Home – Home – Jornals – Russian Geology and Geophysics 2011 number 6

The paper presents the first comprehensive geochemical data, obtained by modern techniques, on the dispersed organic matter (OM) of the Cambrian sediments in the Upper Proterozoic-Paleozoic Cis-Yenisei sedimentary basin (parametric wells Vostok-1 and Vostok-3). The carbonaceous rocks of the Churbiga and Paidugina Formations have been established to be the most OM-enriched in the studied Cambrian sections. The OM in these sections is of aquagene (bacterial, plankton-derived) origin. The detailed characteristics of biomarker hydrocarbons has been given for the fraction of saturated hydrocarbons. Also, the high degree of OM catagenesis (apocatagenesis) has been established. Finally, geochemical criteria for predicting the petroleum potential have been considered.

Study of the Holocene sediments of Lake Beloe (West Siberia) in the depth range 0-137 cm gave an insight into the transformation of organic matter (OM) at the early stages of diagenesis. Analysis of OM was performed by pyrolytic methods (Rock Eval and pyrolysis + chromato-mass spectrometry). It showed that the macromolecular aliphatic structure of the kerogen and the precursors of sterane and hopane geomolecules - sterenes and hopenes - form at the early stages of diagenesis. We suggest that macrophytes and bacteria are the main sources of OM for the lacustrine sediments.

The Dongshen uranium deposit, localized in the northern Ordos Basin in north-central China, is among the largest deposits of this type in China. It occurs in the sandstones of the Middle Jurassic Zhiluo Formation. The crystalline rocks of the Yinshan and Daqingshan orogenic belts in the northern Ordos Basin are the source of uranium for the deposit. The Dongsheng deposit shows the main characteristics of a sandstone uranium deposit of interstratal oxidation zone, including specific hydrothermal rock alteration. It displays distinct lithologic and mineral zoning. The sandstones of the oxidation zone are variegated and oxidized, and the sandstones of the reduction zone are gray and light gray medium- to coarse-grained and underwent albitization, carbonatization, chloritization, epidotization, pyritization, etc. Ore-bearing sandstones occur in the transitional oxidation-reduction zone, being confined mainly to the reduction zone. Uranium minerals are predominant high-temperature coffinite and subordinate brannerite and uraninite. The homogenization temperature of fluid inclusion in the sandstone cement is 150-160 °C. The performed geological analysis suggests that the hydrothermal alteration of the uranium deposit is intimately related to the synchronous Mesozoic regional magmatic events in the Ordos Basin.

Platinum mineralization has been studied within two zoned clinopyroxenite-dunite massifs of the Urals Platinum Belt. Within the Svetloborsky massif, platinum group minerals (PGM) occur directly in serpentinized dunite. The ore-forming system demonstrates the features of an early evolutionary stage: a significant portion of PtFe alloys belongs to ferroan platinum; native osmium is depleted in Ir; an isoferroplatinum-osmium paragenesis is present. Within the Kamenushinsky massif, PGM are genetically related to chromitite and belong to the late-magmatic stage of the ore-forming system evolution: osmium and isoferroplatinum are rich in Ir; both isoferroplatinum-osmium and isoferroplatinum-iridium magmatic parageneses are present. The elevated Pt in the dunite of the Svetloborsky massif and in the chromitite of the Kamenushinsky massif is the result of two events: the primary-magmatic crystallization of PGM and later hydrothermal overprint.

The Cheder meteorite, found 23 km south of Kyzyl, has been studied in terms of mineral composition and internal structure. Structurally, it is a medium octahedrite, which measures 26 × 9 × 8 cm and weighs 5.39 kg. Its surface is streamlined and features flow lines. One of the sides, probably that facing the flight direction, features regmaglypts and a sinuous cavern (9 × 1.5 cm). The meteorite consists of kamacite admixed with taenite. The inside of Widmanstatten bands is of complex kamacite-taenite composition and flanked by taenite lamellae. Phosphides are present in kamacite in small amounts as variously sized and shaped bodies. They are divided into three types depending on the Ni/Fe ratio. Small shapeless wormlike grains, which are usually assigned to rhabdites, consist of nickelphosphide (Ni-rich variety). All the minerals in the meteorite contain Co (up to 67 wt.%) and, sometimes, Cu and P (slightly above the detection limit). The total PGE content of the meteorite is several times higher than that of chromitites, which are one of the terrestrial PGE sources. The REE in the meteorite are dominated by HREE. The total REE content of the meteorite is lower than that of chondrite C 1 by two orders of magnitude.

The studied massifs in western Transbaikalia (Arsent'ev and Orongoi) are composed of pyroxenites, high-Ti subalkalic gabbro, gabbro-diorites, monzodiorites, anorthosites, and syenites. The Orongoi massif also includes small olivinite and plagioperidotite bodies. The gabbroids have high contents of Sr, Ba, Nb, Ta, Zr, and Hf, which is typical of intraplate basites. The REE pattern shows the rock enrichment in LREE (La/Yb_N = 5.35-25.82). The ⁸⁷Sr/⁸⁶Sr values vary from 0.7050 to 0.7054, and ε_Nd, from 1.44 to -1.18. The presence of radiogenic Nd in the gabbroids and their enrichment in ⁸⁷Sr suggest their formation as a result of the melting of the EM-II-type lithospheric mantle.

The subaqueous sediments of the first terrace above the Ob' River floodplain near the town of Kolpashevo are referred to as the Sartan cryochron. New data show that they formed mainly in the Early and Middle Holocene. The key section is on the right bank of Novoil'inskaya Staritsa - a branch of the Ob' River 10 km south of Kolpashevo. Radiocarbon dating of the peat at the terrace base yielded an age of 10,200 ± 55 years, which is close to the Pleistocene-Holocene boundary. The studied sample from the middle section has a ¹⁴C age of 5870 ± 55 years corresponding to the Atlantic Holocene optimum. The palynological data confirm the geologic structure of the section - the recognized spore-pollen spectra distinctly mark the first half of the Holocene and reflect the landscape changes. The results obtained help to refine stratigraphic charts and maps of Quaternary deposits and reconstruct the landscapes (including geochemical ones) and lithologo-facies and neotectonic conditions.

A 3-D model of the seismic heterogeneities of P - and S -velocities has been constructed down to 1100 km beneath the Kurile-Kamchatka and Aleutian subduction zones on the basis of the regional tomographic inversion of data from global seismic catalogs. Particular attention is paid to verifying the data by different tests. A clear image of a classic subducting oceanic slab is observed along the entire Kurile-Kamchatka arc, which coincides in the P - and S -models and with the distribution of deep seismicity. These data served as a basis for a parametric model of the upper and lower slab boundaries beneath the Kurile-Kamchatka arc. According to this model, the slab has various thicknesses and maximum penetration depths in different arc segments. In the southern part of the arc, between depths of 600 and 700 km, the slab moves horizontally and does not penetrate the lower mantle. Beneath the North Kuriles and southern Kamchatka, it subducts down to 900 km. These data suggest that the subducting slab becomes a viscous and nonelastic body and the changes in its shape may be due to phase transitions with increasing temperature and pressure. We attribute its gentler dipping and thickening beneath the South Kuriles to the oceanic "pushing" mechanism. The lithospheric thinning, steeper subsidence, and penetration into the lower mantle beneath the North Kuriles are due to the predominant "gravity sinking," or "slab pull," mechanism.
Unlike some other researchers, we have obtained a high-velocity anomaly beneath the western Aleutian arc (not as clear as beneath the Kurile-Kamchatka arc, yet quite reliable). It suggests the presence of a slab subducting down to 200-250 km. In the eastern Aleutian arc, we clearly observe the Pacific slab subducting down to 500-600 km (somewhat deeper than in the previous studies).

Terrigenous reservoirs are studied by a joint analysis of processes of various physical nature. This study is urgent because new methods for formation evaluation (first of all, permeability) from electric and electromagnetic logging data are required. We propose a method for the complex processing and interpretation of time-lapse well logging electromagnetic measurements, which show the dynamics of processes in the well influence zone. The constructed electrohydrodynamic model of the borehole environment is used to estimate the hydrophysical (petrophysical) parameters of the formation.

Active fault zones are considered open systems. Tectonic stress energy accumulated and redistributed in the areas of its active dynamic influence under the action of various energy sources is spent mainly in slow tectonic and discrete rapid seismic dissipative processes. Based on the results of physical modeling of the formation of a large fault zone, the spatial and temporal relations between tectonic and seismic dissipative processes are analyzed and their controlling factors are estimated. Tectonic and seismic dissipative processes are in particular spatial and temporal relations during the structural evolution of the fault zone and occur in antiphase. As a rule, the maximum development of one process coincides with the minimum occurrence of the other, and these moments correspond to the beginning or end of stages, phases or other shorter evolutionary periods. Depending on the scale of the deformation process, the frequency of repeatability of these periods determines the periodicity of activation of tectonic and seismic dissipations and their spatial features.