Faculté des sciences

The evolution of the Urgonian platform in the Western Swiss Jura realm and its interactions with palaeoclimatic and palaeoceanographic change along the Northern Tethyan Margin (Hauterivian – earliest Aptian)

Godet, Alexis ; Föllmi, Karl (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2006 ; 1904.

During more than twenty years, a controversy appeared about the age of the Urgonian formation (lower Urgonien Jaune and upper Urgonien Blanc) from the Western Swiss Jura. Depending on previous works, these formations are considered to be Late Hauterivian or Late Barremian in age. This divergence mainly results from different calibration of orbitolinids distribution, as well as divergent sequence... Plus

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    Summary
    During more than twenty years, a controversy appeared about the age of the Urgonian formation (lower Urgonien Jaune and upper Urgonien Blanc) from the Western Swiss Jura. Depending on previous works, these formations are considered to be Late Hauterivian or Late Barremian in age. This divergence mainly results from different calibration of orbitolinids distribution, as well as divergent sequence stratigraphic models. Because these formations, as well as the underlying Pierre Jaune de Neuchâtel, are linked to the historical succession for the Hauterivian stage, and because they represent the proximal part of a carbonate platform that rose on the northern Tethyan margin during the Early Cretaceous, we developed new approaches to date, as precisely as possible, the Urgonian formations in the Neuchâtel area. At the base of the Urgonien Jaune, high reworking implied that the biostratigraphy did not give an accurate age model. Moreover, biostratigraphical dating can greatly differ as a function of the used taxon. Consequently, we developed sedimentological and geochemical approach. The boundary between the Pierre Jaune de Neuchâtel and the Urgonien Jaune is clearly marked by erosional and reworking processes, suggesting the presence of a sedimentary gap. Indeed, strontium-isotope dating performed on rhynchonellids shells rather indicate a Barremian age for both the Urgonien Jaune and the Urgonien Blanc. The comparison of the phosphorus evolution between the Western Swiss Jura and hemipelagic sections from the Vocontian Trough helped to precise this age to the Late Barremian, as oligotrophic conditions allowing the rise of the Urgonian platform in a photozoan mode only occurred during this period. Finally, sequence stratigraphic correlation of the Western Swiss Jura with the Helvetic realm, the Subalpine Chains and the Northern Vercors implied that the base of the Urgonien Jaune is characterized by the stacking of several sequence boundaries, which may mirror an episode of condensation linked to a carbonate platform drowning event previously described in the Helvetic realm. Moreover, there is no evidence for an other break within the geological record in the Western Swiss Jura after the sequence boundary B3 of the late Early Barremian Coronites darsi ammonite zone. In addition, these results are coherent with the fact that the rise of the Urgonian platform began from the maximum flooding surface of the depositional sequence B3 upward in the Northern Tethys. The Urgonien Blanc of the Western Swiss Jura may thus correspond to the lower Urgonian and to the lower Schrattenkalk formations of the Northern Vercors and the Helvetic realm, respectively. The stable isotopes study of several (hemi-)pelagic sections of the Northern Tethyan margin and their mineralogical contents revealed that the Urgonian platform had a role in paleoceanographic changes that occurred during the Barremian. Whereas the 13C curve exhibits negligible changes during the latest Hauterivian – Early Barremian, it is shifted toward more positive values from the sequence boundary B3 upward. This behaviour may be linked to the production and the export of 13C-enriched aragonitic material by benthic organisms present on the platform. Moreover, the correlation of the kaolinite content along a platform to basin transect through the Northern Tethyan margin showed that the main part of the Barremian was characterized by a humid climate, whereas a seasonally-contrasted climate dominated during the Hauterivian. This correlation also highlighted a differential settling of clay particles, as high amounts of kaolinite were measured in shallow-water carbonates of the Neuchâtel area, whereas hemipelagic limestones from the Vocontian Trough were depleted in this mineral. Thanks to this multidisciplinary approach, the Western Swiss Jura is better integrated within the history of the Northern Tethyan margin. Finally, interactions between carbonate platforms and basinal environments are clearly highlighted.