Corals and Brachiopods of the Siluro-Devonian boundary beds of western Siberia

Abstract The beds of the Siluro-Devonian boundary in western Siberia contain an abundant brachiopod and rugose coral fauna, as in the underlying Silurian. Here, at the southwestern edge of the Kuznetsk basin and in the northern Altai (Tom-Chumysh and Borovushkina sequences, Kamyshinka limestone), the Siluro-Devonian limestones form a continuous succession with Silurian strata below and Devonian above. The brachiopod and coral assemblages differ from those of the known Ludlovian below and from the Krekov sequence above. The rugose corals are represented by three groups: genera persisting from the Silurian (Phaulactis subcyathophylloides Zhelt., Spongophylloides dubroviensis Zhelt., Philophyllum angustum Zhelt., P. insolutum Zhelt., Stortophyllum subcruciatum Zhelt., Petrozium); genera diagnostic of the Siluro-Devonian (Zelophyllum subdendroideum Zhelt., Tryplasma karcevi Bulv., Tr. tomchumyshensis Zhelt., Tr. altaica (Dyb.), Tr. hercynica (Peetz), Neomphyma roziformis Zhelt., Mycophyllum); and Devonian genera (Spongophyllum, Fasciphyllum). The establishment of the position of this assemblage in the biostratigraphic scale, and comparison with the rugose coral faunas of contemporaneous deposits of the other regions is difficult because of the generally scanty knowledge of the rugose corals of this stratigraphic interval. The following brachiopods are typical of, and widely distributed in, the Tom-Chumysh sequence (southwestern edge of the Kuznetsk basin): Schizophoria striatula Schloth., Gypidula pelagica (Barr.), Stropheodonta (Brachyprion) cf. costatula (Barr.), Chonetes mediocostalis Kozl., Lanceomyonia borealiformis (Siem.), Camarotoechia daphne profunda Rzon., Atrypa lazutkini Aleks., Howellella laeviplicatus Kozl., Altajella contorta Kulkov, Protathyris praecursor Kozl., P. sibirica Zintch. The Borovushkina sequence contains, in addition, Resserella aff. elegantuloides (Kozl.), Spirigerina marginalis (Dalm.), Eospirifer radiatus Sow., Gypidula nux Khod. Representatives of the genera Protathyris, Lanceomyonia and Resserella are essentially Silurian; the remainder range from Silurian into Devonian. The beds of the Siluro-Devonian boundary of western Siberia (the Tom-Chumysh zone), on brachiopod evidence may be compared with the Borshov and Tchortkov horizons of Podolia, the Lochkov Stage of Czechoslovakia, and the Ispharian and Kunzhak zones of Central Asia. On the basis of both rugose corals and brachiopods, equivalency with the lower part of the Petropavlovsk sequence of the Urals is established. These and equivalent deposits are unequivocally Gedinnian in age, and probably Early Gedinnian, at least in the case of the carbonate facies. The Siluro-Devonian of western Siberia comprises, faunally, a mixture of Silurian and Devonian faunal elements with a preponderance of Silurian elements insofar as the rugose corals are concerned.