BOREAL ZONAL STANDARD AND BIOSTRATIGRAPHY OF THE SIBERIAN MESOZOIC

We offer a boreal zonal standard scale in order to achieve an effectiv e panboreal correlation and efficient geological dating of Mesozoic bo real sediments. The necessity for establishment of boreal standard is caused by difficulties of the direct zonal correlation between most bo real stratigraphic sections with the Mesozoic type sections that are l ocated in Western Europe. The standard section of boreal Mesozoic repr esents the most complete succession of mollusk (mainly ammonites) zone s that are known on the territory of the boreal realm. It is composed of fragments of zonal successions that are established on more than 30 Triassic, Jurassic, and Cretaceous sections from Northern Eurasia, No rth America and Greenland. Boreal standard of the Triassic system cons ists of 36 ammonite zones and 2 bivalve zones; Jurassic -- 70 ammonite zones; Cretaceous -- 37 zones, subzones and beds on ammonite, 6 bival ve zones, and 11 zones and subzones on belemnites. Siberia and Northea stern Asia provide stratigraphic sections many of which are the best i n the boreal realm regarding their stratigraphic completeness and deta iled zonal subdivision. In addition, there is a type section of the Ol enekian stage and one of the most complete boreal sections of the Indi an stage. Infrazonal scales are constructed for both Indian and Olenek ian stages as well as for the Middle and Upper Anisian. Successions of ammonite zones from Hettangian, Sinemurian, Toarcian, and Upper Plien sbachian sections, which are located in Northeastern Asia as well as L ower Oxfordian substage from Northern Siberia and Kimeridgian stage of Subpolar Urals, are selected as boreal standards. There is a more com plete ammonite zonal scale of the Upper Volgian substage in Northern S iberia than that of the Gorodishche type section from Volga River regi on. The most complete and continuous successions of ammonite zones in boreal Berrisian and Valanginian are established in northern Siberia. They are accepted as a composite boreal standard. Studies of the North -Siberian Upper Cretaceous sections in stratigraphical interval from U pper Cenomanian to Santonism-Campanian boundary revealed equivalents o f all the East- and West-European Inoceramid zones. In addition to amm onoid zones for all the Mesozoic systems in Siberia and Northeastern A sia, we used the same sections to construct independent parallel zonal scales on the parastratigraphical groups of fauna and flora. We also constructed a bivalve-based <<through>> scale which was incorporated i nto the boreal standard for some intervals such as Upper Triassic and Upper Cretaceous. For the entire Triassic interval we offer zonation b ased on nautiloids as well as those on conadonts and foraminifera for some other intervals. For Jurassic interval we developed a system of ? he following biostratigraphic scales: on belemnites, foraminifera, ost racods, dinocysts, spores and pollen, Dinocysts and foraminifera scale s are available for the Cretaceous interval. In combination, the paral lel scales mag. provide a very detailed regional correlation. However, many of them like a zonal bivalve-based scale make it possible to ach ieve a direct panboreal, and even boreal-peritethyan correlation for t he intervals such as Upper Triassic, Lower Liassic, Upper Jurassic, Ne ocomian, and Upper Cretaceous. The offered boreal Mesozoic zonal stand ard needs an improvement and calibration. In particular this is true f or a greater part of a zonal scale of Cretaceous system. Nevertheless, even in the present version it is useful as a tool to improve panbore al and boreal-tethyan correlation, It is also particularly useful far operative stratigraphic work on boreal sections in specific regions.