Upper Jurassic Foraminiferal Zones and Position of the Jurassic-Cretaceous Boundary in Western Siberia

摘要

The biostratigraphy of Upper Jurassic deposits of Western Siberia is based on zones of foraminifera (hereafter abbreviated F-zones) with widespread foraminiferal assemblages. The description of F-zones corresponding to Oxfordian, Kimmeridgian and Volgian stages is given. The Upper Jurassic-Cretaceous boundary coincides with the top of the Upper Volgian F-zone Ammodiscus veteranus-Evolutinella volossatovi. The Volgian foraminiferal assemblages cannot be further subdivided; they differ systematically at the species level from the Early Berriasian faunas of the Early Cretaceous.