| mhorn | Дата: Sunday, 19.09.2010, 09:50 | Сообщение # 1 |
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| Nakrem H.A., Hammer Ø., Hurum J.H., Little C.T.S. (2010) Hydrocarbon seeps from the uppermost Jurassic, Western Spitsbergen, Svalbard // NGF Abstracts and Proceedings of the Geological Society of Norway, no. 1. P.133.
Pdf: http://www.nhm.uio.no/om-muse....NGF.pdf
Carbonate mounds, interpreted as being the result of hydrocarbon (or cold) seepage, outcrop in the Upper Jurassic dark silt‐ to paper‐shale succession of the Agardhfjellet Formation, Slottsmøya Member, Knorringfjellet area, Spitsbergen, Svalbard. Four mounds were mapped during field work in 2008, the largest being 3‐4 m high and 7 m wide. Field work in the Knorringfjellet and Janusfjellet area in 2009 revealed 10 more mounds. Analyzed samples from these mounds include zoned (botryoidal) carbonate of varying yellow to brown colour, fissure‐infilling sparite, and various fossil shell material. The macrofauna consists mainly of small to medium sized bivalves (<10 mm, up to 70 mm wide), rare brachiopods as well as worm tubes. Bivalves include at least eight species, including the largest known Nucinella, Oxytoma (or Meleagrinella), Pseudolimea, a solemyid, a lucinid and possible arcticids and ‘thyasirids’. Large accumulations of the bivalve genus Buchia are present in all mounds as well as in the dark surrounding shale. Gastropods, usually only preserved as internal moulds, are not common, but a species of Amberleya has tentatively been identified. Brachiopods are represented by probably three terebratulid species, two rhynchonellid species, as well as more common lingulids. Vestimentiferan as well as serpulid worms tubes are also present. A rich microfauna consisting of uncompacted agglutinated foraminiferan specimens of Recurviodes scherkalyensis, Evolutinella schleiferi and Ammobaculites cf. gerkei has been retrieved from dissolved carbonate samples. Calcareous foraminiferans and radiolarians have also been observed in several thin sections. Embedded ammonites and large (up to 40 x 1500 mm) wood pieces are considered to be from surface waters and not related to hydrocarbon seepage. Stable isotope analyses show highly negative δ13C values (~ ‐43‰ VPDB) in the zoned carbonate whereas the sparite, ammonite and bivalve samples have δ13C ~ ‐22‰. The 13C depletions indicate a methanogenic carbonate origin, in the range typical of thermogenic, rather than biogenic (< ‐60‰ PDB) methane. The ammonite and bivalve shells would originally have had normal‐marine isotopic compositio. nsTheir observed negative δ13C values can be explained by recrystallization with introduction of light carbon from the authigenic carbonate. Large depletion of 13O in the sparite, ammonite and bivalve material (δ13O ~ ‐18‰) indicates precipitation and recrystallization involving hydrothermal fluids, either synsedimentary, or in connection with sill emplacements in the Cretaceous. The isotope data from the carbonates will be compared with data from surrounding shale samples from where organic carbon isotope logs have been made.
Middle Jurassic - Lower Cretaceous ammonites & aptychi
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