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The Tommotian Stage Base as the Cambrian Lower Boundary in Siberia
V. V. Khomentovskii and G. A. Karlova
Institute of Oil and Gas Geology, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia
Date article received August 29, 2004
Key words: stages, zones, small-shelled fossils, zonal assemblages, facies, biostratigraphy, correlation, isotopic chemostratigraphyThe Cambrian lower boundary defined at the Tommotian Stage base in Siberia is more plausible than that accepted in the International Stratigraphic Scale. Disadvantages of Siberian sections, which were referred to when the boundary stratotype was selected in Newfoundland, are overstated in fact. A supposed considerable gap in sedimentation between the Tommotian and Nemakit-Daldynian stages was considered as principal shortcoming of Siberian successions. As is established recently, however, the gap at this level in the Siberian platform is not larger than one biostratigraphic zone and cannot be recognized based on paleontological records. The Cambrian lower boundary in Siberia is well substantiated based on small-shelly fossils used to depict in detail the Vendian–Cambrian biostratigraphy. In distinction from the international standard, the boundary level established in Siberia is consistent with available data on the C-isotope chemostratigraphy. The other drawbacks of the international standard are as follows. (1) It is of a doubtful paleontological substantiation. (2) A possibility that biozones of skeletal fossils can be extended downward by zonation of ichnofossils is unproved. (3) Boundaries of the Nemakit-Daldynian Stage identified with the Manykaian Stage are defined incorrectly. (4) The uncertain position of the Nemakit-Daldynian lower boundary means that the Ediacarian Stage is largely or entirely above this level. At present, the Tommotian lower boundary is the reliable oldest level of a high biostratigraphic rank, which is adequate for establishing and tracing the Cambrian lower boundary. References 1. S. A. Bowring et al., “Calibrating Rates of Cambrian Evolution,” Science 261, 1293–1296 (1993).
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