The results of semi-quantitative and qualitative studies of nannofossil assemblages in dark to light grey, rhythmically bedded, calcareous mudstones of the Upper Cieszyn Beds, Silesian Nappe of the Outer Carpathians are presented. The vertical variation in the carbonate content of these mudstones and changes in the composition of nannofossil assemblages, as well as the Shannon diversity index (SI), result from humid-arid climate changes, driving the trophic conditions of surface waters and nannoplankton biocalcification. These changes were control- led by the variable influences of both the Tethyan and Boreal provinces. The light grey mudstones were deposited during the dominance of warm waters from the Tethyan realm and some nannoconids, typical of the warm, stratified water of lower palaeolatitudes, occur here. The dark grey mudstones represent influences of the Boreal province, indicated by the appearance of Crucibiscutum sp. and a generally higher biodiversity demonstrated by SI. Climatic changes affected the stratification of surface water, expressed also by different linear fits for pairs of SI, Watznaueria barnesiae and Rhagodiscus asper. On the basis of the presence of Eiffellithus striatus, E. windii and signals of both Tethyan and Boreal influences. the section studied represents the uppermost Valanginian- lowermost Hauterivian boundary interval, i.e., the upper part of the BC5/BC6 nannofossil zones, related to the furcillata-radiatus ammonite zones.