New data on the Structure of the Terminal Part of the Volgian Stage of the Upper Jurassic in the Reference Section Near the Vasilyevskoye Village, Yaroslavl Region

The study of new sections of the upper part of the Upper Volgian substage near the Vasilyevskoye village (R-ybinsk district, Yaroslavl region) made it possible to clarify the position of the lower boundary and the infrazonal division of the Volgidiscus singularis Zone. Until recently, the lower part of the Singularis Zone was not characterized by ammonites, but thanks to the study of these sections, it was possible to establish the sequence of craspeditins ammonoids (Volgidiscus and Anivanovia) and to identify a new biohorizon Volgidiscus cf. lamplughi. The lower boundary of the Singularis Zone in the Chudinovskaya Formation sections is determined by the first appearance of Volgidiscus above the Craspedites milkovensis biohorizon. The infrazonal volume of the Singularis Zone is represented by three biohorizons: V. cf. lamplughi, V. pulcher and V. singularis. At present, the Singularis Zone is characterized by the most complete structure in the Panboreal Superrealm, which allows it to be considered as an infrazonal standard for the upper part of the Upper Volgian substage.