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In this study we analyze the importance of new magnetostratigraphic data on the Nordvik section for solving the problem of detailed Tethyan–Boreal correlation around the Jurassic–Cretaceous boundary with a special emphasis on the aspects of interpretation of the paleomagnetic data in magnetostratigraphic studies and the need for the integrated (paleontological and paleomagnetic) approach to recognition of the base of the Berriasian.

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The aim of this project was to study the paleomagnetic stratigraphy of the Hauterivian-Aptian deposits in the Middle Volga region, namely in the areas of the Saratov, Samara, and Uliyanovsk right-hand Volga areas. Original data were obtained for the first time for the magnetic polarity of the Hauterivian and Barremian rocks of the Russian Platform. The results obtained for the Aptian rocks agree very well with the results of the previous paleomagnetic studies which had been carried out in the Uliyanovsk region earlier [Baraboshkin et al., 1999] and in the vicinities of Saratov City [Grishanov, 1984]. Based on the complex correlation of the bio-and magnetostratigraphic data for the rocks from the Middle Volga region, the North Caucasus, the North Mediterranean area, and South England, it is shown that the stage and substage boundaries of the Hauterivian, Barremian, and Aptian stages differ in terms of the absolute age from the similar boundaries in the Boreal Belt by a value of some 10 5-10 6 years, which is comparable with the duration of the Early Cretaceous ammonite zones. The diachronism of the Lower Aptian ammonite zones (Deshayesites volgensis-D. forbesi) and of the Middle Aptian Parahoplites melchioris zone in different regions. The results of this study called for a complex substantiation and tracing of the units of the General Stratigraphic Scale (GSS). The basic GSS unit must be a stage, rather than a zone, because where boundaries were traced over a large territory, the asynchronism of the boundaries between the zones is comparable with the duration of the zones, whereas in the case of a stage it is 1-2 orders of magnitude lower. Geomagnetic inversions or isotopic data can be used to test the isochronism of stratigraphic boundaries.

This article is concerned with nannofossil study of Tithonian–Berriasian sediments of Eastern Crimea. The NJT 16, NJT 17a, NJT 17b, NKT, and NK 1 nannofossil zones were determined. The occurrence of Nannoconus kamptneri minor, one of the potential marker-types of the Tithonian–Berriasian boundary (the base of the NKT Zone) of the Tethyan sequence in the Feodosiyan section is assigned here to the Pseudosubplanites grandis ammonite Subzone and the magnetic Chron M18n. The base of the NKT Zone is closer to the Grandis Subzone base than to the base of the Jacobi Subzone. Contradictions in the interpretation of magnetic chrons obtained by the present authors (Arkadiev et al. 2018) and by Bakhmutov et al. (2018) might be caused by mistakes admitted in the latter work on the compiled section.

Previous studies made in different parts of the world have shown that Barremian–Aptian times imply many difficulties in deciphering the biostratigraphy, microfossil evolution and correlation of bioevents. In an attempt to improve our knowledge of this period in a particular area of the Tethyan realm, we present the first integrated study of microbiota (including planktonic foraminifera, calcareous nannofossils, ostracods and palynomorphs) and magnetostratigraphy of the upper Barremian–Aptian sediments from south-eastern Crimea. The nannofossils display the classical Tethyan chain of bioevents in this interval, while the planktonic foraminifera demonstrate an incomplete succession of stratigraphically important taxa. Our study enabled the recognition of a series of biostratigraphic units by means of four groups of microfossils correlated to polarity chrons. The detailed analysis of the microfossil distribution led to a biostratigraphic characterization of the Barremian/Aptian transition and brought to light an interval, which may correspond to the OAE1a.

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