Geological Features of the Bajocian-Bathonian in the Reference Section of the Izhma River Basin (European North of Russia) and the Succession of Ammonites of the Subfamily Arctocephalitinae Meledina

The article discusses the main disagreements between two published models (the authors' model and the previously published model by V.V. Mitta et al.) of the structure and stratigraphic subdivision of the Bajocian-Bathonian reference section in the north of European Russia (near the Dreshchanka River mouth, Izhma River basin). Calculation of dip and strike makes it possible to verify mutually exclusive correlation schemes for key outcrops exposed near the Dreshchanka River mouth. It is shown that the correlation model for outcrops proposed by V.V. Mitta is inconsistent. According to this model, the actual thickness of the Bajocian part of the succession is understated. Some disagreements in ammonite stratigraphy are also discussed. Ammonites occurring in the lower Bathonian Ishmae Zone of the Timan-Pechora region belong to a single species, Arcticoceras ishmae, characterized by a wide range of intraspecific variability. The available data for the studied region does not allow any further division of the Ishmae Zone into either subzones or biohorizons. The classical succession, including four consecutive biohorizons in East Greenland, is based on the change of chronosubspecies/chronovariations of the species A. ishmae, which differ in the ratio of intraspecific morphs of the proposed variability. Currently, it is impossible to define these horizons in the Timan-Pechora region sections because representative sets of well-referenced samples are absent.