THE LOWER BOUNDARY OF THE FRASNIAN STAGE IN THE RUSSIAN PLATFORM

The position of the conodont Lower asymmetricus Zone in the Devonian s ection of the Russian platform is discussed. The International Subcomm ission on Devonian Stratigraphy suggested the base of this zone to be the lower boundary of the Frasnian Stage (and of the Upper Devonian, c orrespondingly). Previously, the base of the Lower asymmetricus Zone w as correlated with the boundary between the Timan and Sargai regional horizons. However, analysis of conodonts from stratotypic sections of the Timan Formation (the stratotype of the Timan regional horizon) and the Ust'-Yarega Formation (corresponding to the Sargai regional horiz on) of the southern Timan reveals the following: (1) The species compo sition of the conodont assemblage from the upper part of the Timan For mation is similar to that from the lower part of the Ust'-Yarega Forma tion. Both assemblages correspond to the shallow marine Polygnathus bi ofacies. The studied section interval is correlative with the Lower as ymmetricus Zone. (2) Conodonts Ancyrodella rugosa, A. mouravieffi, and A. alata, typical of the upper part of the Lower asymmetricus Zone, w ere found in the upper beds of the Ust'-Yarega Formation. Thus, the lo wer boundary of the Lower asymmetricus Zone (and the lower boundary of Upper Devonian) should be placed within the Timan Formation (Timan re gional horizon).