Home – Home – Jornals – Russian Geology and Geophysics 2010 number 2

This study provides a comprehensive sedimentologic analysis of the J₂ prospective horizon (Middle–Late Bathonian) in the central West Siberian Basin. Based on the extensive dataset, it was established that structural complexity of this horizon is largely caused by a variety of depositional environments. Sedimentary facies of this horizon pass upward from widespread continental at the base, through deltaic or coastal-continental, to marginal-marine at the top. Change in the paleohypsometry of the study area reflects the distribution of depositional environments, in terms of the proportions of continental, transitional, and marine deposits. The study also shows that facies variability and change in depositional settings can have a significant control on reservoir quality, as well as its vertical and lateral distribution. Using a combination of detailed GDE reconstructions, structure contour mapping on the top and base of the J₂ horizon, analyses of well test data, probability of reservoir presence, distribution of average porosity and permeability within different subenvironments, a composite map showing variations in the reservoir effectiveness in the J₂ horizon was generated.

The distribution of hydrocarbon accumulations in a basin is modeled as a nonstationary Poisson field of points with the average density of accumulations as a function of distance fr om the basin margin. The model, in which this distance is a unique parameter to define the intensity function, is suitable, in a first approximation, for Monte-Carlo simulation of the real pattern of accumulations. The Poisson random field of points is described with a power function, wh ere the power is a fractal dimension used as an integral numerical parameter of the distribution.

The Late Quaternary history of climate and deposition environments in the western Todzha basin has been reconstructed fr om the respective lacustrine and fluvial deposits. Data from sedimentary sections have provided clues to the causes and timing of sudden deposition changes, including those associated with Holocene tectonic activity in the Taskyl fault. Especially informative are permafrost, diatom, spore-pollen, and tree-ring data from a section at the Merzlyi Yar locality, wh ere frozen ground has preserved the pristine rhythmic pattern of sediments. The Merzlyi Yar section stores a record of dramatic consequences of large earthquakes generated by the fault, which produced a large damlake. The revealed pattern of discontinuous limnic deposition, with prominent tree markers and syngenetic ice wedges, places exact constraints on the stratigraphy of the section and allows its correlation with Holocene climate events within the intervals of fossil soils and vegetation.

Cobalt behavior during the oxidation of sulfide ores, unlike that during the oxidation of Co ores, is poorly known. Moreover, cobalt sulfates are rare in the world. Complex hydrous Co-containing and cobalt sulfates have been found in technogenic zones at the Letnee chalcopyrite deposit (Southern Urals). They have been identified at pit bottoms, in the ore stockpile, as well as directly on ore fragments and the evaporation barriers of underdump water puddles. The paper reports the first experimental data on the oxidative leaching of Co-containing sulfide ores in the laboratory. Also, parts of a thermodynamic model for behavior in oxidized zones are presented.
Experiments have revealed an increase in acidity up to pH = 4.14, along with transport of sulfate sulfur and metals into solution. They suggest acid mine drainage during the development of the Letnee deposit. The published stability diagrams for hydrous Cu, Mg, Zn, Co, and Ni sulfates were analyzed and compared with mineralogical finds in a technogenic oxidized zone. This made it possible to explain the precipitation sequence of minerals from solutions during their concentration by evaporation. As salts of these elements are highly soluble, significant contents of toxic metals inevitably remain in equilibrium solution, necessitating additional waste-water treatment (for example, creating sorption geochemical barriers). Therefore, the paper describes regularities in Co behavior during its sorption on solid phases.

Results of U-Pb (SHRIMP-II) and Rb-Sr dating of sillimanite schists in southwestern Transbaikalia are presented, which testify to high-temperature metamorphism in the Late Mesozoic. The metamorphism might have been related to the formation of metamorphic cores or tectonoplutonic activity accompanied by the formation of rift structures in Transbaikalia and Mongolia.

We consider the structural position and petrology of eclogites in the North Kokchetav accretion-collision zone located north of the Kokchetav metamorphic belt formed by high- and ultrahigh-pressure rocks. In the Early Ordovician North Kokchetav tectonic zone, thin sheets of mylonite and diaphthoric gneisses with eclogites are tectonically conjugate with the volcanic and sedimentary rocks of the Stepnyak paleoisland-arc zone. Eclogites have been revealed at two sites of the North Kokchetav tectonic zone — Chaikino and Borovoe. The Chaikino eclogites formed at 800—850 °C and 18—20 kbar, and the Borovoe eclogites, at 750—800 °C and 17—18 kbar. Study of pyroxene-plagioclase symplectite replacing omphacite of the eclogites at both sites has recognized three stages of regressive magmatism: (1) formation of coarse-grained clinopyroxene-plagioclase symplectite at 760—790 °C and 11—12 kbar, (2) formation of fine-grained clinopyroxene-plagioclase symplectite at 700—730 °C and 7—8 kbar, and (3) amphibolization of pyroxene at 570—600 °C and 5—6 kbar. The Ar-Ar age of muscovite from the Borovoe mica schists hosting eclogites is 493 ± 5 Ma, which corresponds to the time of cooling of metamorphic rocks to <370 °C. Hence, the peak of high-pressure metamorphism and all recognized stages of retrograde changes are dated to the Cambrian. The geological data evidence that eclogite-schist-gneiss sheets were localized in the accretion-collision zone and became conjugate with sedimentary and volcanic rocks no later than in the Middle Ordovician.

TEM responses of conducting ferromagnetic materials to a complex current pulse have been modeled for a magnetic cylinder typical of well casing. The behavior of the responses is investigated as a function of different model parameters.

Concentrations of helium isotopes were measured in gas and water samples from 28 thermal mineral springs in Tuva and adjacent regions of Buryatia and Gorny Altai. It is shown that fluids from 16 springs are rich in mantle helium (4—35%). With regard to the air contamination of the samples, the corrected ratios of helium isotopes (R _cor = ³He/⁴He) in these springs vary from 5.3·10^–8 to 422·10^–8. Using these Rcor values, we estimated the heat flow; these estimates were then applied to calculate the deep-level temperatures and thickness of thermal lithosphere. According to these parameters, the Tuva region is divided into two parts. Eastern Tuva (from ~96o E to the boundary with Buryatia) is characterized by abnormal helium isotope ratios and heat flow values indicating the intense heating of the Earth’s crust in eastern Tuva. At a depth of 50 km, a temperature reaches 1000—1200 oC, and the thickness of thermal lithosphere is reduced to 70—50 km. This testifies to a rift process west (probably, up to 96o E) of the Baikal Rift Zone. In western Tuva, the average heat flow is much lower, ~45—50 mW/m², which is commensurate with that in the Altai-Sayan folded area as a whole. The deep-level temperatures here are twice lower, and the lithosphere thickness increases to 150 km.

We suggest a model of radon emanation under compression or extension strain from a medium equivalent to rocks containing pores and cracks. The model is shown in several examples to be suitable for simulating the nucleation of rock bursts in deep mines and earthquakes. According to correlation of strain and radon measurements at the same sites, a relative strain change of n · 10^-7 corresponds to a 200 % change of radon activity concentration. This high sensitivity means that radon data can be good tracers of tectonic movements.

Damage and parameters of the earthquakes of October 14, 1800, September 18, 1826, and July 7, 1842, in Santiago de Cuba, southeastern Cuba, have been studied. A quantitative re-evaluation of the size of the events is not feasible due to a lack of data. Hence, we have reinterpreted existing data to establish likely intensities and determine the epicentral region of occurrence. Available data do not permit accurate depth determinations. Intensities estimated from contemporary documentary sources give maximum values of 8, 8—9, and 9 (MSK scale), respectively. These seismic shocks were located in the Southeastern Seismotectonic Province of Cuba, Caribbean—North American Plate Boundary Zone. The earthquakes are associated with the Oriente fault system.