Revised ammonite succession of the Upper Volgian of Nordvik section: zonal boundaries and uncertaintiesкод для вставки
Revised ammonite succession of the Upper Volgian of Nordvik section: zonal boundaries and uncertainties Rogov M.A., Alifirov A.S., Igolnikov A.E. Russia; Geological Institute of RAS, Moscow; Trofimuk Institute of Petroleum Geology and geophysics of SB RAS, Novosibirsk Geographic position of Nordvik section (Northern Siberia) Biostratigraphical and paleoecological characteristics of the JurassicCretaceous boundary interval at Nordvik Biostratigraphy, palaeomagnetic and geochemical characteristics of the Jurassic-Cretaceous boundary interval at Nordvik Jurassic-Cretaceous boundary in the Nordvik section Boundaries of the Upper Volgian zones in Nordvik were not coincides with ranges of their index taxa, but historically defined at the base of the members characterized by zonal assemblages Nordvik Kheta Type sections of the Upper Volgian Siberian ammonite zones are located at the Kheta river basin. Their description (Sachs et al., 1969) was unclear, and information about position of ammonite records within thick members was missing Upper Volgian deposits located along the Kheta river now are covered by glacial boulders and hardly accesible Type section of the Taimyrensis zone Criteria for definition of zonal boundaries Okensis zone: lower boundary defined by FAD and upper boundary by LAD of Craspedites (C.) ex gr. okensis Taimyrensis zone: lower boundary is coincides with LAD of Craspedites (C.) ex gr. okensis and FAD of C. (Taimyroceras) ex gr. taimyrensis, upper boundary defined by FAD of Chetaites chetae Chetae zone: its boundaries defined by range of the index species Chetaites chetae Upper Volgian ammonites of the Nordvik section Craspedites (C.) praeokensis Rogov (in msc) Upper Volgian ammonites of the Nordvik section Craspedites (C.) okensis (d’Orb.) Upper Volgian ammonites of the Nordvik section Chetaites cf. chetae Schulg. Craspedites (Taimyroceras) sp. nov. Cretaceous Jurassic Conclusions Subdivision of the Upper Volgian Substage of the Nordvik section by ammonites, even including data about the newly recorded specimens, remains unclear in the Okensis – Taimyrensis transitional interval, which is lacking ammonite finds. Thus the position of the Jacobi Zone lower boundary in terms of Boreal ammonite zones is also indefinite, as this level could be either in the uppermost partof the Originalis Subzone of the Okensis Zone or in the lower part of the Taimyrensis Zone. Additional palaeomagnetic studies of ammonites-rich Boreal sections are strongly required, and the sections of the Subpolar Urals, Spitsbergen and the central part of the Russian Platform are among the most awaiting aims for further investigations. Thanks for attention!