Aim. To analyse the marine influences of the Rhenish sea in the distal part of the Swiss Molasse Basin (Jura Molasse) during the Early Oligocene, in order to reconstruct the palaeoenvironmental and the palaeogeographical evolution. Location and geological setting. In the Jura Molasse, the Rhenish sea deposits are mainly preserved in three sedimentary basins: Laufen, Delémont and Ajoie. They belong to the following stratigraphical groups of the Jura Molasse: Conglomérats de Porrentruy, Septarienton and Meeressand. Methods. The sedimentological series have been studied in detail for stratigraphy, sedimentology and palaeontology. Three stratigraphical columns are proposed for the Laufen, Delémont and Ajoie basins, corresponding to the southern extension of Rhenish sea in the Swiss Jura. Various fossil groups including foraminifers, ostracods, fishes, dinoflagellate cysts, spores and pollens were studied in terms of taxonomy, biostratigraphy and palaeoecology. These studies lead to reconstruct Oligocene palaeoenvironments and their evolution during the time interval NP21 to base NP25 (corresponding to 34.4–25.0 My). Results. Although three transgression-regression cycles are known in the Rhine Graben, only the first two cycles are recorded in the Ajoie basin and only one in the Delémont and the Laufen basins. New biostratigraphical data allow the dating of the different marine incursions in the Swiss Jura basins. In Ajoie basin, the first recorded transgression corresponds to the biozones top NP21 to base NP22. The second one, the global Rupelian transgression, synchronically invaded the whole northern Swiss Jura area at the top part of NP22–base NP23. The regression is clearly diachronous, occuring during the top part of NP23, in two directions, westward and northward. In the Ajoie basin, the Rhenish sea remained until NP24 to base NP25. Main conclusions. The palaeobathymetrical estimations of the global Rupelian transgression based on ostracods, foraminifers and fishes correspond to the interval 0–50 m. During the considered time interval of this study (34.4–25.0 My), fossil assemblages can be very different, showing a high degree of endemism (especially for ostracods). New palaeogeographical data suggest a possible connection between the south of the Rhine Graben and the Bresse Graben, and show local drainage directions of fluviatile systems. In the Jura, there is no evidence for a third transgression.
Abstract
Aim. To analyse the marine influences of the Rhenish sea in the distal part of the Swiss Molasse Basin (Jura Molasse) during the Early Oligocene, in order to reconstruct the palaeoenvironmental and the palaeogeographical evolution. Location and geological setting. In the Jura Molasse, the Rhenish sea deposits are mainly preserved in three sedimentary basins: Laufen, Delémont and Ajoie. They belong to the following stratigraphical groups of the Jura Molasse: Conglomérats de Porrentruy, Septarienton and Meeressand. Methods. The sedimentological series have been studied in detail for stratigraphy, sedimentology and palaeontology. Three stratigraphical columns are proposed for the Laufen, Delémont and Ajoie basins, corresponding to the southern extension of Rhenish sea in the Swiss Jura. Various fossil groups including foraminifers, ostracods, fishes, dinoflagellate cysts, spores and pollens were studied in terms of taxonomy, biostratigraphy and palaeoecology. These studies lead to reconstruct Oligocene palaeoenvironments and their evolution during the time interval NP21 to base NP25 (corresponding to 34.4–25.0 My). Results. Although three transgression-regression cycles are known in the Rhine Graben, only the first two cycles are recorded in the Ajoie basin and only one in the Delémont and the Laufen basins. New biostratigraphical data allow the dating of the different marine incursions in the Swiss Jura basins. In Ajoie basin, the first recorded transgression corresponds to the biozones top NP21 to base NP22. The second one, the global Rupelian transgression, synchronically invaded the whole northern Swiss Jura area at the top part of NP22–base NP23. The regression is clearly diachronous, occuring during the top part of NP23, in two directions, westward and northward. In the Ajoie basin, the Rhenish sea remained until NP24 to base NP25. Main conclusions. The palaeobathymetrical estimations of the global Rupelian transgression based on ostracods, foraminifers and fishes correspond to the interval 0–50 m. During the considered time interval of this study (34.4–25.0 My), fossil assemblages can be very different, showing a high degree of endemism (especially for ostracods). New palaeogeographical data suggest a possible connection between the south of the Rhine Graben and the Bresse Graben, and show local drainage directions of fluviatile systems. In the Jura, there is no evidence for a third transgression.
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Picot, L