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Ben Ammar, Sabrine; Layeb, Mohsen
Arabian journal of geosciences, 11/2021, Letnik: 14, Številka: 22Journal Article
Within the Valanginian–Hauterivian series of the northern Tethyan margin, two organic-rich laminated black shale intervals corresponding to the Valanginian Weissert and the late Hauterivian Faraoni oceanic anoxic events (W-OAE and F-OAE, respectively) are well documented. However, in Tunisia as part of the southern Tethyan margin, despite several biostratigraphic and structural studies awarded to this period, geochemical characterization based mainly on wireline logs data was undertaken. Here, we aimed to confirm the extension of these Valanginian–Hauterivian organic-rich deposits throughout a bulk and biomolecular geochemical (Rock–Eval pyrolysis and gas chromatography–mass spectrometry analysis) study carried out on selected surface outcrops in northern Tunisia. To that end, three hemi-pelagic sections belonging to the Tunisian backbone (Jebel Zaghouan), the Tunisian trough (Jebel Oust) and the diapiric zone (Jebel Boulahouajeb) have been chosen and analysed with highly interest to their organic matter (OM) contents and biomolecular composition. Rock–Eval pyrolysis shows that Jebel Oust section presents total organic carbon ( TOC ) contents ranging from 0.18 to 0.57 wt% with an average of 0.38 wt% and low hydrogen index ( HI ) values ranging from 15 to 74 mg HC/g TOC indicating a type IV OM. Its hydrocarbon generation potential ( HGP ) is very low where the average value is around 0.19 mg HC/g rock, while Jebel Zaghouan section reveals TOC values reaching up to 0.56 wt% with a low HGP up to 1.01 mg HC/g rock and high maturity levels attested by T max values ranging from 449 to 460 °C. Jebel Boulahouajeb section shows low TOC values ranging from 0.22 to 0.69 wt% with an average of 0.35 wt%. Its HGP is very low with a maximum value of 1.67 mg HC/g rock and presents marginally mature to mature stage where T max values ranging from 438 to 450 °C with an average of 443 °C. This maturity distribution was guided by the basin architecture marked by the presence of subsiding zones in local faulted blocks and palaeohighs occupied by Triassic salt domes and horsts. Molecular biomarker analyses from the Valanginian–Hauterivian Boulahouajeb and Jebel Zaghouan facies indicated a mainly marine origin of the OM and a suboxic depositional environment in a normal marine water column. Throughout this study, we confirm the installation, in the southern Tethyan margin (northern Tunisia), of dysoxic depositional conditions during the early Valanginian and late Hauterivian times corresponding to the Weissert and Faraoni events, respectively. However, these depositional environments were not favourable for preservation of high quantities of OM. This was mainly controlled by the presence of an oxygen minimum zone generally disturbed by detrital discharges. Additionally, the results highlight the important interplay of the basin architecture, the various faults developed with an extensional tectonic regime during the Valanginian–Hauterivian period, the warm climate conditions and the eustatic sea-level variations where transgressive systems tracts coincide with black shales deposited majorly under dysoxic conditions.
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JCR | SNIP | JCR | SNIP | JCR | SNIP | JCR | SNIP |
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