A complete Hirnantian sequence comprising the Normalograptus extraordinarius and N. persculptus biozones is well developed at the Mirny Creek section in the Omulev Mountains. The underlying beds are assigned to the Appendispinograptus supernus Biozone, and in the overlying strata the lower boundary of the Silurian is precisely defined at the base of the Akidograptus ascensus Biozone. Due to the completeness of the sedimentary and palaeontological record, the Mirny Creek section can be considered as a reference section for the Hirnantian Stage. The succession, about 100 m thick, is composed of calcareous siltstones and marls with pebble-shaped limestones, deposited at a high sedimentation rate in shallow shelf settings. The regional stratotype section and point (RSSP) for the lower boundary of the Hirnantian is established at the base of member 68, where Normalograptus extraordinarius first appears. This level can be precisely correlated with that at the GSSP section in Yichang and with the sections in Kazakhstan and North America. The position of the Ordovician-Silurian boundary is redefined and placed at the FAD of A. ascensus at the base of member 74.
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Koren’, T. N., Sobolevskaya, R. F., Mikhailova, N. F. & Tsaj, D. T. 1979. New evidence on graptolite succession across the Ordovician–Silurian boundary in the Asian part of the USSR. Acta Palaeontologica Polonica, 24, 125–136.
Koren’, T. N., Oradovskaya, M. M., Põlma, L. J., Sobolevskaya, R. F. & Chugaeva, M. N. 1983. The Ordovician and Silurian boundary in the Northeast of the USSR. Trudy Mezhvedomstvennogo Stratigraficheskogo Komiteta SSSR, 11, 1–205 [in Russian].
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Oradovskaya, M. M. & Sobolevskaya, R. F. 1979. Field Excursion Guidebook on Tour VIII. XIV Pacific Science Congress, Khabarovsk, August. Magadan Publishing House, 103 pp.
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