Dinoflagellate cysts and microforaminifera of the Lower Cretaceous Yatria River section, Subarctic Ural, NW Siberia (Russia). Biostratigraphy, palaeoenvironmental and palaeogeographic discussion

Rich assemblages of phytoplankton and microforaminifera (foraminifera linings) are found in the marine Lower Cretaceous section from the Yatria River (Subarctic Ural). The Lower Cretaceous (Upper Volgian to Lower Hauterivian) dinocyst and microforaminifera biostratigraphy is calibrated against the ammonite zones. The dinocyst stratigraphy differs from Arctic Canada zonation, except for the Berriasian Paragonyaulacysta? borealis zone, and shows significant similarity with the zonal subdivision of Boreal regions of Europe. The distribution of phytoplankton and microforaminifera is related to relative sea level variations. Consecutive changes of phytoplankton associations (2 in the Berriasian, 3 in the Valanginian-Early Hauterivian, 4 in the Early Hauterivian), lateral zonations of microforaminifera (2 in the Berriasian, 2 in the Valanginian) and their relation to environmental changes in the Lower Cretaceous seas of the Subarctic Ural Basin are established.