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Barite concretions as evidence of pauses in sedimentation in the Marnes Bleues Formation of the Vocontian Basin (SE France) |
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| Authors: | Jean-G Brhret Hans-J Brumsack |
| Institution: | a Université François Rabelais, Laboratoire de Géologie, Faculté des Sciences, Parc de Grandmont, F-37200 Tours, France b Institut für Chemie und Biologie des Meeres (ICBM), Postfach 2503, D-26111 Oldenburg, Germany |
| Abstract: | Several intervals rich in barite nodules occur in the mid-Cretaceous marls of the Vocontian Trough (SE France). These concretions, which grew on heterogeneities in the sediment, are arranged as horizons parallel to the bedding, and are often associated with carbonate and phosphate nodules. Detailed sedimentological and stratigraphical observations suggest that they represent the consequence of short-term pauses in sedimentation during early diagenesis, but the most important occurrences of barite are below prominent discontinuities. This may be explained by the frequency of pauses in sedimentation caused by a progressive starvation, or an increase in the energy of bottom-water currents. The high number of barite occurrences on the basin margins is directly linked to the recurrence of sedimentary breaks. The geochemical data show that the host sediment corresponds to a pelagic setting, far from hydrothermal influences, but generally in a restricted, dysoxic to suboxic, depositional environment. The sulphur isotopic composition of the barite sulphate (+16 to +76‰ rel. CDT) supports the hypothesis of an early diagenetic origin of the barite nodules. Their genesis may be explained by the fixation of a diagenetic front corresponding to the penetration of a sulphate reduction zone from the seawater–sediment interface down in the sediment which is in contact with barium-rich porewaters expelled upwards as a consequence of compaction. The barium of biogenic origin, initially stored in the underlying black shale, was mobilized under strongly reducing conditions. Such fixation is normally induced by a pause or a stop in sedimentation due to starvation or renewed bottom-water circulation. Together with other concretions, the barite nodules mark condensed stratigraphic intervals and often represent major breaks. As such they shed light on variations in sedimentary activity and their importance in the sedimentology and stratigraphy of pelagic marly sequences cannot be ignored. |
| Keywords: | barite concretions discontinuities middle Cretaceous sulphur isotopes Vocontian Trough major elements trace metals palaeoenvironment diagenesis |
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