Barite concretions as evidence of pauses in sedimentation in the Marnes Bleues Formation of the Vocontian Basin (SE France)

Several intervals rich in barite nodules occur in the mid-Cretaceous marls of the Vocontian Trough (SE France). These concretions, which grew on heter ogeneities in the sediment, are arranged as horizons parallel to the beddin g, and are often associated with carbonate and phosphate nodules. Detailed sedimentological and stratigraphical observations suggest that they represe nt the consequence of short-term pauses in sedimentation during early diage nesis, but the most important occurrences of barite are below prominent dis continuities. This may be explained by the frequency of pauses in sedimenta tion caused by a progressive starvation, or an increase in the energy of bo ttom-water currents, The high number of barite occurrences on the basin mar gins is directly linked to the recurrence of sedimentary breaks, The geoche mical data show that the host sediment corresponds to a pelagic setting, fa r from hydrothermal influences, but generally in a restricted, dysoxic to s uboxic, depositional environment. The sulphur isotopic composition of the b arite sulphate (+16 to +76 parts per thousand rel. CDT) supports the hypoth esis of an early diagenetic origin of the barite nodules. Their genesis may be explained by the fixation of a diagenetic front corresponding to the pe netration of a sulphate reduction zone from the seawater-sediment interface down in the sediment which is in contact with barium-rich porewaters expel led upwards as a consequence of compaction. The barium of biogenic origin, initially stored in the underlying black shale, was mobilized under strongl y reducing conditions. Such fixation is normally induced by a pause or a st op in sedimentation due to starvation or renewed bottom-water circulation. Together with other concretions, the barite nodules mark condensed stratigr aphic intervals and often represent major breaks, As such they shed light o n variations in sedimentary activity and their importance in the sedimentol ogy and stratigraphy of pelagic marry sequences cannot be ignored, (C) 2000 Elsevier Science B.V. All rights reserved.