Upper Jurassic foraminifera, dinoflagellates and terrestrial sporomorphs from the Gorodishche Section (Ul'yanovsk Region, East European Platform, Russia): Biostratigraphic, palaeoenvironmental and palaeobiogeographical implications

The Upper Kimmeridgian to Lower Volgian successions of the East European Platform are well represented in the clay formations of the Gorodishche Section (Ul'yanovsk Region). It displays very rich foraminiferal and palynological assemblages. A review of foraminiferal and microphytoplankton biozonation is proposed in this study. Foraminiferal as well as marine and terrestrial palynomorph assemblages were also analyzed to reconstruct environmental and climatic conditions during the transition from the Late Kimmeridgian to the Early Volgian. The depositional setting corresponds to the transition from the upper to the middle sublittoral zone mostly associated with oxic conditions. In addition, facies analysis allows the reconstruction of short term sea-level curve. Curves of the Jaccard similarity Index have been constructed between the Gorodishche Section and different regions of Eurasia in order to understand palaeobiogeographical changes. The results of the analyses indicate significant similarities of foraminiferal and dinocysts associations across East European Platform, Western Europe, and Siberia probably associated to good connections between palaeobasins. The wide geographical range of foraminiferal and dinocyst zones indicates the possibility for long distance correlations in the latest Kimmeridgian probably associated with palaeoenvironmental or/and palaeogeographical conditions favorable for the marine biota exchange. Finally, palaeoclimatic reconstructions are derived from the analysis of spore-pollen associations. They indicate a warm arid to semi-arid climate with alternations of wet and dry seasons. The nature of the near-shore land vegetation was strongly influenced by humidity fluctuations.