Home – Home – Jornals – Russian Geology and Geophysics 2025 number 3

Litho- and biostratigraphic analysis of a number of Ordovician reference sections has been completed in five different blocks of Salair, including West-Berd’ (Medvedkovo), East-Berd’, Krasnoe, Gurievsk and Eltsovka. Modern data on nine mainly terrigenous and, less often, carbonate-terrigenous sections bearing fossils and composed of carbonate units and volcanic rocks including lavas, as well as tuff and tuff sandstones have been presented. The Ordovician terrigenous sedimentation in the study area was complicated by stages of volcanic sedimentation. Volcanic-sedimentary sequences were documented at four biostratigraphically dated stages in Salair: 1) the Tremadocian, 2) the late Floian, 3) the middle - late Darriwilian, 4) the late Katian - Hirnantian.

The aim of this work is to elucidate the specifics of recent sedimentation in the Chukchi Sea, based on the study of the material composition of Late Holocene bottom sediments. Analytical methods included grain size and mineralogical analyses. The results of ²¹⁰Pb and ¹³⁷Cs dating were used. The sediments are clayey silt and silty clay containing sand admixture and single gravel grains and pebble. The content of coarse-grained material increases slightly in the upper parts of the studied sections. This is probably due to the greater contribution of ice rafting to the recent Chukchi Sea sedimentation as a result of climate warming. The ultrafine- and fine-grained (0.25-0.05 mm) sandy material is dominated by light-fraction minerals. Volcanic glass was discovered in the sediments sampled in the southern and central areas of the Chukchi Sea. It might have been transported from the volcanoes of Alaska, the Aleutian Islands, and the Kamchatka Peninsula through the Bering Strait by the Pacific currents directed from south to north. The sediments collected in the southern, central, and northern areas of the Chukchi Sea differ in grain size and mineral composition. This is probably due to their remoteness from the coastline and provenance areas and is also caused by different rates of sedimentation.

A digital elevation model of the North Stanovoi metallogenic zone located in the north of the Stanovoi volcanic-plutonic belt is comprehensively analyzed. The correlation of the calculated parameters with some geological and geophysical characteristics of the territory is shown. Spatial relationships of relief features with the location of gold deposits and ore occurrences in the metallogenic zone are analyzed in detail. Many features are determined that can serve as a basis for identifying potentially gold-bearing regions of the ore cluster rank.

Petrographic and mineralogical studies have been carried out for major varieties of rocks composing the Burpalа massif: peralkaline (nepheline), alkali, and quartz syenites. The emphasis was put on the major and trace element composition of mafic minerals present in all types of rocks: clinopyroxenes, micas, and amphiboles. The studies have shown that the compositions of mafic minerals fractionate within each group of rocks but do not form a single trend of evolution in the groups of peralkaline, alkali, and quartz syenites. This suggests the formation of the massif under pulsed intrusion of variably differentiated magma, the contamination of the first portion of magma, and the subsequent formation of quartz syenites.

The geochemical and mineralogical features of gypsum of the Kungurian Stage in the southern Cis-Urals have been studied. Elevated contents of Sr (up to 1.6 wt.%) were detected, and its sedimentary genesis has been established. The main Sr-containing mineral in gypsum is сelestine of two generations differing in morphologic features and chemical composition. The presence of early diagenetic celestine in pelitomorphic zones of gypsum is evidence for lagoon sedimentation in the Kungurian. Barium-containing celestine of late generation often develops in assemblage with magnesite in recrystallized zones of gypsum. The weak positive correlations of Sr with Fe, Mg, and К indicate the enrichment of magnesium chloride and calcium-sodium sulfate-chloride brines with these elements during their interaction with rocks, including argillaceous ones, during halocatagenesis. During diagenesis, nodular and microcrystalline pyrite 1 with impurities of Cu, As, Co, and Ni formed. During catagenesis, the recrystallization of gypsum resulted in pure coarse-crystalline pyrite 2. The rocks near the boundary of the Kungurian and Ufimian stages are characterized by elevated Cu contents (up to 283 ppm). The catagenetic celestine is characterized by low Sr/Ba ratios, which, along with the data on the isotopic composition of sulfur of celestine (δ³⁴S = 24.8‰) and with the presence of pyrite inclusions in gypsum, indicate widespread sulfate reduction processes during catagenesis. The increase in the share of ³²S in gypsum and anhydrites of the southern Cis-Urals (δ³⁴S = 7.4-13.4‰) is due to the increasing role of continental runoff in the Permian salt basin.

Gas composition of bottom sediments of the Laptev-Siberian Sea geostructural zone (LSZ) was determined to include: helium and hydrogen in concentrations of 0.0028-0.2092 and 0.0012-0.8727 cm³/kg, hydrogen sulfide - 0.0006-0.0072, carbon monoxide - 0.0016-0.0577, carbon dioxide - 0.1482-21.1602, methane - 0.0017-8.3047 and its homologues (in total) - 0.00001-0.0355 cm³/kg. It was found that the values of average concentrations of gases in bottom sediments increase with the depth of their occurrence and exceed the anomaly criteria identified for the East Arctic shelf by 2.6-27.7 times, including: CO₂, CO and H₂S - by 2.6, 3.0 and 3.4 times, ΣC₂-C₅ and He - by 9.3 and 10.8 times, CH₄ - by 27.7 times; this actually indicates a high degree of gas saturation of bottom sediments of the region. The formation of CO₂, CO, H₂S, H₂, CH₄ and ΣC₂-C₅ anomalies in sediments of the upper and the base of the middle sampling horizon is determined by relatively high contents of sapropelic and humic organic matter and gas influx from the lower horizon. Distribution of anomalies with maximum concentrations of natural gases and He in sediments of the lower horizon, with minimum contents of C_org, is associated with zones of faults, large dislocations, tectono-magmatic and seismic activity, which are the main routes of diffusion-migration of gas transfer to bottom sediments. Formation of concentrations of gases and their anomalies in sediments of the LSZ follows the rules of additivity, i.e. successive accumulation of migratory natural gases of different genesis with prevalence of gas phase and isotopic indicators of more gas-saturated parental source, epigenetic and syngenetic to sediments. It was found that maximum values of average gas saturation with CH₄, ΣС₂-С₅, He and H₂ in LSZ are typical for bottom sediments of the flanks of troughs, CO and CO₂ - for the arch parts of uplifts. In sediments of monoclinal folds (structural terraces), minimal values of average gas saturation are observed, with the exception of the South Anyui Fault Zone. In general, gas saturation of LSZ sediments is determined by the complex impact of geological factors, with the major ones being: gas content of rock complexes and gas saturation of underlying sediments, fault and fold tectonics, a high degree of tectono-magmatic and seismic activity, a geostructural position, coal content, oil and gas content, as well as the depth of occurrence, organic saturation and material composition of OM in bottom sediments. The influence of the latter predetermined the formation and distribution of five geochemical fields of CH₄, three of CO, ΣС₂-С₅, He and one of CO₂, and one of H₂ in LSZ.

The results of a geochemical study of rocks and dispersed organic matter (OM) of the middle member of the Vendian Kharayutekh Formation of the Chekurovka Anticline (Kharaulakh anticlinorium of the Verkhoyansk fold-and-thrust belt) close to the Siberian Platform border are presented. Its rocks are found to be unevenly enriched with OM, the maximum present organic carbon content (C_org) reaches 2.6%. The composition and distribution of chemofossils in saturated and aromatic bitumen fractions were studied using gas chromatography and gas chromatography-mass spectrometry. The distributions of alkanes, steranes, terpanes, dibenzothiophenes, and aromatic steroids as well as light isotopic composition of C_org indicate a bacterial-algal origin of OM. Rocks of the middle member of the Kharayutekh Formation originally possessed a high generative potential. Analysis of pyrograms, bitumen characteristics, and the distributions of phenanthrenes and dibenzothiophenes allowed assessing the regional catagenesis of the OM in Vendian deposits in this area corresponding to the early dry gas «window». It was established that the studied bitumens are classified as residual and bare evidence of autochthonous and paraautochthonous origins, with biodegraded hydrocarbons. The study demonstrates the local effect of magmatism on the content and composition of bitumens in the near-contact zone of the section. Intrusion of a thick diabase sill resulted in depletion of generative potential of these rocks in the upper part of the section already in the Cambrian period. The considered information gives us more grounds to assume that accumulations of highly viscous heavy oils and solid bitumens, genetically related to the OM of the middle member of the Kharayutekh Formation, might be expected in the Vendian and Cambrian carbonate and terrigenous reservoirs in the north of the Verkhoyansk Trough. The preservation of gas and gas condensate accumulations is unlikely to be good.

Variations in ambient seismic noise generated by the Kyngarga river (Tunka Valley) in the calm state and during significantly rising water levels were analyzed, based on the Arshan short-period seismic station data, observations of water level fluctuations at the Kyngarga Rv. hydrological station, and the amount of rainfall measured with precipitation gauge at Peak Lyubvi mountain (Mt). The methods used in the work include the correlation, spectral, spectral-temporal and polarization analyses. Ambient noise vibrations recorded at the Arshan seismic station show instable over time and show significant seasonal variations. During warm periods, the ice cover melting increases the water level in the Kyngarga River, thus causing amplification of seismic vibrations in the frequency range from 2 to 20 Hz. Occurrence of heavy rainfalls and subsequent significant rise in the Kyngarga river water levels (floods) lead to changes in the amplitude-to-frequency response characteristics of ambient seismic noise. This translates into amplification of seismic vibrations in the frequency range from 4 to 20 Hz, with maximum seismic radiation (radiative transfer of seismic waves) in the 6-10 Hz frequency range. For different floods, the amplification varies significantly and depends on the magnitude of the water level rise. Variation of amplitude-frequency characteristics of the ambient seismic noise in summer relative to winter, as well as during floods, is driven by increased water flow turbulence with affiliated increase in water flow velocity and discharge, and sediment dynamics. Diagrams showing particle motion in the medium in a calm state and during floods do not show any dominant orientation of oscillations, which is associated with the length of the source of vibrations whose extent (in this case) is equal to the entire river channel.