Ordovician deposits occur in marginal uplifts of the Kolyma medial massif (different facies zones of the Omulevka Mountains, Selennyakh Ridge, Tas-Khayakhtakh Ridge), middle part of the Kolyma River, in the Verkhoyansk fold-mountains region (Sette-Daban Ridge), in the Chukotsk Peninsula. Recent data indicate the presence of Ordovician in the Omolon and Okhotsk medial massifs. A correlation chart was prepared, and it shows the correlation of Ordovician deposits of all principal regions of the northeastern part of USSR by ostracod, brachiopod, trilobite, and graptolite associations. The biostratigraphic zones corresponding to the Upper Ordovician of the international stratigraphic standard are accepted as principal units of regional stratigraphic importance. However, the st atigraphic positions of some biostratigraphic suites have been revised. This resulted in the revision of the contents and names of some zones.

The oldest Ordovician deposits (Inaya horizon) are correlated conditionally with the Tremadocian of England, the Gasconadian Stage of North America, and the Ust'kut Stage of the Siberian platform. The correlation is based on brachiopod and trilobite associations and stratigraphic position. The Khita horizon is compared with Beekmantown Stage of North America and the Chunya Stage of the Siberian platform on the basis

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of trilobite and brachiopod associations. The Elgenchak horizon is comparable with the Llanvirnian on the basis of graptolite associations, and is comparable with the Whiterock Stage and probably a part of the Marmor Stage of North America on the basis of the brachiopods. The Lachug horizon corresponds to the Llandeilian; it definitely is comparable with the Krivoluk Stage of the Siberian platform and is conditionally compared with Ashby Stage of North America. The Lachug horizon is characterized by rich graptolite, brachiopod, ostracod, and trilobite associations. The Kharkindzha horizon unites heterofacies deposits and corresponds to the lower and middle Caradocian of England, the Mangaseya Stage of the Siberian platform, and conditionally to the Porterfield Stage of North America. he base of this zone is the most distinctive faunal boundary in the Ordovician of northeastern Asia and is characterized by the appearance of rich graptolite faunas of the Nemagraptus gracilis zone and by numerous ostracod, brachiopod, and trilobite associations. The Omuka horizon correlates with the deposits corresponding to upper Caradocian and Ashgillian. The upper part of the Omuka horizon definitely is comparable with the Ashgillian on the basis of graptolite associations, and with horizon 5b of Norway on the basis of corals and brachiopods.

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