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Revised ammonite succession of the Upper Volgian of Nordvik section: zonal boundaries and uncertainties

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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!
Автор
Nastya
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