Relevance. Geological prospecting carried out recently throughout the Lena-Tunguska oil-and-gas province makes studies of carbonates of the Osinskaya subformation particularly relevant, as they make it possible to establish the patterns of hydrocarbon trap formation. Aim. To present the results of the lithological and facial, biostratigraphic and petrophysical studies of the Osinskaya subformation in the southern and central part of the Nepa-Botuoba anteclise. Objects. Carbonate rocks of the middle Osinskaya subformation of the Usolskiy Formation of the Lower Cambrian Tommotian Stage, studied from well cores. Methods. GIS complex, detailed lithological and sedimentological description of the core, sequential stratigraphic analysis, petrographic analysis of 300 samples and examination of paleontological remains taken from the core; analysis. Results. Based on these studies, three sedimentation zones were identified, traced, and described for the first time: deep-water, marginal, and shallow-water. The deep-water zone is characterized by uncompensated sedimentation, the presence of single reefs, and confinement to depressions and troughs. It was flanked by an edge zone of carbonate platform associated with elevated thicknesses of the subformation and distribution of reefs over the area. It was followed by a shallow-water zone, represented by medium and reduced thicknesses of the Osinskaya subformation, its sediments are composed of carbonate grained and clayey rocks with small single reefs. The localized marginal zone of the extended carbonate platform agrees with the previously identified Chambinsko-Altybsko-Mirninskaya and Verkhnetokhomsko-Katsko-Pilyudinskaya zones of distribution of reef-like organogenic structures. On the basis of the studied wells, the structure of three formations according to the lithologic-facial profile was described. The paper introduces the authors’ variant of conducting the boundaries of the members. It was revealed that in the deep-water zone the prospects of searching for reservoir rocks are associated with single carbonate structures, in the marginal zone with area bioherms, and in the shallow-water zone with detrital deposits and shallow bioherms. The paleontological finds are shown to be confined to the selected sedimentation zones and beds. It was determined that algae diagnosed in the boudstones (autochthonous) were involved in the formation of the second and third strata, the edge zone of the carbonate platform. Archaeocyaths were diagnosed in detrital carbonate rocks of the second pack (allochthonous) in well no. 7 and autochthonous in boudstones of well no. 2X. Cribriciates were diagnosed in the boudstones of the third member (autochthonous). Archaeocyaths, cribriciates, namacalatusses, and calcareous algae studied from the cores of wells West Yaractinian no. 45, 361, Bolshetyrskaya no. 7, 3X, 2X, 4X, 5X, 6X were major edificators of Early Cambrian organogenic structures (biostromes, bioherms, bioherms massifs, reef formations).

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.

The section of the Osa Subformation of the Usolka Formation of the Lower Cambrian Tommotian stage penetrated by the Bolshetirskaya 7 well in the south of the Nepa-Botuoba anteclise has been studied. As a result of lithological-facies investigations, seven lithological types of rocks were identified, they are regularly replaced by each other along the section. Facies environments of the reef complex were dеtected: bar bank, rear part of the reef, organogenic bioherm bildup, reef rear bars, reef buildup, core of reef buildup. In the section of the subformation, three fourth-order sequences are identified. The sequence boundaries are fixed by subaerial hiatuses represented in the core by sedimentation breccias, karst zones. The largest lowering of sea level, in terms of amplitude and duration, is confined to the border of the second and third sequences. The conducted biostratigraphic analysis confirms the Lower Cambrian age. Remains of calcareous algae Renalcis polymorphum, R. gelatinosum, R. granosum, Botomaella zelenovi were found. Small shell fauna of the genus of Namacalathus Grotzinger, Watt ers et Knoll. also was found. It has global distribution in the Lower Vendian (Ediacaran) deposits in Namibia, Brazil, Canada, Oman, Spain, China, Russia, etc.

This paper discusses the structure and facies variability of the Silurian mixed terrigenous-carbonate strata and presents new paleontological and biostratigraphic data. It attempts to refine the stratigraphic position and provide new lithologic evidence (markers) for regional manifestations of global sedimentary events (Mulde transgressive-regressive and Linde regressive events) in the Altai sections. The chronostratigraphic position of the Kuimov Horizon, a regional stratigraphic unit, has been defined based on graptolite and conodont faunas.

Tommotiida finds from the Lower Cambrian of the Khairkhan Section located within the Central Tuva trough were characterized for the first time. The assemblages of small shelly fossils are represented by species of the genera typical for the Upper Atdabanian age, such as Tannuolina, Kelanella, Lugoviella, and Microdictyon. Sclerites of four different morphotypes were identified for the Lugoviella ojmuranica species.

Complex researches of the pyrite Early Cambrian Kyzyl-Tashtyg deposit (East Tuva) have allowed to reconstruct paleohydrothermal systems, to find out physical and chemical conditions of hydrothermal processes, and also to establish features of existence of ancient hydrothermal biota in the operating zone of the solutions, participating at all stages of formation of ore-bearing structures, since influence of postmagmatic fluids and finishing low temperature hydrotherms. The representative data on biota has been received at studying of the objects, which were in a zone of influence of rather low temperature hydrothermal systems. In one cases findings of microorganisms are dated for basalt complexes, containing ore body, and in other cases the found fauna is connected with ferruginous-siliceous sediments. It has been found out that microorganisms in the tonsil cavities of basalts evolved at the temperatures nearby 110-140°С under the influence of hydrothermal solutions. Much lower temperatures (to 100 °С) existed at development of micro-fossils in the quartz-hematite hydrothermal constructions of Kyzyl-Tashtyg deposit.

The Vendian–Cambrian transitional strata of the Igarka Uplift (northwestern Siberian Platform), originally erected as the Sukharikha Formation, are one of the key objects revealing evolution of marine ecosystems in the late Vendian. Herein we describe dwelling burrows Skolithos and Arenicolites from dolostones in the middle Sukharikha Formation (Nemakit-Daldynian Regional Stage). High density and local distribution of these burrows in the Sukharikha Formation demonstrate that the terminal Vendian shallow-marine sediments became occupied by vertical-burrowing benthic organisms, mirrored in the geological record as an archetypical Phanerozoic ichnoassemblage ( Skolithos ichnofacies).

Проведено изучение строения и фациальных особенностей силурийского смешанного терригеннокарбонатного типа седиментации. Получены новые палеонтолого-биостратиграфические данные. Высказаны предположения о стратиграфическом положении и литологических свидетельствах (маркерах) региональных проявлений в алтайских разрезах глобальных седиментационных событий трансгрессивно-регрессивного Мульде (Mulde Event) и регрессивного Линде (Linde Event). По граптолитам и конодонтам оценено хроностратиграфическое положение регионального стратона – куимовского горизонта.

The distribution of the main faunal groups of the Atdabanian Age of Early Cambrian in the Altai–Sayan folded area (ASFA) are analyzed. It is shown that Late Atdabanian assemblages of small shelly fossils of the Kuznetsk Alatau and Altai Mountains include in general the same taxa. At that time, these territories were probably situated rather close to each other and connected with the Siberian Platform. A similar result was obtained analyzing the distribution of trilobite assemblages. It is shown that the Western and Eastern Sayan Mountains have more similar forms with Mongolian assemblages. It is also shown that, in the Atdabanian, the greatest taxonomic diversity of trilobites was confined to the areas of the paleobasin where there was accumulation of organogenic carbonate sediments. It is shown that archaeocyaths from ASFA are older than the taxa characteristic of the assemblage from the Nochoroicyathus mariiskii Zone. At the boundary of the Kiya and Kameshki horizons, archaeocyaths sharply increase in abundance and many taxa with the ring and complex internal walls appeared. This boundary is traced throughout the territory of ASFA.

The aggregated section of Middle Cambrian at the Khorbusuonka River, northeastern Siberian Platform, is described. The aggregated sequence is compiled on the basis of investigations at four bank outcrops. The findings of trilobites at different stratigraphic levels allowed us to identify the intervals comparable to regional zones of the Amgan and Mayan stages (Middle Cambrian). New findings and analysis of stratigraphic distribution of trilobites helped to specify the position of the Middle Cambrian lower boundary. The lower boundary of the Mayan Stage is drawn by the first appearance of Corynexochus perforatus in the section, 9 m from the base of the Yunkyulyabit–Yuryakh Formation. Trilobites of Cyclolorenzella sp. and Kuraspis sp. found in the upper Tyussala Formation are characteristic of the boundary layers of the Middle and Upper Cambrian. It was supposed earlier that the Upper Cambrian part of the sequence began in the overlying Lapar Formation; however, the aforementioned findings suggest that the Middle Cambrian lower boundary can be located within the limits of the upper Tyussala Formation.

The lower Ust'-Kundat Formation has been studied in detail in a section along the Kiya River (Kuznetsk Alatau, Altai-Sayan folded area). The section was deposited during the Tommotian stage and is a stratotype for the Lower Cambrian Ust'-Kundat sequence comprising several members of the Ust'-Kundat Formation. Member 2 contains newly discovered and previously known species of archaeocyathids near its top found stratigraphically lower than the oldest known archaeocyathan assemblage of the Nochoroicyathus mariinskii biozone. The found archaeocyaths cannot be dated precisely and may have either Tommotian or Atdabanian ages. Small shelly fossils from the same section cover a large stratigraphic range as well. The findings call for updating the age of the upper Ust'-Kundat sequence and the Tommotian upper boundary in the regional stratigraphy of the Altai-Sayan folded area. (c) 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.