Volgian stage of the Upper Jurassic and Ryazanian Stage of the Lower Cretaceous of the Panboreal biogeographic Superrealm

The history of the study of the Volgian Stage of the Upper Jurassic and Ryazanian Stage of the Lower Cretaceous, their geographical distribution and use in the world practice, subdivision and correlation are considered. The data on the distribution of different groups of macro- and microfossils (ammonites, bivalves, radiolarians, dinocysts, and marine reptiles) in the Volgian and Ryazanian stages are reviewed. In all these groups, despite significant differences in lifestyle, the Volgian and Ryazanian assemblages, on the one hand, show a high degree of similarity throughout the Panboreal Superrealm, and on the other hand, significant differences from the coeval Tethyan faunas. Biostratigraphic scales of this age based on the sequences of ammonites, bivalves, radiolarians and dinocysts are analysed. It is shown that the substage boundaries of the Volgian Stage are reliably traceable throughout the Panboreal Superrealm. At the same time, no boundary other than the Volgian Stage base corresponds even to the zonal boundary in the Tethys-Pantalassa Superrealm. A similar situation is observed in the Ryazanian Stage: its lower boundary, as well as the boundary of the Lower and Upper Ryazanian substages do not coincide with any clear boundary in the Tethys-Pantalassa Superrealm. The necessity of using the Volgian and Ryazanian stages in the practice of geological research on the territory of Russia for all regions where Boreal deposits are distributed, as well as the inclusion of these stages in the General Stratigraphic Scale in parallel with the Tithonian and Berriasian is substantiated. It is proposed to cancel the resolution of the ISC of Russia on the considering the Volgian Stage to the rank of regional stratigraphic subdivisions, as well as to revise the scheme of comparison of the Volgian and Tithonian stages adopted in the same document.