Modern forecast, exploration and development of hydrocarbon deposits within natural carbonate reservoirs of buried bioherm (reef) structures in the north of the Irkutsk region and the Sakha Republic (Yakutia) areas within the NepaBotuoba Anticline apply the most advanced technologies of 3D seismic exploration and geoelectric prospecting supplemented with a set of new interpretation approaches and analysis attributes contrasted against wide range of well logging data. The recent discoveries of hydrocarbon deposits in the northern reefs shoved the bioherms of the Irkutsk amphitheater in the shadow. The purpose of this research is geological study of biohermal carbonate formations of the Lower Cambrian age in the outcrops of the Bozhekhansky megaswell in the Baikal region. The choice of this object of study is due to the fact that the first oil and gas inflows were obtained from the wells of the Atovskaya, Osinskaya, Birkinskaya, Khristoforovskaya, Balykhtinskaya and Tuturskaya areas. Field geological routes and office processing of data on cuts and polished thin sections of sample surfaces were carried out including microscopic studies in the thin sections of samples and archival collections of cores from deep oil and gas prospecting wells. In 2021–2022, while prospecting in the south of the Irkutsk amphitheater, the authors explored stratified, nodular and columnar stromatolites found within the compound Bozhekhan megaswell of Cambrian carbonate outcroppings. Carbonate intraformational sedimentary breccias identified in the area under investigation feature a great variety in their fragment sizes and inclusions. The stromatolites’ structural features were studied by the cuts and polished thin sections of sample surfaces. Microphytolites were found in the thin sections and described. The authors also took advantage of the opportunity to study an archive collection of the thin sections of VendCambrian and Lower Cambrian carbonate strata from the Akhinskaya area’s wells at the Bozhekhan megaswell and from the Upper-Lena uplift to the north-west. The conducted study implies that Cambrian bioherm outcroppings of the Bozhekhan megaswell-type uplift within the Cis-Baikal trough are, in a sense, model geologic objects. The study of biologic and genetic varieties of Lower Cambrian phytolites, diagnostic features of the organogenic structures, their inner structure and formation processes of the reef ecosystem may provide a region-level prognostic key to the research of Cambrian sediment system in the south of the Siberian platform.

The descripton has been provided for the cross-section (12 km) of a thick sediment unit on the left bank of the Vitim River, opposite the Nizhny Orlov and Dannaya rivers. This place is related to the Karalon-Mamakan area of the Baikal-Muya belt of the Baikal folded area. The changes have been made to the names formally assigned to the following stratigraphic subdivisions: Sulban series was renamed Chayangra formation, and Kelyan subseries was renamed Karalon formation. Evidence has been found for the complex folded and faulted structure and litological-facial characteristics of the cross-section. Some evidence has been provided to confirm that the Karalon formation is younger than the Chayangra formation. When this result is comapared to the well-studied subdivisions of the northern areas of the region, it is apparent that the Karalon formation may well correlate with the Dalnetaiga horizon of the Lower Vendian, the Chayangra formation – with the Ballaganakh series of the Late Riphean, and its layers at the bottom are similar in composition to the the Medvezhevsk horizon.

The geological position, composition, and age of detrital zircons of sedimentary deposits of the Nugan Formation of the Western Baikal region underlying the Golousta Formation of the Baikal series of Ediacaran age have been studied. The formation of both stratigraphic units due to the same sources of detrital material, located within the southern flank of the Siberian Craton, has been proved. The deposits of the Nugan Formation have been demonstrated to mark the rifting stage of the formation of the passive margin of the Paleo-Asiatic Ocean: their accumulation occurred in the Late Cryogenian during the interval 720–640 Ma.

Summary Геологический семинар будет интересен для специали-стов нефтегазового сектора. Часть маршрута будет про-?ходить на корабле по Байкалу от пос. Листвянка до пос.Большое Голоустное. В этом интервале протяжённостью?60 км по берегам обнажаются породы, которые отража-ют значительную часть земной истории. Вдоль Листвянки?и за ней вскрываются гранитоидные толщи, возраст фраг-ментов которых датируется от 2,8 до 3,4 млрд. лет. Они?прорываются дайками базитов мезо, неопротерозойскогои палеозойского возрастов. За посёлком Большие К оты эти?породы сменяются конгломератовой толщей юры с про-пластками углей, которые прослеживаются на север в Че-?ремховский угольный бассейн и имеют ориентировочныйвозраст 170 млн. лет. Далее к северо-востоку при прибли-?жении к посёлку Большое Голоустное, в пади Кадильнаяогромными столбами встают известняки с определённым?возрастом 580 млн. лет. Самым замечательным в них яв-ляются прекрасно отпрепарированные строматолитовые?постройки – свидетель цианобактериальной (и водоросле-?вой?) жизни в то время. И при приближении к посёлку поберегу озера вскрываются разрез голоустенской свиты не-?опротерозоя и галечно-песчаные отложения морского днаБайкала, образованные в последний миллион лет. По пути?от Большое Голоустное к Иркутску, вдоль рек Голоустнаяи Правая Ушаковка в придорожных обнажениях наблюда-?ются фрагменты разреза неопротерозойских толщ, частькоторых является нефтематеринскими. Судя по данным?бурения, к северу они фациально меняются и выклинива-ются. При приближении к Иркутску разрез наращивается?поздневендскими и нижнекембрийскими отложениями,которые перекрываются песчаниками юры.

We present the synthesis of new data on detrital zircon geochronology of the Neoproterozoic strata of the southern part of the Siberian craton as well as a comprehensive analysis of previously published stratigraphic, sedimentological and geochronological (LA-ICP-MS) data obtained for key sections in this area that allows us to trace the process of birth and early stages of development of the Paleo-Asian Ocean (PAO). Before the break-up of Rodinia and opening of PAO, Tonian - Cryogenian intracontinental sedimentary basin existed between southern Siberia and northern Laurentia. The detachment of the southern flank of the Siberian craton from northern Laurentia and opening of the PAO between these cratons took place in Cryogenian. The detrital zircon ages from lower parts of Neoproterozoic successions suggest the Siberian craton as the sole provenance area right after the opening of the PAO. The age constraints on the lower parts of the studied Neoproterozoic successions, which are based on correlation of their tillite horizons with the Marinoan glaciation, suggest the late Cryogenian age for these sedimentary rocks. A clear change in the age spectra of detrital zircons from "unimodal" (Early Precambrian only) in older sedimentary rocks to "bimodal" (Early Precambrian as well as Neoproterozoic) in younger sequences of the studied successions marks the next stage of the PAO evolution, The abundance of youngest (630-610 Ma) detrital zircons in the upper parts of the studied sequences reflects a shrinkage of the oceanic basin as a result of the convergence of the craton with the microcontinents and island arcs within the Paleo-Asian Ocean. We suggest that a passive oceanic margin along the southern margin of the Siberian craton has been transformed into a series of foreland basins at similar to 610 Ma. (C) 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

МИКРОФОССИЛИИ СТРАТОТИПА РИФЕЯ ЮЖНОГО УРАЛА И ПРОТЕРОЗОЯ ВОСТОЧНОЙ СИБИРИ (ПАЛЕОБИОЛОГИЧЕСКИЕ АСПЕКТЫ)© 2018 г

The origin of the Central-Asian Orogenic Belt (CAOB), especially of its northern segment nearby the southern margin of the Siberian craton (SC) is directly related to development and closure of the Paleo-Asian Ocean (PAO). Signatures of early stages of the PAO evolution are recorded in the Late Precambrian sedimentary successions of the Sayan-Baikal-Patom Belt (SBPB) on the southern edge of SC. These successions are spread over 2000 km and can be traced along this edge from north-west (Sayan area) to south-east (Baikal area) and further to north-east (Patom area). Here we present the synthesis of all available and reliable LA-ICP-MS U-Pb geochronological studies of detrital zircons from these sedimentary successions.

This paper provides new data on the age of detrital zircons from metaterrigenous rocks of the Khamardaban Group (Kornilovskaya and Shubutuiskaya formations) which crown the succession of the Khamar-Daban terrain. It was established that the accumulation of the protoliths of both formations occurred in the interval between the Late Riphean and Early Ordovician. In this case, there is a difference between sequences of Shubutuiskaya and Kornilovskaya formations due to a sharp change in a provenance area and depositional conditions, which is a consequence of the change in the paleogeodynamic environment. In addition, this indicates the tectonic juxtaposition of these sequences and probability of accumulation of deposits of the Shubutuiskaya Formation within Dzhida island arc system.

The study of the Mesoproterozoic (1473 +/- 24 Ma) dolerites of the Olenek uplift of the Siberian craton basement has shown their petrologic and geochemical similarity to typical OIB produced with participation of a mantle plume. The dolerites are characterized by variations in the geochemical composition explained by different degrees of melting of the same source. A conclusion is drawn that the parental melts of the rocks were slightly modified by crustal contamination, as evidenced from their Nd isotope composition (epsilon(Nd)(T) = +0.6 to -0.8) and the presence of inherited zircons of four ages (2564, 2111, 2053, and 1865 Ma). Since the Siberian craton in the structure of the Nuna supercontinent (Columbia) was located relatively close to the Baltic continent and the Congo and Sao Francisco cratons, we assume that the Early Mesoproterozoic mafic intrusions (1500-1470 Ma) of all these cratons belong to the same large igneous province (LIP). The province formation was related to the activity of superplume (or mantle hot field), which supplied mantle matter to the lithosphere basement. The superplume core was probably located beneath the northern part of the Siberian craton, where basites are compositionally most similar to the primary mantle source. (C) 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

The Paleoproterozoic sedimentary and volcanic-sedimentary sequences of the Urik-Iya Graben at southern flank of the Siberian Craton have been studied. Based on the isotopic U-Pb LA-ICP-MS dating of detrital zircons contained in the clastic fraction of the studied rocks, three main extension stages accompanied by sedimentation are recognized; each stage is characterized by certain types of sediments and conditions of their accumulation. The oldest rocks (Ingashi Formation) mark early extension events (∼1.91−1.87 Ga), which were caused by collapse of the orogen that arose due to collision of the Biryusa and Sharyzhalgai blocks. The basin formed as a result of extension is regarded as an aulacogen. Granitoids of the Sayan Complex were emplaced in the cratonic lithosphere at the final stage of the first extension stage. The second stage of extension started ∼1.75 Ga ago as a response to the effect of the inferred mantle plume on the lithosphere of the Siberian Craton. It was accompanied by deposition of the Daldarma Formation. Stress inversion took place at the final stage (∼1.70 Ga), and an intracratonic fold zone arose at the place of the paleoaulacogen. The third extension stage (1.65−1.60 Ga) corresponds to the time of molasse accumulation in pull-apart basins (Yermosokha Formation). The final stage of rifting was marked by emplacement of granitoids (Chernaya Zima Complex, 1.53 Ga), which completed the active tectonic events in the region. Afterward, the Urik-Iya Graben transformed into a stable intracratonic domain. The available data allow us to revise the tectonic history of the Urik-Iya Graben. In light of new evidence, this structural unit may be interpreted as a long-evolving paleoaulacogen. The series of revealed sedimentation settings reflects the formation of a consolidated continental lithosphere at the southern flank of the Siberian Craton.

The first data on the age of detrital zircons are given for Late Precambrian terrigenous rocks of the Baikal Group and Ushakovka Formation of the southern flank of the Siberian Craton. The ages obtained for 348 zircons cover the Paleoarchean to Late Ediacaran period, demonstrate the dynamics of change of sources of the clastic material in the sedimentation basin, and mark the changes of the Late Precambrian tectonic regimes. The age of the youngest group of detrital zircons extracted from the rocks of the Kachergat Formation allows us to restrict the upper age limit of accumulation of the rocks of the Baikal Group to the Late Ediacaran (Late Vendian).

Kirbia multipartita Mikh. et Volk. и др.) формацию Вангьятан можно отнести к верхам верхнего рифея.For the first time the microfossils from Wangjiatan formation in the northeast of China were obtained and studied. Various organowalls forms of acritarchs and cyanobacteriae were described. Their habitat and disposal area were reconstructed. Among the acritarchs the morphological groups were singled out. The groups have been preliminary correlated with large taxa of the plant world: brown and green algae. The Wangjiatan formation can be referred to the upper layers of the Upper Riphean on a number of forms (Kirbia multipartita Mikh. et Volk.).

Microfossils from metamorphized Lower Proterozoic deposits of the Udokan Mountains (Naminga Formation) and Sayany Region (Daldarma Formation) are investigated in preparations and petrographic thin sections. They include leucoxenized and ferruginous threads varying in size (50–500 μm) and thread accumulations, which are compared to cyanobacteria. Mineralized thalli probably belong to algae. Small, rounded forms from carbonaceous beds are treated as iron and chemolithotrophic bacteria of stagnant zones of paleobasins. Round or pear-shaped nontransparent forms with projections or several fused forms are described from preparations and identified as fungal microfossils. Similar forms have been recorded in the Lower Proterozoic of Karelia.

We present U–Pb (LA-ICP-MS) data on detrital zircon from the Late Precambrian terrigenous rocks of the Baikal Group and Ushakovka Formation, western Cisbaikalia (southern flank of the Siberian craton). The sources of clastic material for the studied sediments are interpreted. The youngest group of detrital zircon grains from the upper Baikal Group and Ushakovka Formation permits assigning these sediments to the Vendian. The lack of Mesoproterozoic detrital zircon in most of the analyzed samples confirms the hypothesis of a global (~1 Gyr) break in endogenic activity within the southern flank of the Siberian craton through the Precambrian. The abundance of Neoproterozoic zircon in sandstones from the upper horizons of the Baikal Group and the Ushakovka Formation might be due to the shrinkage of the ocean basin as a result of the convergence of the craton with the microcontinents and island arcs within the Paleoasian ocean.