Home – Home – Jornals – Russian Geology and Geophysics 2026 number 8

We analyze the composition of : rocks and provide age estimates for detrital zircon grains extracted from sandstones of the tuff-turbidite sequence in the Dzhida Zone of the Central Asian Fold Belt. The sandstones are classified as feldspathic graywackes based on their mineralogical and petrochemical characteristics and are interpreted as first-cycle sediments. The detrital material consists predominantly of volcanic rock fragments, with plagioclase grains being subordinate. Relatively low values of the chemical index of alteration (CIA = 58-68) indicate moderate to weak chemical weathering of lithic components. This reflects the low maturity of the detrital material, which was derived primarily through mechanical breakdown of source rocks. The U-Pb age distribution for detrital zircons from the tuff-turbidite sandstones is bimodal, with major peaks at 530 and 510 Ma. No Neoproterozoic zircon grains were identified in the samples studied. The graywacke composition, low sediment maturity, and the observed U-Pb age distribution of detrital zircons collectively suggest that these deposits accumulated in a forearc basin setting, with detritus supplied from local sources - namely, Early Cambrian volcanic rocks of the Dzhida ensimatic island arc.

LA-ICP-MS dating of zircons from a tuff bed in the Bolshoi Patom Formation in the Ura Uplift yielded an age of 633 ± 7 Ma. The Vendian reference section in the southern Siberian Platform therefore includes the last Cryogenian (Marinoan) glaciation, whereas the postglacial Vendian succession is entirely Ediacaran. Using the obtained age estimate of the Bolshoi Patom Formation, we considered chronostratigraphy of the Riphean-Vendian deposits in the Ura Uplift and in the adjacent areas. The available geochronological and sequence-stratigraphic indicate the presence of a hidden stratigraphic unconformity above the glacial horizon at the base of the Vendian sequence, caused by glacioisostasy.

Located in the north-west of the Murmansk Region, the sulphide Cu-Ni ore deposits were discovered in the 1930s. Nowadays they provide approximately 8-10% of the nickel mining in Russia. The deposits are spatially associated with serpentinite massifs, which have long been assigned to either the ophiolitic complex or basalt magma differentiates. The magma is regionally represented by wide-spread fields of pillow lavas and gabbro-diabase sills. The ultramafic ferropicritic volcanic rocks, which alternate in cross-section with lavas and basaltic tuffs, were discovered in the 1970s. Initially the ore-bearing massifs were intrusions composed of peridotite (wehrlite), clinopyroxenite, and gabbro, which were tectonically metamorphosed during the Svecofennian orogeny. Based on petrological, geochemical, and isotopic research, scientists have proved that ferropicritic volcanic rocks and ore-bearing intrusions are genetically bonded. This paper concerns the processes of differentiation of all the ferropicritic magma facies, i.e., intrusive (ore and ore-free intrusions), volcanogenic (massive and pillow lavas, stratified flows, sills), and dike ones (dike bodies, cross-cutting intrusions, and dike swarms in the Archean surrounding host rocks). Their relations, time and sequence of formation have been revealed. We also discuss the role that the processes of contamination and assimilation of the Archean basement rocks and hosting sulphide carbonaceous schists have played in sulphide ore formation.

We present data on the finds of glendonites (pseudomorphs after cold-water ikaite) in Permian, Jurassic, and Lower Cretaceous sedimentary rocks according to the drilling results of borehole KS-1, located on the coast of the Laptev Sea. This is the only section in the world that contains both Paleozoic and Mesozoic glendonites corresponding to three geological systems. We have found glendonites at all previously known stratigraphic levels in northern Siberia, excluding intervals with relatively rare glendonite finds (Oxfordian, Ryazanian, and Valanginian stages). There are the most numerous, diverse, and large glendonites in the Middle Jurassic interval of the section (Bajocian-Callovian). The KS-1 borehole is a unique natural archive that reflects significant late Paleozoic-Mesozoic events of cooling in the region under study.

We present the first reconstruction of a lacustrine sedimentary environment for the extensive area of the Angara steppe (Baikal peripheral basin) in the late Holocene with decadal temporal resolution. The reconstruction is based on the results of a comprehensive study of lacustrine sediments from Lake Ordynskoe: petromagnetic and granulometric parameters and biogenic silica content. We have determined sedimentation rates and the age of the reconstructed events using radioisotope dating (²¹⁰Pb and ¹³⁷Cs). The age of the core base is 2800 cal yr BP. The results show that the primary contribution to the magnetization of sediments from Lake Ordynskoe, unlike other studied lakes in the region, comes from grains produced by magnetotactic bacteria. However, the behavior analysis of biogenic components of magnetization provides information on the lake bioproductivity and relative changes in redox conditions during the formation of sediments at the water-sediment interface, and the results of granulometry are informative for reconstructing relative changes in the lake level. Despite this particularity in the formation of the lake’s petromagnetic sediments, we have established that noticeable anthropogenic impact on the geosystem of Lake Ordynskoe began approximately 280 years ago, significantly increasing in the last century due to agricultural activity.

This paper describes a system for monitoring the seismicity of specific volcanic regions in Kamchatka (Russia, Kamchatka Peninsula) with an assessment of the seismicity level according to the SESL’09 (Statistical Estimation of Seismicity Level) methodology. The application of the SESL’09 methodology for the purpose of investigating the preparation of volcanic activations is a promising direction in conducting volcano status monitoring. We present the results of identifying seismically active source volumes for 10 active volcanoes in Kamchatka, for which nomograms have been constructed that allow for the estimation of the seismicity level for any given time interval based on the seismic energy released within it. Examples of variations in the seismicity level of these volcanic regions associated with the preparation of an eruption are provided, confirming this conclusion.

The article presents algorithms for numerical modeling of electrical micrologging signals and calculation results in geoelectric models of impermeable rocks and the borehole environment of a porous, permeable reservoir. This research may be useful for developing an integrated approach to the quantitative interpretation of electrical logging data. A joint numerical analysis of electrical logs measured by conventional probes and microprobes will enable refining the radial electrical resistivity distribution in the borehole environment of oil-gas reservoirs. This, in turn, will provide additional information when jointly solving the problem of drilling mud filtration and numerical inversion of electrical logging data, resulting in a more accurate assessment of reservoir porosity and permeability. We have developed software for numerical simulation of electrical micrologging signals - those from a microgradient and micropotential probe and a microlaterolog probe with focusing. We employ a finite element (FEM) algorithm with an arbitrary position of the probes in a cylindrically-layered geoelectric model, and a finite difference (FDM) algorithm for the microgradient and micropotential probe in a horizontally-layered model. Test calculations of the signals in geoelectric models of Western Siberian terrigenous deposits are presented. The models describe impermeable sediments as well as layers with the mud cake and invaded zone at different resistivity ratios. The model parameters correspond to Lower Cretaceous oil-gas reservoirs exposed with polymer-clay drilling muds. An analysis of the calculated micrologging signals reflects a small investigation depth of the microgradient and micropotential probe, and a strong dependence of the signals on the drilling mud and mud cake resistivities. Numerical modeling in the class of a cylindrically-layered model has allowed us to obtain the radial resistivity change in the borehole environment of an oil-gas reservoir.