002)and methane-derived authigenic carbonates(Díaz-del-Río et al., 2003)were found associated with the most active MVs, which act as major migration conduits for deep sourced fluids enriched in hydrocarbons(Magalhães, 2007). While the tectonic setting of the GC is complex and not clearly resolved, the source layers of the MV in these region seem to be related to the Allochthonous Unit of the GC(Medialdea et al., 2009), but their roots are not clearly imagined with the presently existing seismic data. Molecular and carbon isotopic composition of the MV hydrocarbon gases are interpreted as indicating the presence of hydrocarbon
accumulations in the deep subsurface of the GC(Stadnitskaia et al., 2006). Clay mineral
dehydration has also been proposed as a major source of the fluids from the GC MV(Hensen et al., 2007). Some MVs, such as Cap. Arutyunov, also show possible hydrothermal imprint on the geochemical composition of pore fluids(Hensen et al., 2007).
In this work, the clay mineralogy study of MV mud breccia clasts and matrix is used as a
contribution to a better understanding of some of these questions. For this purpose, two MV were selected: Bonjardim and Tasyo. These MVs are located at different settings in relation to the tectonic structure of the GC. Bonjardim is a very active MV, from which gas hydrates were collected, and is located at the lower slope of the margin (3100 m). Tasyo MV is located at the upper slope of the margin (1100 m), at present inactive and covered by pelagic sediments.
Bonjardim and Tasyo MVs were sampled by gravity coring and tv-controlled grab during TTR10, 11, 12, 14 and 16 cruises. 31 different types of rock clasts were identified in Bonjardim and 28 different lithologies in Tasyo. The clasts lithology is dominated by claystones and mudstones, but marlstones, limestones and sandstones were also retrieved. The most frequent ages of the clasts are Miocene (Aquitanian to Messinian), Eocene (Lutetian to Priabonian) and Upper Cretaceous (Santonian to Maastrichtian) for Bonjardim MV(Ovsyannikov et al., 2003);
Tasyo MV clasts are of Miocenne Age for most of the groups, but Pliocene and Maastrichtian Age clasts were also identified.
Clasts representative of the different lithologic types were selected for this study. Mineralogical analysis with X-ray diffraction was carried out on bulk samples, and on suspensions and oriented aggregates of <20 and <2 µm on air-dried, glycolated and heated samples. Scanning electron microscopy with elemental analyses was also done on selected samples. Bulk and clay mineral composition of the mud breccias matrix and clasts are clearly different from the hemipelagic sediments. Smectite is the dominant clay mineral and clay association implies a week thermal maturation of clays, and therefore the parent units appear to be buried no more than a few kilometers in depth, probably less than 3 km.
Díaz-del-Río, V., Somoza, L., Martínez-Frias, J., et al. (2003) Vast fields of hydrocarbon- derived carbonate chimneys related to the accretionary wedge/olistostrome of the Gulf of Cádiz.
Marine Geology, 195(1-4): 177-200.
Hensen, C., Nuzzo, M., Hornibrook, E., et al. (2007) Sources of mud volcano fluids in the Gulf of Cadiz – indications for hydrothermally altered fluids. Geochimica et Cosmochimica Acta, 71(5): 1232-1248.
Magalhães, V.H. (2007) Authigenic carbonates and fluid escape structures in the Gulf of Cadiz.
PhD Thesis, University of Aveiro, 421 pp.
Mazurenko, L.L., Soloviev, V.A., Belenkaya, I., et al. (2002) Mud volcano gas hydrates in the Gulf of Cadiz. Terra Nova, 14: 321–329.
Medialdea, T., Somoza, L., Pinheiro, L.M., et al. (2009) Tectonics and mud volcano development in the Gulf of Cádiz. Marine Geology, 261(1): 48-63.
Ovsyannikov, D.O., Sadekov, A.Y. and Kozlova, E.V. (2003) Rock fragments from mud volcanic deposits of the Gulf of Cadiz: an insight into the Eocene-Pliocene sedimentary succession of the basin. Marine Geology, 195(1-4): 211-221.
Pinheiro, L.M., Ivanov, M.K., Sautkin, A.P., et al. (2003) Mud volcanism in the Gulf of Cadiz:
results from the TTR-10 cruise. Marine Geology, 195(1-4): 131-151.
Stadnitskaia, A., Ivanov, M.K., Blinova, et al. (2006) Molecular and carbon isotopic variability of hydrocarbon gases from mud volcanoes in the Gulf of Cadiz, NE Atlantic. Marine and Petroleum Geology, 23(3): 281-296.
Библиографическая ссылка на эту работу:
Magalhaes V.H., Rocha F., Pinheiro L.M.Clay mineralogy of mud volcano clasts from the gulf of cadiz // Глины, глинистые минералы, слоистые материалы - CMLM2009 - М.: Издатель И.В. Балабанов, 2009, C.223-224
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