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

The history of study of the Upper Jurassic Volgian Stage and Lower Cretaceous Ryazanian Stage, their geographic occurrence, and use in world practice, subdivision, and correlation are considered. The data on the occurrence of various groups of macro- and microfossils (ammonites, bivalves, radiolarians, dinocysts, and marine vertebrates) in the Volgian and Ryazanian are reviewed. In spite of significantly different lifestyle of all these groups, the Volgian and Ryazanian assemblages are highly similar in the entire Panboreal Superrealm, on one hand, and significantly different from coeval Tethyan fauna, on the other hand. The biostratigraphic scales of these stages based on successions of ammonites, bivalves, radiolarians, and dinocysts are analyzed. It is shown that the substage boundaries of the Volgian are reliably traced along the entire Panboreal Superrealm. None of the boundaries, except for the base of the Volgian, corresponds even to a zonal boundary of the Tethys–Pantalassa Superrealm. A similar situation is also observed in the Ryazanian: its lower boundary, as well as the Lower–Upper Ryazanian boundary, does not coincide with any well-traced boundary in the Tethys–Pantalassa Superrealm. The necessity of using the Volgian and Ryazanian in geological studies is substantiated for all Russian regions with abundant Boreal deposits, as well as inclusion of these stages to the General Stratigraphic Scale in parallel with the Tithonian and Berriasian. We suggest the cancellation of the Decree of the Interdepartmental Stratigraphic Committee of Russia about the transition of the Volgian into regional stratigraphic subdivisions, as well as the reconsideration of the correlation scheme of the Volgian and Ryazanian accepted in the same decree.