The Borehole Vostok 3 drilled in the east of the West Siberian Plate (Tomsk Region) revealed a Vendian section in the depth range 5002–3870 m, which was subdivided into the Poiga, Kotodzha, and Raiga Formations based on geological, geophysical, and paleontological data. In the Kotodzha and Raiga Formations, typical Upper Vendian fossils of Cloudina hartmanae and Namacalathus sp. were found along with diverse Platysolenites, which are commonly considered to be of zonal significance in Lower Cambrian strata. Hence, the stratigraphic interval with abundant diverse Platysolenites has a wider stratigraphic range than it was believed earlier and seems to cover the Upper Vendian and Lower Cambrian deposits. The Borehole Vostok 3 is the first Siberian occurrence of the fossils Namacalathus, the world’s fourth occurrence of the Cloudina-Namacalathus association, and the first site where coexisting Platysolenites and typical Vendian organisms have been found. Therefore, the borehole provides one of the most informative (in paleontological context) Upper Vendian sections.
On the basis of available sedimentological, isotope-geochemical, and geophysical data, we have refined the intrabasinal correlation and the structure of Mesoproterozoic deposits of the Baikit anteclise. A pre-Vendian section was stripped within the Baikit anteclise, which can be correlated with the section of the Mesoproterozoic Teya and Sukhoi Pit (pre-Pogoryui) Groups of the Yenisei Ridge. Most of the overlying deposits of the Tungusik Group were destroyed during the tectonic events in the Early Neoproterozoic (~860 Ma) and in pre-Vendian time. Using the results obtained, we have refined the variations in the sedimentation environments of Mesoproterozoic deposits in the Baikit anteclise and Yenisei Ridge and constructed paleogeographic schemes for particular time intervals in the modern frame of references. The predominance of peritidal complexes and numerous signs of subaerial exposition indicate that the carbonate accumulation was periodically interrupted. In the periods (sometimes, long) when this process stopped, fine silicoclastic material transited from the shelf edge into a deep basin localized southwest of the Baikit anteclise. The longest transition periods were in Early Yurubchen, Dolgokta, and Late Vingol’da time, though the main flow supplying silicoclastic material to the Yenisei Ridge passed south of the Baikit carbonate shelf. In the Mesoproterozoic, the basin with predominantly carbonate sedimentation occupied a vast area in the western Siberian Platform, extending to the western margin of the Anabar Shield, where the Mesoproterozoic deposits were similar in structure and isotope-geochronological and isotope-geochemical characteristics to the pre-Vendian deposits of the Baikit anteclise. Most part of the area with carbonate sedimentation in the Baikit anteclise was remote from the shelf edge. The Mesoproterozoic shelf was much wider than its fragments in the recent structure on the western margin of the Siberian craton.
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.
A new type of Cambrian section penetrated by the Vostok-1 stratigraphic test well in the east of the West Siberian Plate, Tomsk Region, is described in terms of paleontology. It can be considered a key section for this region. According to a complex of geological and geophysical characteristics, the section in the depth range of 2766–5010 m was divided into the Churbiga, Paidugina, Pudzhelga, Podelga, Kondes, Shedelga, and Pyzhina Formations, and their first description was made. The Cambrian section in Vostok-1 well is most similar to those of the Kotui-Igarka area of the northwestern Siberian Platform, where Cambrian deposits formed in the pre-reef zone of an open sea basin. In the lower section of the borehole the highly carbonaceous siliceous-argillaceous-carbonate Paidugina Formation was stripped, which is an analog of the Lower-Middle Cambrian Shumnaya and Kuonamka Formations of the Siberian Platform. The Paidugina Formation, which accumulated on the shelf and slope of the open sea facing the Paleo-Asian ocean, is considered oil-producing and suggests the high petroleum potential of the cis-Yenisei basin. Also, a highly promising generation-accumulation petroleum system has been revealed.
It was studied that the possibilities of preservation, migration and transformation of organic matter (OM) in rocks of Precambrian age, the textural and structural characteristics of which are inextricably linked with the physiological characteristics of microorganisms, and the role of stylolithization structures in these processes. The similarity of modern stromatolites with ancient ones in the mechanism of deposition of carbonate micrite and the formation of porous frameworks allowed us to establish that a significant amount OM of cyanobacterial lamins could be preserved after the buildings were immersed in the sedimentation basin and transformed into sedimentary strata. OM was concentrated on numerous relict surfaces between micrite carbonate secretions deposited by microorganisms during life. Subsequent transformations were expressed in the recrystallization of the carbonate substance, accompanied by a significant loss of primary voidness, oustion and redistribution of OM, as well as clay material deposited from the suspension. During detailed petrographic studies, it was observed how clay-organic material (COM), displaced by recrystallization, concentrated on the crystal faces and in intercrystalline pores in certain areas and composed numerous lenticular layers in contact with each other, forming surfaces of complex configurations - rudimentary microstylolites. According to the authors, recrystallization is one of the main catalysts of stillolithization processes, because it creates mineral aggregates in which the pressure on the COM enclosed between them varies greatly at different points. This contributes to the occurrence of the phenomenon of differentiated dissolution under pressure, more intensive transformation of OM in areas with maximum load and, as a consequence, excessive release of CO, which in turn leads to saturation of pore solutions with carbon dioxide and initiation of chemical dissolution. Probably, this is how stylolites and associated fractured voidness are formed in stromatolites, which is available for the migration of OM.
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