Basic problems of the Upper VoIgian, Berriasian and Valanginian stratigraphy of the Boreal zone

Abstract

The ammonite stratigraphy of the Upper Volgian, Berriasian and Valanginian of the Boreal zone of the USSR is presented and compared to other boreal zones of the world. The sediments of the above stages are chiefly silts, clays and siltstones sometimes hardly calcareous. The ammonite assemblages are unique in the Boreal zone and in more southerly peripheral, zones some mixed assemblages with Tethyan forms occur. In the lack of ammonites the stratigraphy is based on pelecypods (Buchia). Attention is paid to the boundaries between the stages.