The structure of the Lower–Middle Ordovician Tuloi and Karasa Formations, which occur widely in northeastern Gorny Altai, has been studied. A bed-by-bed description of a new section of the boundary sediments between these stratigraphic units has been given. The graptolite, trilobite, and brachiopod assemblages typical of the Tuloi and Karasa Formations have been analyzed. Graptolite zones and subzones have been recognized, which permit correlating the stratotype and key sections of these local stratigraphic units with the stages of the International Stratigraphic Chart (ISC). The Karasa Formation occupies a considerably larger stratigraphic volume than presumed: It spans the late Arenig and the entire Llanvirn (including the Llandeilo) of the British standard, which is used in Russia (=Upper Dapingian–Darriwilian of the new Ordovician standard in the ISC). Finally, the paper discusses the suggested changes in the names of regional stratigraphic units (horizons) in the Ordovician stratigraphic chart of the western Altai–Sayan fold belt.

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