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Géochimie des black shales du Jurassique supérieur de la plate-forme russe. Processus de sédimentation et de préservation de la matière organique

Abstract : During the Middle Volgian, the Russian Platform was the seat of important organic matter (OM) deposition. The resulting formation shows alternating levels of clayey marlstones and black shales. This formation was studied in Gorodische and Kashpir areas, located in the middle Volga Basin, in order to determine the processes of OM sedimentation and preservation. A global study of the sediment and OM was first performed on numerous samples from the two places. Very similar bulk geochemical data (HI, TOC, CaCO3...) are obtained, indicating similar conditions of OM deposition and preservation in the whole middle Volga Basin. Palynological observations indicate that the samples are dominated by amorphous OM (AOM) and that TOC and HI variations are not due to differences in relative contributions of terrestrial and marine OM. Gorodische samples were divided into three groups of contrasted bulk geochemical and morphological features : (i) grey AOM for poorly aliphatic, Sorg-poor samples of lowest TOC contents, (ii) heterogeneous orange AOM for relatively aliphatic and Sorg-containing samples of middle to high TOC contents, (iii) pure orange AOM for aliphatic and Sorg-rich samples of very high TOC contents. Detailed geochemical study (spectroscopy and pyrolysis) of three kerogens representative of each group showed the phytoplanctonic origin of the OM. A weak bacterial contribution is also noted. Different OM preservation processes are observed : pure orange AOM was mainly formed by natural sulphurisation of lipids and carbohydrates under anoxic conditions; grey AOM was mainly formed by the degradation-recondensation process with a minor contribution of selectively preserved algal cell-walls under mainly oxic conditions; heterogeneous orange AOM appears in between the other two groups and is constituted of melanoidins and sulphurised material under changing oxygenation conditions, mainly dysoxic. The three kerogens also show an important contribution of lipids incorporated by oxydative reticulation. Combination of bulk and detailed geochemistry, allows to put forward a recontruction of the paleoenvironment and of its evolution during the deposition of the organic-rich formation.
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Submitted on : Tuesday, August 24, 2004 - 11:15:20 AM
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Armelle Riboulleau. Géochimie des black shales du Jurassique supérieur de la plate-forme russe. Processus de sédimentation et de préservation de la matière organique. Géochimie. Université Pierre et Marie Curie - Paris VI, 2000. Français. ⟨tel-00006731⟩

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