Volgian and Ryazanian Stages in the Novoyakimovskaya-1 Well (Western Yenisei-Khatanga Regional Trough, Siberia). Article 1. General Characteristics of the Yanov Stan Formation and Its Molluscan Biostratigraphy

We represent data on lithology, biostratigraphy, and molluscan paleontological characteristics of the Lower Volgian–Upper Ryazanian interval of the Yanov Stan Formation obtained from the core of the Novoyakimovskaya-1 parametric borehole. The formation is represented by the intercalation of mudstones and siltstones with rare sandstone beds and numerous levels with carbonate nodules. For the first time, numerous glendonites are reported from the Yanov Stan Formation, including those from stratigraphic intervals from which they have not been previously known. A total thickness of the Janov Stan Formation is 290 m with almost no missing intervals of the core, and ammonites and bivalves are common throughout the section. This enabled us to work out a detailed biostratigraphic subdivision of the section based on ammonites and bivalves. The following succession of ammonite zones known in Western Siberia and, mainly, in Eastern Siberia (from bottom to top) was established: Paravirgatites lideri, Pavlovia iatriensis, Dorsoplanites ilovaiskii, D. maximus, Epivirgatites variabilis, Praechetates exoticus, Craspedites okensis, Praetollia maynci, Hectoroceras kochi, and Surites subanalogus. The bivalve succession, based on the members of the genus Buchia, includes the following zones: Buchia mosquensis, B. unschensis, B. okensis,? B. jasikovi, and B. volgensis. The molluscan assemblages of the studied interval, starting at least from the latest Middle Volgian, are similar to the assemblages of the eastern part of the Yenisei-Khatanga depression and the Laptev Sea coast but differ from the coeval complexes of Western Siberia. This suggests the existence of some environmental factors that prevented the free dispersal of mollusks from the western part of the Yenisei-Khatanga depression into the West Siberian basin.