The Volgidiscus Singularis Zone of the Terminal Horizons of the Volgian Stage of European Russia and Its Significance for Interregional Correlation and Paleogeography

Abstract

It has been accepted until recently that a regionally developed gap corresponding to the boundary interval between the stages (equivalents of the Chetaites chetae and Volgidiscus lamplughi zones) exists between the upper Volgian Craspedites nodiger Zone and the Ryazanian Stage. Recently, the presence of this interval was established in Yaroslavl oblast. In this paper, we publish for the first time a detailed description of the sections of the terminal part of the upper Volgian Substage, the majority of which have not previously been published, as well as illustrations of characteristic mollusks (ammonites and bivalves) from these deposits. For this stratigraphic interval, the recognition of the Volgidiscus singularis Zone, including the V. pulcher and V. singularis biohorizons, is substantiated. In this zone, ammonites are mainly represented by the genus Volgidiscus, but the pulcher Biohorizon also contains very rare Garniericeras, whereas the glacial boulders together with V. cf. pulcher contained Shulginites. The Singularis Zone has a high correlative potential and, at an infrazonal level, directly correlates with the uppermost zones of the upper Volgian Substage of the Subpolar Urals, northern Siberia, England, and the North and Barents seas. It is most likely that the biostratigraphically confirmed gap between the upper Volgian Singularis Zone and Ryazanian stage is absent. In both the Subpolar Urals and in European Russia, the terminal part of the Volgian Stage and the lower part of the Ryazanian Stage both contain ammonites of the genus Shulginites. Bivalves are represented mainly by the genera Anopaea, Camptonectes (Camptonectes), Entolium (Entolium), and Plagiostoma. The studied sections lack Buchia, one of the most characteristic taxa of the Jurassic–Cretaceous boundary beds of the Panboreal Superrealm. It is proposed that the Volgian–Ryazanian boundary beds in the upper reaches of the Cheryomukha River (Rybinsk district, Yaroslavl oblast) are recognized as the Chudinovo Formation, the upper part of which contains numerous ammonites of the Singularis Zone.