|mhorn||Дата: Wednesday, 15.06.2011, 15:14 | Сообщение # 1|
|Pestchevitskaya E., Lebedeva N., Ryabokon A. (2011) Uppermost Jurassic and lowermost Cretaceous dinocyst successions of Siberia, the Subarctic Urals and Russian Platform and their interregional correlation // Geologica Carpathica, Vol.62, no.3, P.189-202 |
final version of pdf available through web-site http://www.geologicacarpathica.sk after registration
Uppermost Jurassic and lowermost Cretaceous dinocyst successions calibrated against ammonite and foraminiferal zones were studied in five sections from North Siberia, the Subarctic Urals and the Russian Platform. Together with analysis of published palynological material on additional contemporaneous sections from the Russian Platform, our research provides a reliable regional correlation. The obtained biostratigraphic results are compared to palynological data from different regions of Europe, America, Australia and Antarctica using the method of first/last appearances of selected key species and evolutionary trends of dinocyst floras. Four correlative levels are defined in the middle parts of the Volgian and Berriasian and near their tops providing interregional correlation of dinocyst successions. These levels range within 1-1.5 ammonite zones as the first/last appearances of some key species and minor stratigraphic shifts in different sections. Unfortunately, problem of the J/K boundary, Boreal-Tethyan correlation and usage of the different stage names for Boreal and Tethyan successions are not discussed in this article. Instead authors showing 3 J/K boundary levels for Boreal succession, indicating that all these levels drawn by paleontological data. These are: 1) Volgian/Ryazanian (Boreal Berriasian) boundary. Coincidence of the boundary with its Tethyan counterpart were based in the past on identification of some Tethyan ammonite genera (Virgatosphinctes and Berriasella) from the Upper Volgian of Siberia, but during at least two last decades these determinations considered as erroneous and Siberian ammonites re-interpreted as dorsoplanitids (Praehaetaites genus). The next J/K boundary level is showing in the base of the Upper Volgian, as it follows from ISC decision. But all paleontological evidences for such correlation are indirect and coincidence of these boundaries were supported by erroneous opinion about strong changes in ammonite assemblages in both Boreal and Tethyan areas at these levels. The third J/K boundary level (within the Taimyrensis ammonite zone) was not based on paleontology, but on the paleomagnetic correlation. As authors just showing these 3 boundaries without any explanation on their on opinion, it leads to confusion. The same problem exists with J/K boundary in the Russian Platform – only different levels without explanations are shown, sometimes accompanied by erroneous data (such as “ammonite evidences” for J/K boundary in the base of the Fulgens Zone in Hantzpergue et al., 1998). Much more confusion appears from the usage of the incredible stage names. The single article includes 5 different combinations of stage names! These are: 1) Volgian-Berriasian (for Siberia! Authors even indicating Lower, Middle and Upper Berriasian), 2) Volgian – Ryazanian (for the Russian Platform); 3) Portlandian – Ryazanian for Denmark, Norway and England (this strange combination differ from those used by authors for these countries, who prefer Volgian/Ryazanian (Casey, 1973; Abbink et al., 2001) or Portlandian-Berriasian (Hunt, 2004; Wimbledon, 2008)); 4) Tithonian – Ryazanian (for N. America); 5) Tithonian – Berriasian for Tethyan Europe, Australia and Antarctica. Really two combinations is enough: Volgian/Ryazanian for Boreal and Tithonian/Berriasian for Tethys. Unfortunately, two LADs of dinocyst taxa, which are considered by Berriasian WG as J/K boundary markers (LAD of Dichadogonyaulax pannea and Egmontodinium polyplacophorum) and their correlational potential are not discussed.
Middle Jurassic - Lower Cretaceous ammonites & aptychi