Litho- and biostratigraphic analysis of a number of Ordovician reference sections has been completed in five different blocks of Salair, including West-Berd' (Medvedkovo), East-Berd', Krasnoe, Gurievsk and Eltsovka. Modern data on nine mainly terrigenous and, less often, carbonate-terrigenous sections bearing fossils and composed of carbonate units and volcanic rocks including lavas, as well as tuff and tuff sandstones have been presented. The Ordovician terrigenous sedimentation in the study area was complicated by stages of volcanic sedimentation. Volcanic-sedimentary sequences were documented at four biostratigraphically dated stages in Salair: 1) the Tremadocian, 2) the late Floian, 3) the middle - late Darriwilian, 4) the late Katian - Hirnantian.

The territory of the Gorny Altai includes a wide range of bedrock formations from the Ordovician period. The composition and origin of these rocks also varies widely, with both terrigenous layers (from argillite to conglomerate) and carbonate beds (limestone, dolomite and transitional forms) being known. For example, reef-forming bodies are exposed in the western and northwestern parts of the Gorny Altai, while normal-laminated limestone is recorded in large numbers in the northeastern part of the Gorny Altai. The Ordovician of the Gorny Altai has been studied for a long time, but there is very little information about calcareous algae in the literature. However, during field work, rock material was collected from several sections of the northeastern Gorny Altai ("Biya", "Lebed'", and "Bura"). Petrographic thin sections were made from this material. Their study showed a high content of calcareous algal remains in these outcrops. The detected remains were identified as Girvanella Nicholson et Etheridge, 1878, Vermiporella Stolley, 1893, Contexta Gnilovskaja, 1972 and Solenopora Dybowsky, 1878. In addition to these calcareous algal remains, invertebrate bioclasts such as trilobites, ostracods, Acorals, bryozoans, Ashell brachiopods, gastropods and echinoderm fragments were found in the studied strata. In addition to identifying calcareous algae, carbonate rock structures (as classified by R. ADunham with additions) and carbonate microfacies have been identified. It has been determined that microfacies of polybioclastic wackestones to packstones, algal-peloid packstones, polybioclastic-oolitic packstones to grainstones, trilobite-algal wackestones to packstones, Aand recrystallized silty limestones mayAbe present in these sections. Good and average preservation of calcareous algae finds allowed us to perform their brief paleontological description and to establish that some Gorny Altai species are closely related to the apparent remains described in the Ordovician of China, East Kazakhstan, Norway, Scotland and other localities. AThus, the remains of the genus Solenopora were identified as Solenopora aff. spongoides Dybowski, Vermiporella was identified as Vermiporella aff. diffluens Gnilovskaja, and the preserved calcareous covers of Girvanella were identified as Girvanella problematica Nicholson et Etheridge, 1878. Upper Ordovician sedimentation is assumed to have occurred near the source of organogenic remains. However, it is possible that the source of clastic material was located distant from the deposition basin. At the same time, there is evidence that the energy of transport agents may have increased, leading to the formation of clastic packs. A model for the possible formation of these strata is proposed.

We studied a collection of the Silurian (Rhuddanian) multi-branched dendroids, including reticulate, benthic dendroid graptolites of the Callograptidae Hopkinson (in Hopkinson and Lapworth, 1875), from the Gorny Altai, Russia. Species of Dictyonema Hall, Koremagraptus Bulman, Callograptus Hall, and Desmograptus Hopkinson were identified and described. The graptolites are represented by in situ burial of colonies with 3D preservation of tubaria (= rhabdosomes). Complete (intact) colonies of dendroid graptolites are characterized by successive development stages: juvenile, neanic, ephebic (adults), while the general configuration of the tubarium changed from the proximal to distal. In a number of cases, later stage of colony development — the gerontic stage (senilis) is found. We carried out morphological analysis of dendroid graptolite tubaria (Callograptidae) of various development stages, showing eight morphotypes of the colonies, namely, the cylinder, disc, disc on a stem, hemisphere, cone, wineglass, blooming flower, and funnel type.

--New and revised previous data based on detailed description of sections have provided better constraints on fauna taxonomy, lithology, and stratigraphy, as well as on biostratigraphic ages of sediments. The lithology and stratigraphy of several key Upper Ordovician sections in the northern Gorny Altai area show that the local units of Sandbian and Katian Savelieva and Bulukhta formations can be used as reference for the regional Ordovician division. It is suggested to include a newly distinguished Bulukhta facies zone, an area of Late Ordovician coarse clastic deposition, into the mapped Ordovician deposits of Gorny Altai. According to paleogeographic reconstructions, the deposition occurred in a large sea gulf surrounded with elevated landmasses exposed to erosion.

New paleontological and biostratigraphic data were obtained for a series of Middle Ordovician sections in the Charysh-Inya and Uimen’-Lebed’ structural-facial zones (SFZ) of the Gorny Altai. Regional stratigraphic stages, namely, the Kuibyshevo and Kostinsky horizons, are discussed in detail. By graptolites, the complete stratigraphic volume of the Kuibyshevo regional unit is defined as following zonal succession: (1) sinodentatus/Cardiograptus/Oncograptus, sinodentatus, (2) austrodentatus, (3) dentatus; and the Kostinsky regional unit within the lentus/balhaschensis/kirgisicus Zone. Beds with conodonts Eoplacognathus pseudoplanus were distinguished in the Batun Section of the Kostinsky Horizon; the same-age beds with conodonts Semiacontiodus asymmetricus–Parapanderodus striatus were distinguished in the Baraniy-1 Section. For the first time in the Voskresenka Formation, in the stratotype of the Kuibyshevo Horizon (Maralikha Section), a brachiopod assemblage was found and identified.

––The Aeronian clastic sediments in the Gorny Altai area bear signatures of past seismic effects (paleo-seismites). Sedimentary structures produced by brittle deformation (breccias) span the level of the upper convolutus graptolite zone and the lower sedgwicki zone. They coexist with soft-sediment deformation structures (SSDS), such as turbated and rolled layers, bumpy layer boundaries, etc. At the final deposition stage, the rocks were deformed during seismically-triggered landsliding and were shed into a submarine canyon, with formation of rolls (pseudo-conglomerate).

A new version of the Regional stratigraphic chart for the Silurian of West Siberia and explanatory note, compiled in accordance with the Russian Stratigraphic Code, introduce changes, additional and specified data in comparison with the previous chart, adopted as Silurian section of the composite Stratigraphic chart of Paleozoic of the West Siberian Plane approved as the official stratigraphic basis for all types of regional geologic activities. After the meeting in 1998, the Silurian International Standard Stages were completely updated. In accordance with the updated International Standard Stages, in 2012, the Regional stratigraphic chart for Devonian of the West Siberia and a Regional stratigraphic chart for the Cambrian of the Cis-Yenisei part of West Siberia were adopted and approved. In 2020, the Regional stratigraphic chart for the Ordovician of West Siberia was adopted and approved. New paleontological, stratigraphic, lithological, and seismostratigraphic materials on the Silurian deposits of West Siberia have been obtained and generalized. For the first time, an inclusive Regional stratigraphic chart for the Silurian of West Siberia has been compiled.

The paper presents data on the classification, structure, and biota of reef complexes from the Late Ordovician-early Silurian Altai Basin. The early Paleozoic history of the Altai Basin included the early-middle Katian, late Katian, early Hirnantian, late Telychian, and middle-late Sheinwoodian events of large-scale reef formation which produced framework reefs, mud mounds, and coral meadows. The early Paleozoic reefs of the area formed at rates of 40 to similar to 12 cm per thousand years. The late Telychian reef system, which formed in 0.51-1.0 Myr, occupied the largest area of the basin. The lateral extent of reef systems in the Late Ordovician-early Silurian basin of Altai depended more on the rates of vertical and lateral growth, as well as on the previous sea bottom topography and clastic inputs, than on the duration of reef formation. Carbonate deposition in the Altai basins of that time can be simulated with a general model implying formation of distal barrier reefs.

large regional-scale Ordovician continental basin has been revealed for the first time in Siberia. Sedimentary structures in the Ordovician rocks of Tyva represent deposition environments of rivers, oxbow lakes, dunes, river deltas, tidal, and coastal plains. Ichnofossils record a succession from freshwater (limnic-fluvial) to deltaic and tidal facies. Integrated studies of sedimentary rocks from Central Tyva show that they were deposited on a coastal plain within a large river delta. The Central Tyva Ordovician basin was similar to the coeval basins of the Gondwanian continents in the style of clastic and redbed deposition.

A zonal graptolite scale of the Ordovician on Bennett Island is constructed. The circumpolar correlation of Ordovician terrigenous sequences of the Arctic regions of Eurasia and North America is presented. Possible migration routes of pelagic organisms between the Ordovician basins are analyzed. The paleogeographic proximity of the basins of Alaska and the De Long Islands in the Ordovician is shown.

Research subject. The Nenya-Chumysh basin is a long-term Mesozoic intracontinental sedimentary basin confined to the zone of a regional fault separating the Salair from the structures of Gorny Altai, Gornaya Shoria, and the Kuznetsk basin. Aim. To establish the geological and structural-kinematic characteristics of impulses of intracontinental orogeny that took place during the Mesozoic and Cenozoic in the territory of the northwestern part of the Altai-Sayan Folded Area. Materials and Methods. Geological maps of the area, geophysical data on the position of the base of the Paleozoic basement, satellite images and digital elevation models were used. The sedimentary filling of the basin is considered as a chronicle of tectonic movements in the NW part of the Altai-Sayan Folded Area at the intracontinental stage of development. Results. The Early Jurassic, Early Cretaceous, Late Cretaceous-Paleogene, and Neogene-Quaternary tectonic stages of the development of the Nenya-Chumysh basin were distinguished. In the Early Jurassic, the Nenya-Chumysh basin was a pull-apart basin in a left-hand strike-slip zone. This stage is associated with the accumulation of coal-bearing deposits of the Glushinskaya Formation, the thickness of which in the Nenya-Chumysh basin reaches about 1900 m. In the Early Cretaceous, as a result of changes in the stress field, the Nenya-Chumysh basin was transformed into a thrust basin, composed of terrigenous deposits of the Ilek Formation, forming a wedge-shaped sedimentary basin characteristic of foreland troughs. The neotectonic structure of the Nenya-Chumysh basin, formed in the stress field of the Indo-Eurasian collision, inherits an older structural plan in general terms, although differing in details. A non-inherited structure is the transverse neotectonic uplift of the Sary-Chumysh swell. Conclusions. The Early Jurassic stage is related with the closure of the Paleo-Tethys and the collision of a series of Cimmerian terranes with the southern margin of Eurasia, Early Cretaceous stage is caused with Mongol-Okhotsk collision, and the Cenozoic stage proceeds with the ongoing Indo-Eurasian collision. The geological evolution of continental sedimentary basins controlled by regional faults can be used as a source of information about the intensity and kinematic pattern of impulses of intracontinental orogeny in the geological past.

A new version of the Regional stratigraphic chart for the Ordovician of the West Siberian Lowland and the explanatory note, compiled in accordance with the Russian Stratigraphic Code, introduce changes, additional and specified data in comparison with the previous (first edition) chart. Since 1998, the stages of the Ordovician chart were changed completely. New stages – Tremadocian, Floian, Dapingian, Darriwilian, Sandbian, Katian and Hirnantian were adopted by Interdepartmental Stratigraphic Committee of Russia. The independent Regional Stratigraphic Scheme for the Devonian of the West Siberian Lowland and the Regional Stratigraphic Scheme for the Cambrian of the Pre-Yenisei Part of West Siberian Lowland were adopted. The proposed scheme for the Ordovician of West Siberian Lowland fills the lower part of the Ordovician-Silurian interval for the West Siberia. New paleontological, stratigraphical, lithological, seismo-stratigraphical data for the Ordovician of the West Siberian Lowland were obtained and generalized. For the first time independent Regional stratigraphic chart for the Ordovician of the West Siberian Lowland was compiled.

The large spherical structures found in the Upper Ordovician carbonates of the Teletskoe Lakeside structural-facies zone of the Gorny Altai were formed by algae. Oncolites-rhodolithes are indicators of shallow-water environments formed in the conditions with active hydrodynamics.

Dendroid graptolites of the Ona Formation from Lower Silurian of the Western Sayan have been studied. The collection is represented by very well preserved specimens. It includes only benthic forms and is characterized by the complete absence of planktonic taxa. This is extremely rare for graptolite communities. Representatives of dendroid graptolites have retained their lifetime shape. The unbroken shape of rhabdosomes may indicate calm sedimentation environments, since it is benthic dendroid graptolites that are very easily destroyed by underwater currents. Such type of autochthonous burial was previously studied by the authors on the territory of the Gorny Altai, and it is also known in the Lower Silurian of the Czech Republic.

New brachiopod species Eoanastrophia tozodoviensis sp. nov. is described from the Tozodov Body, Upper Ordovician of the Teletskoe Lakeside facies zone, Altai Mountains. Amendments are suggested to the diagnosis of Eoanastrophia Nikiforova et Sapelnikov.

During the Late Precambrian-Palaeozoic the southern Siberian continent bordered on the Palaeopacific and Palaeoasian Ocean tectonic plates. Their interactions led to the formation of the complex Central Asian folded (orogenic) belt containing the Silurian Tuvaella and Retziella brachiopod fauna. The subduction of the Palaeopacific plate under the Siberian continent led to the formation of the Late Precambrian-Palaeozoic accretionary orogen. The subduction of the Palaeoasian Ocean plate, including Gondwana-derived continental blocks, under the Siberian continent, created in the Late Precambrian-Ordovician a composite Kazakhstan-Baikal continent (collisional orogen). The Late Precambrian-Early Palaeozoic Ob'-Zaisan ocean basin was located between the Kazakhstan-Baikal and the Siberian continents. On site of the Ob'-Zaisan ocean basin the amalgamation and collision of these two continents resulted in the formation of the Early Palaeozoic Ol'khon suture-shear zone in the Baikal region, the Middle Palaeozoic Charysh-Terekta-Ulagan-Sayan suture-shear zones in the Altai-Sayan region, accretional and island arcs in Junggar and East Kazakhstan. The Silurian Tuvaella brachiopod fauna inhabited the Ob'-Zaysan ocean basin coastal outskirts, the epicontinental sea and active margins of the Kazakhstan-Baikal continent in subtropical-temperate latitudes. To this continent, the Gondwana-derived continental blocks with the Silurian Retziella brachiopods fauna have been accreting since the Late Palaeozoic. The Silurian Retziella brachiopod fauna inhabited shallow epicontinental marine areas of the continental blocks (Tarim, North China, South China, Australia and New Zealand) during the rifting of Gondwana separated from the Siberian continent by the Mongol-Okhotsk Ocean.

Calyptras are the smallest biogenic structures were found from the Upper Ordovician "Azratkan" Section in the Anui-Chuya structural facies zone. The petrographic and paleontological methods were applied to study caliptra’s internal structure and the host rock. It was found that bacterial-cyanobacterial associations took part in the calyptras development. The species Hedstroemia halimedoidea Rothpletz was identified in the space between calyptras and described.