Home – Home – Jornals – Russian Geology and Geophysics 2023 number 7

We present the results of study of the geologic structure, petrologic composition, and age of plagiogranitoid associations in the east of the Kaa-Khem batholith (Eastern Tuva). The batholith is located in the junction zone of the Tannu-Ola island arc (TIA) of Vendian-early Cambrian age and the Precambrian rocks of the Tuva-Mongolian microcontinent (TMM). Plagiogranitoids of this region formed in an accretion-collision setting in the period 490-450 Ma. Three stages of formation of plagiogranitoid associations have been established (~489, ~476, and ~450 Ma). The plagiogranitoid associations in the east of the Kaa-Khem batholith do not differ in petrologic composition and age from the plagiogranitoid associations (500-450 Ma) in the west. Xenogenic zircons in the studied plagiogranitoids of the eastern Kaa-Khem batholith have ages of 2335-517 Ma. Their ages are divided into several clusters (524-517, 549-536, 615-586, 684-647, 739-735, 810-794, 842-827, and 2335 Ma) reflecting the heterogeneity of the crust in the TIA-TMM junction zone. The wide range of ages and the abundance of xenogenic zircons in plagiogranitoids of the eastern Kaa-Khem batholith testify to the contribution of older crustal sources to the granite formation at all stages of accretion-collision processes (from 490 to 450 Ma). The much narrower age range of xenogenic zircon (616-474 Ma) and its low contents in coeval plagiogranitoids of the western Kaa-Khem batholith are consistent with their localization within the TIA and the relative homogeneity of the crust in the area of felsic-melt generation. In the west, the influence of older crustal sources was insignificant and manifested itself only at the final stage (~450 Ma) of accretion-collision processes.

A 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.

The leading role of lithostratigraphic, structural/tectonic, magmatic, metamorphic, and fluid geologic factors in the formation of a graphite deposit is substantiated. We present the petrographic and chemical compositions of rocks and minerals of igneous rocks, the δ¹³С values of graphites and calcitites, the δ¹⁸O values of carbonates, the δ³⁴S values of sulfides, and the ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr ratios in calcitites and microdolerite injections. The thermal effect of magmatic melt on coal led to its degassing and graphitization. The steaming of microdolerite injections with hydrocarbon fluids was probably accompanied by the removal of iron from them and the crystallization of low-Fe clinopyroxene, and the interaction of fluids with basaltic melt in the upper endocontact zone of the intrusion led to the segregation of the melt and the formation of a globular structure of rocks. The presence of a calcite lens and a sulfide vein between the roof of the intrusion and the graphite bed is due to the earlier intrusion of sulfate salt melt and its subsequent interaction with hydrocarbon fluids, which resulted in sulfate reduction and the formation of calcites. The sulfate reduction terminated with iron sulfurization in the basaltic melt and the formation of a sulfide vein. Sulfate salt melt was a source of heavy sulfur isotope (δ³⁴S = 14.9-17.5‰) for sulfides and of calcium for calcites. The similar isotope compositions of calcites (δ¹³С_PDB = -22.5 to -23.5‰), coals (-22.6 to -25.5‰, n = 25), and graphites (-23.5 to -25.0) suggest that coal is a source of carbon for calcitites. In the isotopic compositions of strontium (⁸⁷Sr/⁸⁶Sr ≈ 0.70834-0.70885) and oxygen δ¹⁸O ≈ 15‰) the calcites are similar to the Devonian evaporites of the Siberian Platform. The isotope data (δ³⁴S, δ¹⁸O, δ¹³С, and ⁸⁷Rb/⁸⁶Sr) point to the participation of the coal seam and anhydrite as sources of substance in the formation of calcitite and sulfide mineralization.

The behavior of fluids during plastic deformation is studied from the morphology and distribution of fluid inclusions in quartz grains of different microstructure types from a vein system controlled by thrusting and strike-slip faulting in the eastern Sayan-Baikal fold area. The analytical work includes electron backscatter diffraction (EBSD) for quartz microstructure and crystallography, as well as Linkam heating-and-freezing analysis and Raman spectroscopy for the composition of fluid inclusions. The studied fluid inclusions are of seven types that differ in morphology and position in the deformed quartz structure. A model is suggested to describe successive structural changes of quartz aggregates during dislocation sliding and subsequent creep-related recrystallization associated with redistribution of fluid. Fluid inclusions undergo qualitative and quantitative changes due to water leakage at all stages of plastic deformation. The changes occur by two main mechanisms: (i) mass transfer during dislocation sliding at medium temperatures and strain rates and (ii) diffusion creep at low strain rates and high temperatures. The contribution of creep increases gradually with temperature, which maintains the interaction of inclusions with migrating grain boundaries.

We studied the bottom sediments of lake systems located in the taiga landscape of Siberia. In the south of Western Siberia, there are 11 lakes in the subzone of the south taiga and 9 lakes in the subtaiga subzone. On the territory of the south of Eastern Siberia in the taiga zone there are 10 lakes on the southern coast of Lake Baikal, 5 lakes on its east coast and 6 lakes are located on the territory of the national park “Alkhanai” in Transbaikalia. Studies of the chemical composition of water, soil, and sediment samples were conducted at the Analytical Center for multi-elemental and isotope research SB RAS, Novosibirsk. The use of a complex of modern analytical methods in the study of the same lake samples made it possible to obtain more reliable information (1861 samples of bottom sediment were studied). Quartz and feldspars of pelitic dimension dominate in the mineral composition of the terrigenous fraction, organic matter and authigenic minerals (opal, pyrite, illite and sometimes calcite) are added to them during the bottom sediments formation. The absolute contents of the studied elements in the bottom sediments of all lakes are lower than their concentrations in the upper continental crust and sedimentary rocks of the Russian Plate, with the exception of Si, Hg, Cd. In the lakes of the subtaiga of the south of Western Siberia, an exception was established for Ca in the bottom sediments of which carbonates predominate. Contents variations of a number of elements in the bottom sediments of taiga lakes in different regions do not significantly differ. The dispersion of their concentrations between lakes with a total catchment area and lakes from different regions are comparable (Ca, Sr, Al, Mn, Fe, Cd, Hg, etc.). The current sedimentation rate in natural occurrence in the lakes of the taiga zone (southern Siberia) vary from 1.1 to 2.9 mm/year. Moreover, it is comparable to the current sedimentation rate in the lakes of the forest-steppe and steppe zones of southwestern Siberia.

The study considers the distribution of organic carbon in different formations and facies of Cambrian age identified in the cis-Yenisei subbasin. Two groups of depositional environments were recognized in the deepwater perioceanic and epicontinental basins, which accumulated sediments with elevated and sometimes anomalously high concentrations of planktonic and benthic organic matter. These are 1) local centers in the Tommotian (Oksym Formation) and Botomian stages of the lower Cambrian, Mayan (Elogui and Pudzhelga Formations) and Ayusokkanian (Kondes Formation) stages of the middle Cambrian; 2) regional Kuonamka horizon, which is composed of Toyonian and Amgian deposits and occurs along the margins of the Siberian Platform; it is represented by the Inikan and Kuonamka formations in the east of the Siberian Platform and by the Paidugina Formation in the study area. The study shows that the organic matter has undergone a high degree of transformation and reached the apocatagenesis grade. The results of the study assume that the hydrocarbon generation intensity was high in the Cambrian formations of the cis-Yenisei subbasin in the geological past.

We report results of an integrated interpretation of geophysical data (2D CDP seismic acquisition and gravity and magnetic survey) conducted by Gazpromneft Science and Technology Center in the North Wrangel lease block on the East Siberian Sea shelf. The newly obtained geophysical data have allowed us to update the geological model of the sedimentary cover within the North Wrangel lease block, to clarify the limits of the Drem-Khed Trough, to reconstruct the conditions under which sedimentary complexes formed, and to identify possible promising oil and gas accumulation zones. We were able to determine the position of the main stratigraphic unconformities, to propose several approaches to their interpretation, and to identify the clinoform architecture of the Aptian-Albian and Cenozoic complexes within the Drem-Khed Trough. The integrated review of geological and geophysical data has been helpful in refining the framework of the main structures adjoining the trough and predicting the extent of potential reservoirs and oil and gas source rocks throughout the sedimentary cover. The Upper Cretaceous and Cenozoic complexes are regarded as oil and gas exploration targets.

The integral Sumudu transform was established as an alternative to the Laplace transform in the 1990s. The Sumudu transform fundamental properties include preservation of the dimensionality of a function, ensuring measurement units both in the function and its image to be equivalent. Among its disadvantages is the absence of an explicit formula for calculating the inverse transform. The transforms are inverted by solving the corresponding Fredholm integral equation of the first kind, which is reduced to solving an ill-conditioned system of linear algebraic equations. We apply Tikhonov’s method for regularization of this type system. The paper proposes a technique for constructing a parametrized regularizing matrix that takes into account the properties of the Sumudu images obtained by modeling the electromagnetic sounding process of the Earth’s interior. A method for choosing the Tikhonov regularization parameters and the regularizing matrix is considered. The effectiveness of the proposed method for the Sumudu transform inversion is examined on a model problem of electromagnetic sounding of the Earth’s interior by a measurement system consisting of two loops spaced apart.

This paper describes a three-dimensional inversion of magnetovariational tippers calculated for the Dalat observatory in South Vietnam on the basis of the geomagnetic variation records obtained from the INTERMAGNET network. The inversion of the frequency dependences of the tippers is performed using the ModEM software, which makes it possible to construct a geoelectric model of the Dalat zone in a 300 × 300 × 150-km spatial region along the x , y , and z axes, with its center located at the observatory. The resulting model of the geoelectric section contains both surface and deep conductive blocks located at the top and bottom of the Earth’s crust and in the upper mantle. The central part of the upper local conducting block with a resistivity of 0.5-1.0 Ohm·m is located near the observatory at a depth of 12-14 km. A more massive conductive block is located to the east of the observatory in the coastal region. Its central part with a resistivity of 0.3 Ohm·m is located at a depth of 24-28 km and extends under the continent along the coast for a distance of about 80 km. The roots of this block are visible to depths of ~100 km in the South China Sea shelf region. It is assumed that the high electrical conductivity of these blocks is provided by melting in the presence of aqueous fluids and granite series rocks, which are widely distributed in the Dalat zone. The crystallization depths of the samples of these granites estimated from the geothermobarometry data are close to the depth intervals of conductive blocks in the model section, which was also observed in the interpretation of magnetotelluric soundings in Southern Tibet, where granitoid rocks are widespread.