This paper presents the results of a detailed study of the Bajocian-Bathonian sections in the "Sokur" Quarry near the city of Saratov and of a borehole drilled in the same quarry. The study of these Bajocian-Bathonian boundary beds, unique for the Boreal-Tethyan ecotone, offers a solution to the much-debated problem of the Middle Jurassic Boreal-Tethyan correlation. The Bajocian-Bathonian boundary beds of this section contain both Peri-Tethyan and Boreal ammonite species, allowing recognition of the Boreal ammonite zones and associated "Boreal" Bathonian zones based on belemnites, bivalves, and foraminifers. This integrated biostratigraphy allows correlation of the Late Bajocian-Early Bathonian boundary interval of the Boreal Jurassic scale with the Submediterranean standard scale.E eschweizerbart_xxx 96 V. Mitta et al.

Пополнена палеонтологическая характеристика рязанского регио яруса (нижняя часть нижнего мела) в разрезах страторегиона (бассейн р. Ока в окрестностях г. Рязань, Центральная Россия) на основе исследования головоногих и двустворчатых моллюсков. Полученные сведения важны для анализа стратиграфического положения в суббореальных разрезах таксонов, известных в Арктике, и последующей качественной разработки зонального уровня шкал по белемнитам и бухиям приграничного юрско-мелового интервала Русской плиты. Ключевые слова: рязанский региоярус, белемниты, двустворчатые моллюски, Русская плита, биостратиграфия.

No abstract

The Maurynya section exposed on the northwestern margin of West Siberia is one of few continuous sections of the Jurassic–Cretaceous boundary deposits in Boreal regions. In the meantime, it can be considered a reference section for the transitional Volgian–Ryazanian beds formed in shallow water environments of the West Siberian sea basin. This study is a synthesis of the new data with previous results of lithologic, paleontological, biostratigraphic, and (O, C, Sr) isotope studies of the Upper Volgian–lowermost Ryazanian of the Maurynya section. It has been established that the beginning of the Cretaceous (corresponding to the middle Late Volgian) is associated with a sharp increase in species number and diversity of cephalopods and bivalve mollusks on the northwestern margin of the West Siberian sea basin. This can be explained by an increase in its depths and their stabilization at a level which appeared optimal for the habitat of nekton, nektobenthos, and benthic fauna communities, superimposed on the general trend of warming. At the same time, the percentage of phytoplankton significantly increased, indicating the onset of transgression, which affected largely coastal landscapes and type of vegetation: The forests dominated by conifers were gradually succeeded by forests mostly composed of Ginkgoaceae.

The discovery of Tethyan and Boreal fossils in Bathonian deposits of the Sokur section in the vicinity of Saratov (Central Russia) reopened the possibility of direct correlation of the Bathonian sections in different paleobiogeographic provinces, which require further integrated studies involving both paleontological (based on different groups of fauna) and physicochemical (paleomagnetic and isotope-geochemical) methods. Stable carbon isotope data for belemnite (Cylindroteuthididae) rostra from the Sokur section show that the Subboreal Oraniceras besnosovi Zone and Boreal Arcticoceras harlandi Subzone are correlated with the Zigzag Zone of the standard scale, whereas the Boreal A. ishmae Subzone can be correlated completely or, even partly, with the Tenuiplicatus Zone. We identified equivalents of chrons of the geomagnetic polarity time scale (GPTS): e-Bath N, m-Bath R, and lt-Bath N, corresponding to the Lower–Middle Bathonian and, possibly, lowermost Upper Bathonian.