Depositional environment of a Kimmeridgian carbonate 'black band' (Akkuyu Formation, south-western Turkey) |
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| Authors: | François Baudin Nicolas Tribovillard Fatima Laggoun-Défarge Eric Lichtfouse Olivier Monod & Silvia Gardin |
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| Institution: | CNRS-ESA 7073 et FR 32, Département de Géologie Sédimentaire, UniversitéP. et M. Curie, case postale 117, 4 place Jussieu, 75252 Paris Cedex 05, France (E-mail;), Laboratoire de Géochimie des Roches Sédimentaires et CNRS-URA 723, UniversitéParis-Sud, Bat. 504, 91405 Orsay Cedex, France,;CNRS-UMR 6531 et FR 09, Universitéd'Orléans, BP 6759, 45067 Orléans-La Source Cedex 2, France,;Laboratoire de Biogéochimie isotopique, UniversitéP. et M. Curie, 75252 Paris Cedex 05, France,;CNRS-UMR 6530 et FR 09, Universitéd'Orléans, BP 6759, 45067 Orléans-La Source Cedex 2, France |
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| Abstract: | The Upper Jurassic Akkuyu Formation of south-western Turkey consists of pale-coloured carbonate facies, but includes a peculiar ‘black band’ at its base. In order to characterize the depositional environment of this level, a sedimentological, organic and inorganic geochemical study was performed on 30 samples. Three units are distinguished. Unit 1 consists of black marlstones, very rich in organic carbon (22–30%) with HI (hydrogen index) values around 600. The organic matter is an amorphous homogeneous material even at nanoscopic scale. This unit is also characterized by a very high concentration of V ( >1000 p.p.m.) and a δ13Corg value around ?26·5‰. Unit 2 shows thin-bedded black limestones with chert layers. Its organic content varies from 2% to 6%. As in unit 1, the HI values are uniformly high (600), and gas chromatography shows a high quantity of hopanes. The V concentration is also high (≈400 p.p.m.). The unit has a high Sr/Ca ratio, an organic carbon isotopic ratio close to ?28·7‰ and diffuse, nanoscopically amorphous organic material that is closely associated with pyrite. Unit 3 corresponds to the white limestones overlying the ‘black band’. This calcitic facies is almost devoid of organic carbon (<0·1%). The δ13Corg values are similar to those of unit 1 (?25·5‰ to ?26‰). Deposition occurred in a basinal environment surrounded by a carbonate platform. Primary productivity was high but not exceptional; terrestrial organic inputs were scarce. Bottom waters were probably euxinic during the deposition of unit 1. The presence of both hopane-rich and sulphur-rich nanoscopically amorphous organic matter may be the result of strong sulphate-reducing bacterial activity. Coupled with low amounts of iron, this led to intense vulcanization of the organic matter and thus good preservation. With the onset of unit 2 deposition, environmental conditions became less extreme, perhaps because of cooler water temperatures. Finally, the depositional conditions became oxic, leading to the accumulation of pale-coloured organic-poor limestones of unit 3. |
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| Keywords: | Anoxia carbon isotopes organic geochemistry organic matter palynofacies trace elements vulcanization |
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