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Russian Geology and Geophysics

2007 year, number 6

1.
Physicochemical modeling as applied to study of sulfoarsenide complexes in hydrothermal solutions

N.V. Vilor a , L.A. Kaz'minb
a Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
b Research Geotechnological Center, Far Eastern Branch of the RAS, 30 Severo-Vostochnoe shosse, Petropavlovsk-Kamchatsky, 683002, Russia
Keywords: Thermodynamic constants; solubility; orpiment; inverse problem; arsenic; sulfoarsenides
Pages: 457-467

Abstract >>
System As-Na-S-Cl-H-O was studied. The research was carried out in three stages: (1) selection of the most likely complexes resulting from arsenic sulfide dissolution, (2) calculation of their thermodynamic constants, and (3) comparison of calculated data with thermodynamic database obtained in tests with the solution of inverse thermodynamic problems using the Selektor program complex. The system As-Na-S-Cl-H-O included more than 230 dependent components, which were divided into two groups, base and functional. The former group includes components of the solution (NaCl, NaOH, Na2S, NaHS, HCl, H2S, H2SO4, sulfates, H2SO3, sulfites, thiosulfates, Na+, Cl-, HS-, S2-), gas phase (43 components), and solid phase (orpiment, red arsenic, arsenolite, claudetite, arsenic, sulfur, sodium salts). Thermodynamic constants of the base components are contained in the Selektor database (they were borrowed from reference-books). The latter group includes 77 complexes labile in the solution but determining the solubility of arsenic and stability of its solid phases. Physicochemical modeling was performed in H2S (≤0.01 m, pH = 1-10), Na2S, and NaHS solutions at 25-250
																								



2.
A METHOD FOR DETERMINATION OF THE ISOTOPIC COMPOSITION OF AUTHIGENIC URANIUM IN BAIKAL BOTTOM SEDIMENTS

E.P. Chebykin, E.L. Goldberg, N.S. Kulikova, N.A. Zhuchenko, O.G. Stepanova, Yu.A. Malopevnaya
Limnological Institute, Siberian Branch of the RAS, 3 ul. Ulan-Batorskaya, Irkutsk, 664033, Russia
Keywords: Bottom sediments; authigenic uranium; paleoclimate; Lake Baikal
Pages: 468-477

Abstract >>
A simple, reliable, and high-performance method has been proposed for direct determination of the isotopic composition of authigenic uranium in silica lacustrine sediments. The method is based on studying the kinetics of the selective extraction of authigenic uranium from sediments with weak solutions of ammonium hydrocarbonate followed by the ICP-MS analysis of the nuclides. To estimate the contamination of authigenic uranium by terrigenous one, the contents of 232Th and some other clastogenic elements in the extracts were measured simultaneously. The selectivity of extraction of authigenic uranium from the sediments treated with a 1% NH4HCO3 solution appeared to be no worse than 99%. The method was used to analyze the isotopic composition of authigenic uranium at several key horizons of a core dated before. The measurements directly prove that the 234U/238U values in Baikal water varied depending on climate, which contradicts the previous statements. The measured 234U/238U ratios in paleo-Baikal water match the values reconstructed from isotopic data for total uranium in the sediments on the supposition that the U/Th ratio is constant in the terrigenous part of the sediment. Direct experimental determination of total and authigenic nuclides in sediments enhances the potentiality of the method for absolute 234U-230Th dating of carbonate-barren lacustrine sediments, including those from Lake Baikal, within the intervals corresponding to the periods of glaciation, where the sediments contain a large fraction of terrigenous component. Given the fractions of terrigenous and authigenic uranium are accurately determined, we have an opportunity to study the variability of the sources of terrigenous matter and to refine the previous model for reconstructing the climate humidity in East Siberia.
																								



3.
Composition and conditions of formation of andalusite-kyanite-sillimanite pegmatoid segregations in metamorphic rocks of the Tsel block ( Mongolian Altay )

V.P. Sukhorukov
Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Polymorphous modifications of Al2SiO5; polymetamorphism; Mongolian Altay
Pages: 478-482

Abstract >>
Pegmatoid segregations containing three polymorphous Al2SiO5 modifications have been revealed in metamorphic rocks of the Tsel block localized in the Hercynian belt on the southern flank of Mongolian Altay. Petrographic study showed a successive substitution of polymorphs in the sequence: andalusite-kyanite-fibrolite-sillimanite. Estimated parameters of the host-rock metamorphism indicate that the mineral assemblage of pegmatoid veins formed at two successive stages of metamorphism: andalusite-sillimanite and kyanite-sillimanite. It is suggested that the transformation of Al2SiO5 from one to another polymorphous modification occurs by the ion exchange mechanism with the participation of muscovite.
																								



4.
MODERN ACCUMULATION RATES OF BIOGENIC SILICA IN BAIKAL BOTTOM SEDIMENTS: SIGNIFICANCE OF DIATOM SPECIES COMPOSITION

O.V. Levina, E.B. Karabanov
Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: Biogenic silica; mass accumulation rates; diatom plankton; species composition; production; Lake Baikal
Pages: 483-490

Abstract >>
The modern distribution of biogenic silica (SiO2biog ) in Baikal surface (0-0.2 and 0-0.5 cm) sediments, its mass accumulation rates (MAR), and the processes responsible for this distribution were studied. The contents of SiO2biog were determined in samples from 87 stations evenly distributed throughout the lake water area. These data were used to calculate the MAR of SiO2biog and compile SiO2biog and MAR distribution maps. The maps showed that the MAR varies significantly throughout the study area. Its highest values have been established in North Baikal. In Central and South Baikal the MAR values are much lower, and the minimum values are observed in the Selenga shoal. This MAR distribution pattern is consistent with the distribution of frustules of Aulacoseira baicalensis endemic diatoms throughout the surface sediments. Compared with other modern diatom species, this one has the most massive and heaviest frustules, which undergo the least dissolution when settling. This suggests that the above species plays a leading role in the accumulation of biogenic silica in modern Baikal bottom sediments, whereas the role of thin-walled species is minor because of their small mass and partial or complete dissolution in water on settling. Dilution of the sediments with terrigenous material is less significant for the SiO2biog accumulation.
																								



5.
Paleozoic facies megazones in the basement of the West Siberian geosyncline

E.A. Yolkin a , A.E. Kontorovich a , N.K. Bakharev a , S.Yu. Belyaev a , A.I. Varlamov b , N.G. Izokh a , A.V. Kanygin a , V.A. Kashtanov a , N.P. Kirda a , A.G. Klets a , V.A. Kontorovich a , V.I. Krasnov b , V.A. Krinin c , S.A. Moiseev a , O.T. Obut a , S.V. Saraev a , N.V. Sennikov a , V.M. Tishchenko d , Yu.F. Filippov a , A.V. Khomenko a , V.G. Khromykh a
a Institute of Petroleum Geology and Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
b Siberian Research Institute of Geology, Geophysics and Mineral Resources, 67 Krasny prosp., Novosibirsk, 630091, Russia
c Vankorneft' Ltd., 51 ul. Respubliki, Krasnoyarsk, 660075, Russia
d Nordimperial Ltd., 7 ul. K. Marksa, Tomsk, Russia
Keywords: Facies zonation; biochronology; paleogeography; geodynamics; terrane; continent; Paleozoic; West Siberian geosyncline
Pages: 491-504

Abstract >>
Facies zonation of the Paleozoic basement of West Siberian geosyncline and its surroundings is presented. Facies megazones are distinguished according to types of sedimentation. Analysis of lateral and successive sedimentary sequences shows that the available data are insufficient to map the facies distribution over the whole territory of the geosyncline for short time slices. Only the Late Devonian section is supported by data sufficient for the proposed facies zonation. Five megazones, I, II, III, IV, and V, are distinguished in the westward direction. First three megazones make up a single lateral facies succession and represent sedimentary environments on and around the Siberian continent. Megazone IV includes shallow-water volcanic and sedimentary rocks that compose the Kazakhstan continent bounded by Early and Middle Carboniferous sutures in the west and east. Megazone V comprises fold-thrust (island arc) complexes of the eastern Urals.
The main events in the geologic history of the region were associated with the interaction of two major crustal masses (Siberian and East European continents) and the young Kazakhstan continent in the oceanic space called the Paleoasian ocean. Only few fragments of this space occur in the present-day framework of the territory, the greatest part being sunk in subduction zones, especially in the large zone of the Main Uralian Fault.
Production and accumulation of organic matter in pre-Mesozoic deposits occurred on continental shelves, which are most promising for Precambrian and Paleozoic oil and gas.
																								



6.
Seismicity and seismoactive faults of Cuba

M.O. Cotilla Rodriguez a , H.J. Franzke b , D. Cordoba Barba a
a Universidad Complutense de Madrid, Facultad de Ciencias Fisicas, Departamento de Fisica de la Tierra y Astrofisica I,
Ciudad Universitaria, s/n, 28040 Madrid, Spain
b Institut fur Geologie und Paleontologie, TU Clausthal, Leibnizstrasse 10, D38678 Clausthal-Zellerfeld, Germany
Keywords: Crust types; fault; microplates; neotectonics; seismicity; seismic hazard; Cuba
Pages: 505-522

Abstract >>
The first catalogue of active regional faults of Cuba is presented. The seismotectonic map of Cuba is a base for studying the seismicity in this region. Of the 30 faults studied, only twelve are active. The main seismotectonic structure is the Bartlett-Cayman fault system, which borders the eastern and southeastern seismotectonic units in this region. Approximately 70% of Cuban seismicity is concentrated here. The Cauto-Nipe, Cochinos and Nortecubana faults border other seismotectonic units. The Nortecubana fault is the only one associated with a tsunami. All the faults are segmented. The faults described are related to the current tectonic stress regime of the Northern Caribbean. All the available information (maps, sections and profiles, photos, geological and neotectonic data on seismicity and focal mechanisms) is supported by a GIS.
																								



7.
Electromagnetic processes in a conducting magnetic casing string

M.I. Epov, G.M. Morozova, E.Yu. Antonov
Institute of Petroleum Geology and Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Transient electromagnetic field; resistivity; ferromagnet; magnetic permeability
Pages: 523-531

Abstract >>
We investigated the electromagnetic filed of a current loop in a conducting ferromagnetic cylinder for applications of casing monitoring in petroleum wells. We tested different transmitter-receiver configurations for excitation and recording of TEM and stationary magnetic fields, studied the responses to changes in electromagnetic and geometrical parameters of casing strings, and determined the intervals of highest sensitivity.