Home – Home – Jornals – Russian Geology and Geophysics 2023 number 11

This special issue is dedicated to discussing the possibility of the stage division of the Vendian system, specifically, a composite section of the Siberian platform is proposed as a stratigraphic standard. For the Ordovician system of the Siberian platform and Northeastern Russia, certain changes in stratigraphic charts are proposed, nomenclative issues and different variants of correlations of regional horizons with the GSS of Russia are considered. A geochemical analysis of Bajocian and Bathonian belemnite rostra of Arctic Siberia was carried out and for the first time a comprehensive C, O and Sr isotope characterization of the sections was obtained, making it possible to propose a comparison of the studied sections with the European standard and to develop paleogeographic models. Comprehensive studies of the Upper Jurassic and Lower Cretaceous series of the Anabar Bay deposits (Laptev Sea) led to an improved biostratigraphic division for this reference section and to the identification of the levels for interregional and global correlation. The magnetostratigraphic division of the Jurassic-Cretaceous boundary interval and its comparison with the Tethyan regions are discussed. Taking into account biostratigraphic data, the δ¹³C_org variation curve can also serve as a correlation tool. The clinoform structure of Upper Jurassic strata was revealed in the northwest of the Anabar-Khatanga depression, based on the interpretation of seismic profiles calibrated against studied sections. The formation of this complex allows us to consider this stage as the initiation of the Verkhoyan orogeny in this folded area. Analysis of variations in the thickness of sediments allows us to deduce that intense tectonic-sedimentary processes took place during the Late Jurassic epoch, along with subsidence of the bed of the sedimentary basin. Analysis of a specific assemblage of lacustrine fossils from the reference section of the upper part of the Lower Cretaceous series of Transbaikalia and of published data showed that this fossil assemblage was widespread in East Asia. Biofacies analysis made it possible to reconstruct the paleoenvironments of this time.

The way towards the stage subdivision of the Vendian starts with the assemblage of a complete composite geological section of this interval of the Earth’s history, a succession of geological bodies each reflecting a certain ecosystem state. A Vendian composite section of the Siberian Platform, which is a succession of regional to sub-global scale geological bodies reflecting particular states or unidirectional transformations determined by episodic expansion of relatively oxygen-rich environments onto the shelf and alternating with large-scale oceanic anoxia and euxinia events is proposed as a reference for constructing the stage subdivision of the Vendian. The redox instability had to be accompanied by changes in nutrient availability and could not but affect the course of macroevolution and macroecology. The geological record of the Vendian of the Siberian Platform is also marked by episodic increase in alkalinity of the World Ocean. At least five such alkalinity events could be provisionally identified in the composite section of the Vendian of the Siberian Platform. A hypothesis is proposed suggesting that the alkalinity events could control the appearance and disappearance in the geological record of a distinctive suite of sedimentary structures in carbonates, the discrete nature of early diagenetic cementation of aluminosilicoclastic sediments, and fossilization of soft-bodied organisms in the Vendian.

The chronozonal structure of the Mangazeya Superhorizon of Eastern Siberia, which includes the Listvyazhny and Baksan horizons and their chronozones, is considered. Their volumes and stratotypes are given and their place in the regional and global stratigraphic structure is shown, where Mangazeya occupies the middle parts of the Kuzmovsky regional complex and the Caradocian stage of the Upper Ordovician. Proper names for the Lower Dolbor Horizon (Amutkansky) and the Upper Dolbor Horizon (Cherlechinsky) are introduced for the first time.

In the Middle Ordovician sections of the Eriekhe River basin, which are of crucial importance to correlate the regional stratigraphic units of Northeast Russia and the General Stratigraphic Scale (by graptolites), we identified a diverse ostracod assemblage. The new data substantially supplement the Middle Ordovician paleontology of this region. The analysis of the stratigraphic distribution of these assemblages made it possible to verify the correlation of the Eriekhe Structural Facies Zone (SFZ) suites with regional stratigraphic units of the Northeast Russia and, based on homotypic ostracod assemblages, with the horizons of the Siberian Platform. The joint findings of ostracods and graptolites in the studied section allowed us to correlate the horizons of the Northeast Russia and the Siberian platform with the stages of the General Stratigraphic Scale.

The Middle Jurassic sedimentary strata of Siberia remain poorly studied chemostratigraphically. We contribute to the knowledge with pioneering C, O, and Sr isotopic data for carbonate material of belemnites from the Yuryung-Tumus Peninsula and the lower reaches of the Lena River, as a basis for comprehensive description of the north Siberian Bajocian and lower Bathonian. The obtained chemostratigraphic constraints, with new ⁸⁷Sr/⁸⁶Sr ratios and previous δ¹³C and δ¹⁸O estimates, also include data for the lower Bathonian in the Sokur section (Central Russia). Despite the limited amount of material, chemostratigraphy, along with the available biostratigraphic data, allows reliable correlation of the Boreal sections with the primary standard of Northwest Europe, which is impossible for these strata with any of the two methods alone. The δ¹³C, δ¹⁸О, and ⁸⁷Sr/⁸⁶Sr patterns correlate with the records of eustatic, climatic, tectonic, and paleogeographic events. The new δ¹⁸O data and the inferred paleotemperatures for the latest early Bajocian and the Bajocian/Bathonian boundary reveal two excursions of notable seawater warming near the Siberian Arctic coast, which were synchronous with episodes of global sealevel rise. It was presumably during the eustatic events that the N-S Komi Strait (first naming) opened twice in the territory of the Russian Plate. The strait connected the Boreal and Tethyan seas and thus changed the oceanic circulation patterns. Specifically, it opened a gateway for a warm current from the south to northern Siberia responsible for the high seawater temperatures recorded in the δ¹⁸О patterns of belemnites.

The goal of this study was to improve the high-resolution biostratigraphy of the Upper Jurassic and Lower Cretaceous in the type section on the Nordvik Peninsula, Anabar Bay, Laptev Sea. The results were used to identify a succession of 13 foraminiferal biostratigraphic units, 9 dinocyst units, and 8 palynofloral biostratigraphic units in the rank of zones and local zones. Based on new data, the stratigraphic continuity of this section is proved for the Lower and Middle Volgian, where a major stratigraphic hiatus was previously assumed. The established zones have variable correlation potential. Different versions of the subdivision of the section based on ammonites are discussed and the proposed scale is justified. A reference level based on calcispheres/calcareous dinocysts is defined providing global correlations and calibration of beds near Jurassic-Cretaceous boundary in the Tethyan and Arctic regions. The magnetostratigraphic subdivision of the studied section and its comparison with the Tethyan regions are discussed. Together with biostratigraphic data, the proposed δ¹³C_org curve in the Upper Jurassic and lower part of the Valanginian can be a tool for detailed correlations in different regions of the Northern Hemisphere. The cyclicity of carbon isotope composition in the Upper Volgian and Boreal Berriasian and isotope events in the Volgian and Lower Valanginian are determined within the Arctic region. The distribution of geochemical parameters (C_org, δ¹³C_org and hydrogen index) is determined in studied section, geochemical stratification of the section and variations in major oxide compositions are demonstrated. A sharp change in geochemical parameters is identified at the base of the Paksa Formation (Upper Volgian). The distribution interval of the “subsurface chlorophyll maximum” is determined. The study shows that high concentrations of isotopically light organic carbon in the first diasterene geochemical subhorizon were caused by high phytoplankton productivity and were deposited under reducing conditions favorable for preservation of organic matter. The analysis of major oxide compositions of the studied rocks revealed that variations in the chemical composition of the studied rocks reflect changes in their mineralogy and petrography.

New materials on the stratigraphy and structural features of the Bathonian-Neocomian sections of the Anabar-Lena basin are presented. In the territory adjacent to the Taimyr mountain-folded system, a clinoform-like structure of the Upper Jurassic strata has been revealed, which are characterized by high thickness compared to adjacent sections in the east of this region. The lectostratotype of the Chernokhrebetnaya Formation (uppermost Lower Oxfordian-lowermost Volgian) is proposed. The most complete and continuous stratigraphic section is compiled, its litho- and chronostratigraphic correlations are given comparing to the sections of adjacent territories characterized in detail by fossil assamblages. The evidences for a new lithostratigraphic division of the territory are provided, facies zoning of sediments is clarified, a thickness chart of the Upper Bathonian-Boreal Berriasian strata of the Khatanga-Olenek interfluve is proposed.

The age of the Turga Formation in the Middendorf’s outcrop is determined as latest Barremian-earliest Aptian based on fauna and microfossil assemblages. The composition of the fauna and palynomorph assemblages allows correlations between the Turga Formation and the Yixian Formation and lower part of the Jiufotang Formation of Northeast China. Results of the petrographic and geochemical analysis as well as microfossil studies showed that sediments accumulated in a shallow lake with low-energy hydrodynamics, sometimes under low-oxygen conditions. Climate features (warm temperate climate transitional to subtropical) and type of vegetation (coniferous forests with admixture of hardwoods) are reconstructed taking into consideration the composition of spore-pollen assemblages.