Home – Home – Jornals – Russian Geology and Geophysics 2010 number 11

With U-Pb zircon dating, the ages of the Ul'degit (228 ± 1 Ma) and Chek-Chikan (203 ± 1 Ma) mafic massifs were determined. These massifs were earlier considered to form at the Early Precambrian stage of the geologic evolution of the Dzhugdzhur-Stanovoi superterrane. In geochemical features the igneous rocks of the massifs show relation with a within-plate source, on the one hand, and are similar to igneous rocks of subduction zones, on the other. They might have formed after subduction, which caused the intrusion of gabbroids of the Lucha massif (248 ± 1 Ma) and diorites of the Tok-Algoma complex (238 ± 2 Ma), followed by the fracturing of the subducted plate.

Geochemical studies of the ecosystems of 184 Siberian lakes in three largest zones of northern Asia (humid, arid, and semiarid) and in mountainous area were carried out. The contents of natural radionuclides, radiocesium, and rare-earth elements in conjugate components of the systems and the types of the main sources of the bottom sediment material have been determined.
Dating of the bottom sediments was made by the activity of radioisotopes ¹³⁷Cs and ²¹⁰Pb, which permitted estimation of the sedimentation rates in lakes in different regions of Siberia: 0.35 cm/year in the south and 0.25-0.3 cm/year in the north.
Six main ions have been determined in the waters of the studied lakes: Ca²⁺, Mg²⁺, Na⁺, HCO^-₃, SO₄^2-, Cl^-.
The distribution of natural radionuclides in the stratified sections of bottom sediments of Siberian lakes evidences the stable sedimentation and characterizes their contents in the soils of water-catchment areas, which can be considered background contents there. Sediments enriched in organic matter have higher concentrations of U and lower ones of Th and K. The Th/K ratio in the studied bottom sediments is the same as in the soils. The Th/U ratios are somewhat lower than those in the soils because U is accumulated by chemogenic and organic components.
The overall ¹³⁷Cs pollution of bottom sediments of Siberian lakes is close to the global background (40 mCi /km² in 2000), but in the Altai Territory and Buryatia and Altai Republics it is twice higher. The uneven areal and temporal distribution of residual radiocesium is observed not only in the lacustrine sediments but also in the lake water areas.
The REE patterns of bottom sediments of different mineral types are similar to those of continental crust and clays of the Russian Platform, though organogenic and carbonate sediments have higher absolute REE contents than terrigenous ones. Pelitic fraction is the main REE concentrator in the bottom sediments.

The northeast of the Russian Arctic is a deep-water basin underlain by the Lomonosov and Mendeleev Ridges, with the Makarov basin in between. In most of this area, the water depth is ~1-4 km and the crust is thick (20-30 km), with a well-pronounced granitic layer. Therefore, some researchers regard this crust as continental. Others think that this is the oceanic crust, the same as that on the hotspots like Iceland in the Atlantic or Ontong Java in the Pacific. After their activity ceases, such structures must subside as a result of the crust and mantle cooling, in the same way as the oceanic crust on a spreading axis. As regards the Lomonosov and Mendeleev Ridges, they subsided in quite a different way. In the absence of volcanism, they remained near sea level, almost not subsiding, for a long time (at least 70 and 190 myr, respectively). In the late Early Miocene, these areas subsided rapidly and deep-water sediments overlay shallow-water ones. In the same epoch, the Makarov basin subsided rapidly, which also used to lie near sea level. Its subsidence was several times that which could have taken place over the same period of time as a result of lithosphere cooling on an extinct hotspot. Such tectonic movements were possible only for the continental crust. The data on the structure of the sedimentary cover preclude considerable lithospheric stretching in these areas. Therefore, the rapid subsidence is accounted for by the transformation of gabbro in the lower crust into denser rocks (garnet granulites and eclogites), catalyzed by infiltration of a mantle-derived fluid. Dense, deeply metamorphosed mafic rocks with a thickness of up to 10-20 km and P -wave velocities of ~8 km/s underlie the Moho in the area under study.

A microfossil assemblage has been found in the widely spaced (300 km apart) Baikit and Katanga areas (inner Siberian Platform) in stratigraphically coeval strata in the lower part of the petroliferous Kamo Group (Riphean). It comprises complex protists Tappania plana Yin; acritarchs Valeria lophostriata Jank., Satka sp., Lophosphaeridium sp., Pulvinosphaeridium sp., and Miroedichia sp.; porous vesicles of Tasmanites sp. and Osculosphaera sp.; vesicles with anastomosing processes, and spiral filaments of Obruchevella and Glomovertella . Siberian fossil microbiota is considerably more diverse than biota with Tappania protists known in Australia and China. A taxonomically similar fossil biota found 1500 km northeast of the well-studied Riphean section in the Olenek uplift suggests that the host rocks, too, are stratigraphically coeval. This permits refining the age of remains of the first complex eukaryotes from the inner platform and is an additional correlation criterion for the lower Kamo Group.

The peculiarities of the distribution of high-molecular normal alkylbenzenes in Vendian-Cambrian oils of the Siberian Platform were studied according to the GC-MS data. A predominance of high-molecular homologues with the odd number of carbon atoms in the molecule has been established. Possible biochemical precursors and the probable mechanism of formation of "odd" monoalkylbenzenes are considered.

The paper discusses modern views of the structure of the main producing plays in the southern Siberian Platform: Riphean, Vendian, and Lower Cambrian. The major zones of oil and gas accumulation have been distinguished. Forecast resources have undergone differentiated expert evaluation, and methods for their development have been recommended.

We discuss the results of a field experiment in the Malaya Botuobiya area (West Yakutia) at a site where earlier surveys revealed slowly decaying transient responses. That time-dependent voltage decay indicated magnetic viscosity effects associated with magnetic relaxation of superparamagnetic grains in rocks. In this study, we have applied a high-resolution array TEM survey to contour the anomaly and parametric soundings with systems of different configurations to explore the vertical pattern of magnetic viscosity. The parametric data have been inverted, by means of manual and automated fitting, with a reference model of a layered magnetically viscous earth, using, respectively, analytical formulas and simulation based on a forward solution by separation of variables. According to both automated and manual inversion, the section at the center of the anomalous site fits a three-layer earth model with an intermediate magnetically viscous layer between two nonmagnetic layers. This model is consistent with a priori evidence for local geology and might provide more details of the latter. The inversion results have been further used to estimate the volumetric percentage of superparamagnetic grains in the magnetically viscous layer, assuming magnetite to be the main ferrimagnetic phase.

On August 20, 1852, an earthquake caused widespread destruction in the city of Santiago de Cuba and its surroundings. A comprehensive search for contemporary documentation was made. The information gathered was used for a detailed analysis of the damage and characteristics of the earthquakes. Intensities were evaluated at 45 localities, and an isoseismal scheme has been drawn. Maximum intensity reached 8 degrees (MSK), and 6.4 was the estimated magnitude. Damage in the city of Santiago de Cuba has been studied in detail. The low quality of construction contributed to the damage. The total number of casualities was 2 dead and approximately 200 injured. The shock was felt within 80,000 km². The epicentre was determined as 19.75 N, 75.32 W, h = 30 km. This study shows that contemporary Cuban documents must be studied with care in their historical and cultural background to avoid overrating earthquake intensities.