LOWER JURASSIC BLACK SHALE SUCCESSIONS FROM EUROPE: DISCUSSIONS ON THE OAE

During the last six years Lower Toarcian black shales from SW-Germany have been investigated at several localities in high stratigraphic resolution. A multidisciplinary approach combining organic geochemical methods, stable carbon and oxygen isotope analysis, palaeoecological and sedimentological investigations (e.g. faunal distribution, lamination-type, grain size distribution), spectral gamma-ray analysis was used to reconstruct the controlling factors of black-shale deposition within the SW-German Epicontinental Basin. The results indicate that the depositional environment became silled during regression. Subsequent sea-level rise changed palaeoceanic conditions progressively resulting in a more open-marine situation. The comparison of the results with data from other European epicontinental sections collected from literature evidences that the initial force of black-shale sedimentation within the Central European Epicontinental Basins (CEB) was regression which started during Pliensbachian and lasted until the earliest Toarcian. The subsequent early Toarcian transgression obviously was minor so that long-term stagnant conditions established within the CEB leading to widespread black-shale sedimentation. To compare the results from the CEB with the Tethyan region we investigated three sections from different depositional environments of Northern Italy supposed to be of early Toarcian age. These sections are biostratigraphically poor dated and thus do not allow a correlation with the sections from the CEB. For instance, sequence stratigraphic interpretation of the sections from the Lombardian Basin reveals that black shales were deposited during sea-level lowstand and their occurrence most likely represents a correlative conformity of an unconformity at proximal locations. Therefore, the stratigraphic correlation based on stable isotopic data is not convincing and it is much more plausible that stable isotope geochemistry reflects the regional palaeoenvironmental conditions within a basin rather than a global atmospheric signal. Conclusively, it is most likely that the OAE postulated by JENKYNS (1988) represents a diachroneous "event" and stable isotope curves are deeply facies dependent.