Home – Home – Jornals – Russian Geology and Geophysics 2025 number 8

New sedimentological, lithochemical, and biostratigraphic data were applied to describe the main part of the Inikan Formation, which is comprised in the Kuonamka Complex of the lower and middle Cambrian of the Siberian Platform. A classification of carbonaceous rocks in the Inikan and Chaya formations was proposed, based on a comprehensive approach, including textural analysis, study of geochemistry of rock-forming oxides and organic carbon, lithochemical calculations, optical petrographic, and X-ray structural data. This made it possible to clarify the structure of the overall section and the conditions under which Cambrian deposits formed. The widespread influence of volcanic activity during sedimentation in the Kuonamka Complex in the southeastern Siberian Platform was identified for the first time. The distribution of paleoseismic breccia in the section was also described, with horizons that represent good regional stratigraphic markers. Limitations in the use of the Strakhov modulus and the Nesbit-Young chemical index of alteration for characterizing sedimentation conditions of Domanikoids were revealed.

This article presents results of a multidisciplinary study of late Pleistocene deposits exposed at the newly discovered Nozhii locality in eastern Transbaikalia. New data on small mammals have filled a gap in the fossil record of the late Pleistocene faunas in the studied region. The faunal remnants originate from loessial deposits, which are rarely distributed in the region, where spore-pollen samples were collected. A rich, diverse faunal assemblage has been identified, and information on the paleoflora of one of the late Pleistocene ages has been obtained. The fauna is predominantly composed of species characteristic of open steppe landscapes: Ochotona dauurica , Marmota sibirica , Lasiopodomys brandti , Cricetulus barabensis , and Allactaga. The species composition, the quantitative ratio of taxa, and evidence on paleovegetation indicate that, during the existence of the vertebrate community at the Nozhii site, forest-steppes with open steppe landscapes were widespread along with patches of desertified areas and forb meadows as well as sparse forests with a herbaceous cover.

The taxonomic composition of acritarchs from Albian-Cenomanian deposits recovered by boreholes in the southwest of Western Siberia has been studied. Two new genera and species of acritarchs have been described monographically: Pyramidinium annulatum gen. et sp. nov. and Trilobatina angusta gen. et sp. nov. The species P. annulatum is found both in the Albian, Khanty-Mansiysk Formation, and in the Cenomanian, lower part of the Uvat Formation. The species T. angusta is found only in the Cenomanian, Uvat Formation.

Metasomatic reactions involving garnet lherzolite and SiO₂, Al₂O₃, and alkali-rich supercritical fluids or melts have been experimentally studied at pressures of 3.0 and 5.5 GPa and a temperature of 1200 °C. These fluids and melts might have formed during the dehydration, decarbonation, and melting of metasediments in subduction zones. The reaction of garnet lherzolite with model subduction-related mobile phase is shown to lead to changes typical of modal metasomatism in the lithospheric mantle. The reaction with the melt at 5.5 GPa yields phlogopite-bearing garnet lherzolite, while at 3.0 GPa it produces phlogopite-bearing garnet harzburgite. The reaction with a fluid rich in volatiles, mainly CO₂, at 3.0 and 5.5 GPa proceeds through intense carbonation of peridotite, leading to the consumption of olivine and the formation of orthopyroxene and magnesite, which results in the transformation of lherzolite into carbonated pyroxenite. At 3.0 GPa and 1200 °C, a carbonate-silicate melt rich in alkalies forms. In general, the presence of a significant amount of dissolved CO₂ in SiO₂-rich fluid or melt induces metasomatic transformations in peridotite, which are very similar to those occurring during its reaction with carbonatite melts. Specifically, at the molar CO₂ /(CO₂ + H₂O) ratio of <0.23, phlogopite forms, while at its value of >0.51, magnesite is produced. Phlogopite and magnesite do not form simultaneously in metasomatic reactions.

Quartz veins and quartzites of the Unda quartz vein zone (southern part of the Borshchovochnyi Ridge) of the Argun block of the Mongol-Okhotsk Orogenic Belt have been studied using modern high-precision methods. The mineralogical, petrographic, and geochemical features of quartz of the Unda zone have been first described in detail. The vein quartz and quartzites sampled at the tectonic contact between the Middle-Late Jurassic igneous granitoids of the Borshchovochnyi complex and the overlying volcanosedimentary rocks of a Middle Jurassic accretionary complex (Talangui Formation) are intensely deformed and mylonitized. The quartz veins that formed in metamorphic rocks located at a distance from the tectonized zone (Dzhida site) are weakly granulated. The parameters of fluid inclusions in vein quartz at the Borshchovochnyi and Talangui sites were determined. The quartz veins and quartzites are genetically related to the host rocks and inherit their geochemical and isotopic features. Deformation processes led to a decrease in the content of impurity elements and a change in their proportion in the quartz veins and quartzites.

The brine and long drilling core (4.8 m) of bottom sediments of the hypersaline Lake Maloe Yarovoe were studied. For the first time, the detailed chemical composition of pore waters and microbial diversity of bottom sediments were studied along the section. A complex mineral composition of sediments was revealed, reflecting the evolution of salinity and changes in sedimentation conditions in the Holocene Epoch. In the upper horizons, the mineral composition of the sediment is dominated by halite, quartz, and plagioclase, while in the chemical composition, Na and terrigenous elements (Si, Al, Fe, K) are associated with greater salinity and a high level of lake waters, while in the lower intervals, an increase in gypsum, calcite, magnesite, as well as Ca, Mg, Sr, U, S_tot, S (VI) is noted, which indicates a drop in salinity and the level of lake waters in the past. Authigenic minerals (pyrite, hydrotroilite, siderite, ankerite) are formed in reducing environmental conditions, in the process of bacterial sulfate reduction. Distribution of biogenic elements along the section is stratified: maximum TOC (up to 3.2%) and TN (up to 0.52%) in the upper sediment layers is associated with accumulation of OM of planktonic genesis, and a decrease with depth reflects destruction of OM during diagenesis and changes in the bioproductivity of the lake. In pore waters, in comparison with brine, an increase in the average contents of ions SO₄^2-, PO₄^3-, Ca²⁺ and dissolved elements - Si, Al, Fe, Mn, Mo - is noted. A trend is observed for a decrease in Cl^- concentrations with depth and an increase HCO^-₃, Ca²⁺, Sr²⁺, and Mn²⁺ in pore waters, which indicates some decrease in the salinity of lake waters and more intense processes of carbonate sedimentation. Cyanobacterial mats play an important role in the biogeochemical cycles of the lake, they accumulate Zn, S and Mg in their composition, and their microbial communities ( Pseudomonadota, Cyanobacteriota, Rhodothermota ) create unique conditions for the formation of zinc sulfides, copper carbonates, as well as sulfides and intermetallics of noble metals (Ag-Au). Microbial processes play a key role in the formation of authigenic minerals and the destruction of organic matter during diagenesis.

The Laptev Sea sedimentary basin is located in the eastern part of the Russian Arctic shelf and has been fairly well studied by geophysical methods, but drilling data have been lacking until recently. In 2021, six new stratigraphic wells were drilled on the Laptev Sea shelf north of the Anzhu Islands by the order of PAO NK Rosneft. As a result, a unique core material was obtained, which was studied by a number of analytical methods. This paper is concerned with analysis of the geochemical characteristics of the petroleum source rocks of a Cretaceous-Cenozoic complex in the Laptev Sea sedimentary basin, which can be regarded as promising for hydrocarbon generation. The paper presents the main pyrolysis parameters characterizing the share of free hydrocarbons in the rock (S₁), oil generation potential (S₂), and maturity ( T _max) as well as the maceral composition of rocks and vitrinite reflectance ( R ^o). We have established that the carbonaceous and clay-siltstone rocks of the Paleocene-Eocene complex are enriched in organic matter (OM) containing up to 20% lipid component and are capable of generating not only gas but also liquid hydrocarbons. In contrast to the OM of the rocks of the Paleocene-Eocene complex, the OM of the Lower Cretaceous strata is at a much higher stage of transformation, which corresponds to the oil peak zone and the beginning of the wet gas zone. Organic matter accumulated in different lithofacies conditions; therefore, both humic type III kerogen and mixed type II-III kerogen are found in the rocks. Based on the drilling results and studies performed in the section of the Laptev Sea sedimentary cover, we have obtained the first factual data on the characteristics of petroleum source rocks, which can be used to predict the petroleum potential of both the Laptev Sea sedimentary basin and the entire East Arctic region.

The distribution of more than three dozen strongest intraplate crustal earthquakes (with M_w ≥ 6.9) recorded in Central Asia (20°-55° N, 55°-120° E) for the years 1973-2022, including the level of their aftershock activity and background seismicity in the field of velocity heterogeneities of the medium, is considered. The analysis of background and aftershock seismicity with m_b(isc) ≥ 4.5 (according to International Seismological Center isc.ac.uk) was analyzed at the sites of 3° × 3° focal areas. The total scalar seismic moment of the background seismicity and aftershock sequence was calculated within the sites. The background seismicity was estimated for the 50-year time period before and after the main earthquake and its aftershocks. The duration of the aftershocks series is limited to one year. Against the background of the generalized seismotomographic model of Asia (at a depth of ~50 km), crustal foci of 24 strong earthquakes mainly tend to the zones of sign change of P -wave velocity anomalies, three earthquakes are recorded in the low velocity zones and 4 in the high velocity zones. The largest values of the total scalar seismic moments released during the implementation of background seismicity are located within the negative P- wave velocities anomalies, as well as along the boundaries of anomaly sign change. When scalar seismic moments are released during aftershock and background activity, as well as the main event, the prevailing values remain within E+27. Increased values (E+28) are typical for zones of anomalies sign change and areas of positive P -wave velocity anomalies. Low values of total scalar seismic moments of background and aftershock events are observed in the focal zone of six sites (3° × 3°). It is supposed that exactly the limiting magnitude for 100-years (1901-2022) was realized in these zones, and the release of accumulated stresses could be carried out exactly due to the strongest earthquakes.

We assess seismic impacts at various levels of seismic hazard zoning of the Central Baikal area using geophysical, seismotectonic and engineering-seismological studies and our methodological approaches. The implemented approaches help to identify zones of probable large earthquakes and specify their main features. This is necessary to properly assess input signals corresponding to the parameters of predicted large earthquakes. The basis for the reliability of the selected approaches and methods, relative to refining of baseline seismicity, are the manifestations of previously registered relatively large earthquakes in the study area. As far as seismic hazard zoning of particular territories is concerned, on a probabilistic basis, the registered data on the behavior of coastal water-saturated strata of loose sediments, according to local earthquake records, are quite reasonably accepted. The implemented approaches using a set of geophysical methods in the study areas allow us to improve the methods to predict maximum seismic impacts for different ground conditions in order to obtain a set of seismic data in the form of accelerograms, acceleration spectra, frequency curves and their parameters necessary to design earthquake-resistant constructions in the coastal areas of Lake Baikal. The studies are presented for the site located in the seismically active southeastern part of Central Baikal, in seismically worst ground conditions, within coastal areas.

As shown in recent decades, additional information about the structure and composition of rocks can be extracted by studying the transient induced polarization (TIP) in a large time interval from 1 ms to 10 s and more. In this study I present the comparison of study results of TIP of the Yasnoe gold occurrence, obtained via field and laboratory research on 37 rock samples. The samples differed in terms of composition, degree of variation of hydrothermal-metasomatic processes, type of electron-conducting minerals (pyrite and graphite), and features of their distribution in the rock. Mineralogical-petrographic studies and X -ray microtomography (μ-CT) were performed for representative samples of the collection. This made it possible to determine the features of the mineral composition and structure of the samples, as well as to quantitatively describe electron-conducting minerals (pyrite and graphite). The results of field and laboratory research of induced polarization (IP) were applied to obtain distribution of the relaxation times (DRT), which turned out to be close in shape for the same types of rocks. The μ-CT data and the Maxwell-Garnett model were used to calculate the DRT of the samples. In the case of certain samples, there was good agreement between the DRT obtained using the results of IP and μ-CT laboratory measurements. In the case of other samples, obtaining a satisfactory agreement between DRT required making assumptions that some pyrite inclusions were passivated or had a flattened shape. There were samples for which no agreement between DRT could be established. This contradiction was explained by the poor applicability of granular models for well-cemented rocks of low porosity. I concluded that there is a need to develop a capillary model of IP of rocks with electron-conducting mineral inclusions, which accounts for the structural features of low-porosity capillary media.