The Ordovician of the Siberian Platform exhibits a wide range of shallow-water facies in thick, normal marine and lagoonal carbonate-clastic sequences. It is illustrated herein by two west-east geological profiles across the Siberian Platform. Stratigraphic and paleontologic studies in this area led to subdivision of the Ordovician into 12 regional stages. Their boundaries correspond to changes in sedimentology and in taxonomic composition of a benthic fauna dominated by brachiopods and ostracodes. In most cases the precise correlation of regional with global stages is difficult, mainly because of the endemic character and sparse distribution of the East Siberian fauna in shallow-water facies, as well as the sporadic occurrences of graptolites. Two important events at the beginning of the Volginian (the Hustedograptus teretiusculus Zone) and Chertovskian (Nemagraptus gracilis Zone) are documented in the Ordovician of the East Siberian Basin. They include large-scale transgressions and significant biotic changes, and are used as the most reliable levels for correlation in the vast area of the Siberian Platform and the Verkhoyano-Chukotka region. The distribution of lithofacies and dominant members of faunal communities are figured on three maps for the Nyaian (Early Ordovician), Volginian (Middle Ordovician), and Bak-sanian (Late Ordovician). Comparative analysis of the Ordovician benthic faunas of the Siberian Platform, Taimyr, Novosibirsk Islands, and the Verkhoyano-Chukotka region shows that they all belong to the Kolymo-Siberian paleobiogeographic province. Available data confirm the location of the Siberian paleocontinent in the Southern Hemisphere during most of the Ordovician and an inverted orientation relative to its present position.

Analysis of Mid-Palaeozoic successions in the northern part of the Alai Range (Kyrgyzstan and bordering Uzbekistan), Southern Tien Shan, Central Asia, has identified a Silurian-Devonian deep-marine depositional system of basin-slope facies-associations. Here, we document the stratigraphy and sedimentology of a region in Central Asia that, through conflict, has become inaccessible for geological research. The turbidite-dominated Pul'gon Formation (Silurian) accumulated in sea-floor depressions and within the inferred basin axis. The large-scale, coarse clastic lenses of the Dzhidala Formation (mostly Devonian) represent the fills of submarine channels, canyons or gullies of the palaeoslope; other slope apron processes include sediment slides, debris flows and olistoliths. The partly time-equivalent condensed sequences of the Mid- to Late Silurian Kursala Formation and Early Devonian Tamasha Formation represent graptolitic mudstone and chert accumulation, respectively, together with thin dolomitic limestones, that accumulated over c. 10-15 Ma, probably on seamounts in the Turkestan (Fergana) Ocean between the continental margin of Kazakhstania and the Alai microcontinent. The graptolitic shale-rich Chakush Formation (early Silurian) is geographically and petrographically different from the Pul'gon and Dzhidala formations and suggests a different provenance (opposing continental margin or seamount talus).

The Skala horizon of Podolia has been under discussion as a possible candidate for the stratotype of the fourth Silurian series (primary subdivision) in the international standard chronostratigraphical scale. After a brief survey of the history of investigation, the section through the basal boundary of the Skala horizon is fully documented. The lithostratigraphy, biostratigraphy, and correlation of the Skala horizon are then discussed.