Home – Home – Jornals – Russian Geology and Geophysics 2014 number 10

This paper is concerned with study of tephriphonolites, rare rocks that have been first found in the pre-Jurassic basement of West Siberia. Based on geological observations of the bedding of different types of volcanics and their geochemical peculiarities, the conclusion about the synchronous occurrence of subalkaline and alkaline volcanism is drawn. This is confirmed by the coexistence of compositionally contrasting volcanics in the borehole section. The hypothesis is put forward that these rocks alternate in the section, as is the case in the Tunguska syneclise of the Siberian Platform. The new data on Sr and Nd isotopy suggest the participation of crustal material in the formation of alkaline rocks. The rift nature of the regional intraplate volcanism is confirmed.

More than 20,000 lakes with sapropel sediments are located in southern West Siberia. The work is aimed at an integrated geochemical study of ten sapropel lake systems: a geochemical description of the lake waters, hydrobiological analysis, and analysis of the sapropels (phase and chemical analyses of inorganic matter and elemental analysis of organic matter). The geochemical landscape and low-flow regime of the waters of Siberian small lakes favor massive growth of living organisms. The anaerobic medium at the water-sediment boundary promotes the burial of organic matter as terrigenous material builds up. Based on the results, the sapropels are classified by chemical and mineral compositions into organic, organic siliceous, organic carbonate, organic carbonate-silicа, and carbonate-siliceous. The sapropels inherit the contents of elements in soils and parental rocks, but the presence of considerable amounts of organic matter in the sediment determines their lower contents with respect to those in the soils of water catchment areas (except U, Cd, Hg, Ca, Sr, and Mn). Biochemical formation of pyrite and calcite is observed in the lake bottom sediments. The calcite formation in the sapropel sediments of Lakes Barchin, Bergul’, Kankul’, and Itkul’ determines their high Ca, Sr, Mn, and Mg contents. Comparison of data on the anion composition of the waters and the organic and mineral compositions of the sapropels has shown that the formation of low-ash biogenic sapropel takes place in lakes whose waters contain a considerable amount of dissolved organic matter and in which HCO₃^– is a predominant anion. No effect of the cation composition of the waters (Na, Ca, and Mg) on the sapropel composition has been detected. Therefore, Lakes Kankul’, Kachkul’nya, Bergul’, Barchin, and Kambala are the sapropel lake systems with the highest potential.

In Eastern Kazakhstan, Sb mineralization is the most widespread in the Irtysh and Bakyrchik ore districts of the West Kalba gold-bearing belt. It is spatially related to disseminated gold-sulfide ores at some deposits and is structurally and spatially isolated at others. The disseminated gold-sulfide mineralization is localized in Carboniferous carbonaceous-terrigenous carbonate rocks. It is marked off by zones of dynamic metamorphism and foliation and is characterized by the ribbon-like-lenticular morphology of ore deposits. Later Sb (predominantly, quartz-antimonite) mineralization is formed in the extension setting as brecciated/veined ores. In combination with gold-sulfide ores, Sb mineralization is more diverse. For example, microparageneses with berthierite, native Sb, aurostibite, ullmannite, jamesonite, and tetrahedrite coexist with pocket-vein quartz-carbonate-antimonite mineralization in the gold-sulfide ores of the Suzdal’skoe deposit. Also, Sb-containing minerals such as arsenopyrite and pyrite are observed. Two temperature regimes of mineralization are established here: 418–300 °C for gold-polysulfide mineralization and 280–200 °C for later Sb mineralization. The isotopic composition of antimonite sulfur at the Suzdal’skoe, Zherek, Zhanan, Bakyrchik, and Dal’nii 1 deposits shows close values within the interval δ³⁴S of –3.8 to +2.5 ‰, suggesting its great-depth origin. No visible gold is found in the antimonite of the quartz-antimonite veins, but atomic-absorption analysis reveals few ppm or more gold. Point X-ray analysis indicates possible presence of the so-called “invisible” gold. Microstructural observations, temporal relationships of the parageneses, and studies of gas-liquid inclusions at the Suzdal’skoe deposit permit assigning Sb mineralization to the second productive gold-polysulfide stage of the ore deposition. The late antimonite stage of mineralization is separated from the gold-polysulfide stage by 7 Myr long intramineralization tectonic shifts. Gold-polysulfide mineralization (248.3 ± 3.4 Ma) was synchronous with Triassic tectonomagmatic activity.

The paper deals with the origin of a large number of sand massifs concentrated in the South Minusa basin. The study of the sand deposits was based on field and laboratory work. Cross sections were selected in such a way as to penetrate the deposits of ridgy sands and the underlying surface at different hypsometric levels. Cross-section studies have demonstrated that the sands formed both under subaerial and subaqueous conditions. Analysis of the work of our predecessors has shown that the formation of a thick unit of subaqueous sand deposits, including those at high hypsometric levels, might be related to the removal of large amounts of sediments by catastrophic flooding along the Yenisei River caused by the failure of an ice dam in the Darhad basin at ~17 ka. The overlying sand deposits, which now occur as sand bars, are of eolian genesis. These bars formed in the Late Holocene owing to eolian transportation and redeposition of the underlying sand members.

This study presents new data on transgressive-regressive and accommodation-sedimentation regimes in the eastern Russian Plate during the Middle Jurassic-Lower Cretaceous. The proposed generalized scheme illustrating the combined effects of three major factors (eustasy, tectonic “noise”, and depositional gradient) controlling the deposition of sequences with different stratal architecture allowed us to quantify the parameters of sedimentation (δS) and accommodation (δA) for second- and third-order cycles. A distinctive feature of the evolution of the Middle Jurassic-Lower Cretaceous sedimentary basin is the excess of accommodation space over sediment supply, which was not conducive to creation of clinoforms. The difference between stacking patterns in individual time intervals and the estimated values of δA/δS may be indicative of the presence of unidentified stratigraphic breaks in the Bathonian and Late Tithonian-Berriasian, which were accompanied by erosion and reworking of sand strata. The stepwise regressive-transgressive deepening during the Oxfordian-Early Tithonian and transgressive-regressive shallowing during the Late Tithonian-Berriasian were probably caused by short-term manifestations of local tectonic “noise”, and depositional hiatuses accompanied by the erosion of missing elements in the structure of third-order cycles. The Lower Cretaceous succession exhibits no mismatch between transgressive-regressive and retrogradational-progradational cycling, which provides another supporting evidence for a quiet tectonoeustatic and sedimentation regime during the Early Cretaceous compared to that of Middle-Late Jurassic time.

The Oxfordian Stage of West Siberia contains Boreal ammonites Cardioceratidae. The authors’ bank of paleontological data includes ~500 definitions of Cardioceratinae, permitting a considerable refinement of the official Oxfordian regional zonal scale. The lower substage is divided into the Cardioceras (Scarburgiceras) obliteratum, C. (S.) scarburgense, and C. (S.) gloriosum Zones instead of beds with C. (S.) spp., whereas the C. (Cardioceras) percaelatum and C. (C.) cordatum Zones are recognized instead of beds with C. (C.) spp. We have found new ammonites typical of the Middle Oxfordian C. (Subvertebriceras) densiplicatum and C. (Miticardioceras) tenuiserratum Zones. The first of these zones is divided into two subzones. The Upper Oxfordian includes the Amoeboceras glosense and A. serratum Zones instead of beds with A. spp., and the A. regulare Zone and beds with A. rosenkrantzi are recognized instead of the A. ex gr. regulare Zone. The genus Ringsteadia (Aulacostephanidae) is observed only in the northwestern part of the region, along the eastern slope of the North Urals; therefore, two upper units of the biostratigraphic scale correspond to beds with Ringsteadia marstonensis. In the Oxfordian, West Siberia and northern Siberia belonged to the North Siberian province of the Arctic realm. Only in the latest Oxfordian did the northwestern West Siberian basin become part of the Boreal-Atlantic realm, as evidenced by the distribution of Ringsteadia on the eastern slope of the Cis-Polar Urals.

Experimental and theoretical research into the processes of radon emanation during failure of rocks has been carried out. We propose a physical model for the mechanisms of origin of anomalies of radon volume activity. Based on the obtained experimental data, relative changes in the open porosity and specific inner surface of rock during its failure have been studied.

Earthquake nucleation and fracture propagation in deformed rocks generate elastic waves, within acoustic frequencies. Strain-induced acoustic waves appear both in field tectonic structures and in laboratory samples, thus making laboratory acoustic emission (AE) data from load tests suitable to interpret natural seismic processes. However, laboratory tests are commonly run at room temperature, while the natural rocks at the earthquake origin depths are as hot as hundreds of degrees centigrade. We report AE data for thermally and mechanically loaded granites subjected to impact fracture at different temperatures. The energy distribution in the time series of acoustic signals emitted from fine-grained granite fits a power law of the type of the Gutenberg-Richter relationship at temperatures from 20 to 500 ºC. Medium- and coarse-grained samples behave in this way only within 300 ºC but show a Poissonian (random) AE energy distribution above 300 ºC.

Phase change of dielectric magnesiowьstite in the lower mantle may leave signatures in geomagnetic records of the globally distributed array of observatories. The related features appear in EM induction responses of lower mantle, which are studied theoretically. The surface EM field corresponding to a response of the earth with conductivity anisotropy in a mantle spherical layer is presented as the sum of the magnetic and electric modes. Equations for the fields of both modes and their relationship in a weakly anisotropic earth are obtained by the perturbation method. The two field modes are analyzed jointly and separately to characterize the conductivity tensor of the anisotropic lower mantle. The tensor elements corresponding to the tangential components of the field can be estimated from the magnetic mode alone recorded currently by the global network of geomagnetic observatories. For the tensor data to be complete, observatory data on lateral variations of the electric field are required in addition to three-component geomagnetic records.

The mantle structure in Central Asia was investigated by surface-wave tomography from dispersion of the fundamental mode of the Rayleigh wave group velocities along more than 3200 earthquake-station paths within 40º N to 60º N and 80º E to 132º E. The velocities were processed by the frequency-time analysis at periods from 10 to 250 s to obtain their dispersion curves. Then group velocity maps were computed separately for each period, at different sampling intervals: at every 5 s for the short periods from 10 to 30 s, at 10 s for periods between 30 and 100 s, and at 25 s for the longest periods of 100 to 250 s. Resolution was estimated according to the effective averaging radius (R) and presented likewise in the form of maps. To estimate the depths of the revealed inhomogeneties, locally averaged dispersion curves were calculated using the group velocity maps, with reference to the radius R , and were then inverted to S-wave velocity-depth profiles. The resulting three-dimensional S-wave velocity structure to depths of about 700 km revealed large lateral inhomogeneties through the entire depth range. This pattern may be due to the history of the major tectonic structures, as well as to ongoing processes in the mantle.