Abstract
Ordovician deposits of northeastern USSR are represented by several types of sediments. Carbonate rocks are the most widespread, then terrigenous sediments with graptolites, and volcanic-terrigenous facies with graptolites and parts of corals and brachiopods. The bases for correlations are the sections of transitional lithologic type with corals, trilobites, ostracods, brachiopods, and graptolites. The Tremadocian Stage is identified only in the carbonate sections. The Arenigian deposits are carbonate sediments with distinctive faunas that make it possible to correlate them with the Chunya Stage of the Siberian platform and the Beekmantown of North America. They also correlate with graptolite-bearing terrigenous-carbonate sections. The Llanvirnian includes terrigenous and carbonate lithologic types that extend as far as the Chukotsk Peninsula. The strata have numerous graptolites, which make it possible to correlate across the entire region and with equivalent deposits in England, North America, Australia, and China. The Llandeilian contains predominantly benthonic faunas and some graptolites of the Glyptograptus teretiusculus zone. This stage is definable only where complete sections are present and its boundaries are traced rather conditionally. The Caradocian Stage has the widest distribution. It may be divided into three substages by graptolites and confidently correlated with deposits of other continents. Of peculiar interest are the Ashgillian deposits (the typical section in the Omulevka Mountains). There rich benthonic faunas (corals and brachiopods) permit correlation with the 5b zone of Norway, the Ashgillian of England, and the uppermost parts of the Ordovician of North America. In the Mirnyy Creek section of the Omulevka Mountains, a detailed study was made of the successive transition from Ordovician to Silurian that is marked by the graptolite zones of Dicellograptus ornatus, Climacograptus supernus, Akidograptus ascensus, and A. acuminatus. End_of_Article - Last_Page 2499------------
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