The regional stratotype section and point for the base of the Hirnantian Stage (the uppermost Ordovician) at Mirny Creek, Omulev Mountains, Northeast Russia

Abstract

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.

More about the Ordovician–Silurian transition beds at Mirny Creek, Omulev Mountains, NE Russia: carbon isotopes and conodonts

“…The sedimentary strata are partly strongly folded and faulted, some intervals are covered and some contacts are tectonic (Oradovskaya & Sobolevskaya 1979). Still, the primary authors cited above and Koren′ & Sobolevskaya (2008) showed that the succession of the study interval is not seriously disturbed at key levels and is biostratigraphically continuous. The conodont colour alteration index indicates (Nowlan & Barnes 1987) that the rocks are thermally altered.…”

“…Chitinozoans, except rare poorly preserved fragments, were not found. Shelly fossils like brachiopods and trilobites are quite common at some levels, but graptolites play a leading role in biostratigraphy and are used for the establishment of a clear general biozonal framework of the Mirny section (Koren′ & Sobolevskaya 2008). Despite the occurrence of mixed shelly-graptolite assemblages in the studied section, detailed correlation with shelly faunal successions in the Baltic and some other regions is problematic due to scarcity of microfossils.…”

“…The study history and the modern state of knowledge about the section were reviewed by Koren′ & Sobolevskaya (2008). Based mainly on that paper and our new carbon isotope and conodont data, we will discuss some aspects of regional stratigraphy and a couple of more general topics of the uppermost Ordovician isotope geology, first of all differences in patterns of the δ 13 C trends in Avalonia, Baltica, Laurentia, the South China plate and Kolyma terrain, correlation of bio-and chemostratigraphic events, and dating of carbon isotope excursions in terms of biostratigraphy.…”

Uppermost Ordovician (upper Katian-Hirnantian) graptolites of north-central Nevada, U.S.A.

“…Only a single occurrence of a diplograptine taxon, Paraorthograptus cf. pacificus, was found in the lowest Hirnantian strata (Koren' & Sobolevskaya 2008). On the other hand, Normalograptus ojsuensis is restricted to strata of the uppermost pacificus Subzone.…”

“…-In the Mirny Creek region (Omulev Mountains) of NE Siberia Koren' et al (1983) recognized the Appendispinograptus longispinus and Paraorthograptus pacificus subzones within upper Katian A. supernus Biozone. The biostratigraphy of the uppermost Katian to lowest Rhuddanian part of the succession was updated and slightly revised by Koren' & Sobolevskaya (2008). Only eight sample levels were reported in the supernus Biozone at Mirny Creek (only two in the lower subzone), which does not allow for high resolution correlation.…”