Home – Home – Jornals – Russian Geology and Geophysics 2011 number 7

The Sarkhoi Group localized in the stratotype area is composed of various volcanics and volcanoclastics more than 4 km in total thickness. The volcanics of the lower part are greenschist facies altered rocks ranging from basalt to dacite, and those of the upper part are mainly rhyolitic ignimbrites. The composition, sequence, and facies features of these volcanics show their affinity to the active continental margin. This is confirmed by geochemical data evidencing the participation of the Early Precambrian continental crust in their formation. The U-Pb zircon dating (SHRIMP) of the Sarkhoi volcanics yielded a Neoproterozoic age of the ignimbrites (782 ± 11 Ma, concordant cluster N = 8). The tonalite-trondhjemites of the Sumsunur complex in the Gargan block are plutonic analogs of these volcanic rocks. Coeval granitoids were also revealed in other parts of the Tuva-Mongolian and Dzavhan massifs. The ~1500 km long Sarkhoi-Dzavhan volcanic belt is a fragment of the Neoproterozoic active continental margin separated from the continent. In the Middle Neoproterozoic, the geodynamic system also included the Oka accretionary prism joining the Sarkhoi continental margin and the Shishkhid island arc. Coeval analogs of the above structures are also reconstructed in the Yenisei Range and Baikal-Muya belt. This suggests that the Sarkhoi belt was initially localized along the margin of the Neoproterozoic Siberian continent. But the exotic origin of the Precambrian massifs in southern Siberia is not ruled out either.

The data on the composition of metacinnabar and mercurian sphalerite with an isomorphous Cd admixture fr om some mercury and complex ore objects are summarized. There are two varieties of metacinnabar: (1) cadmium-rich (9.25-15.80 wt.%), with the minimum quantity of Zn admixture (0.67-3.94 wt.), which has the idealized formula (Hg, Cd, Zn)S (Ulandu) and (2) with increased amounts of Zn (2.21-10.83 wt.%) and Cd (6.00-14.10 wt.%), having the formula (Hg,Zn,Cd)S (Arzak, Kadyrel', Murzinskoe, Ravnou-1, Ravnou-2). The maximum content of isomorphous Cd (15.80 wt.%) was determined in the metacinnabar from the Ulandu ore occurrence. The Hg-sphalerites are compositionally divided into two groups: (1) with an increased Cd concentration (up to 7.96 wt.%) and (2) mainly with a low content of Cd admixture (0.0 n -1.63 wt.%). The sphalerites of the first group are typical of mineral assemblages including Cd-Zn-enriched metacinnabar (Arzak, Kadyrel', Murzinskoe, Sarasa). The sphalerite of the Sarasa deposit is included into this group arbitrarily, because it contains more cadmium than the sphalerites of the second group, which are constituents of parageneses only with Zn-metacinnabar or lacking it (Nikitovka, Khaidarkan, Dzhizhikrut, Bayan-Khan, Aktash). There are some Hg-bearing varieties of sphalerite with the idealized formulas (Zn, Hg, Fe)S, (Zn, Hg, Cd)S, (Zn, Cd, Hg, Fe)S, (Zn, Fe, Cd, Hg)S, (Zn, Hg, Cd, Fe)S, (Zn, Fe, Hg, Cd)S, and (Zn, Hg, Cu)S, which transforms into (Zn, Cu, Hg)S. The maximum Cd admixture (7.96 wt.%) was established in Hg-sphalerite from the Kadyrel' ore occurrence. It is the highest content observed in natural Hg-bearing ?-ZnS. The studied metacinnabar and sphalerite varieties are the only cubic phases of the natural system Zn-Hg-Cd-S, which is sometimes expanded because of minor admixtures of other metals. These varieties have a structure identical to the cubic phases of the synthetic systems Me-S and Me-Me-S, wh ere Me = Zn, Cd, and Hg. None of Hg-bearing hexagonal solid solutions were found in the natural system Zn-Hg-Cd-S. The continuity of the natural isomorphous series ZnS_cub-HgS_cub has not been confirmed by finding of natural intermediate solid solutions at the series fragment between (Zn_0.752Hg_0.248)S and (Hg_0.539Zn_0.461)S. The metacinnabar varieties enriched in Zn and Cd show no correlation between these elements. It is assumed that the host rocks with a high content of cadmium compounds were the source of Cd for the formation of specific varieties ?-HgS and ?-ZnS.

The paper examines trilobite assemblages found in the Botomian-Toyonian (Lower Cambrian) boundary stratotype section in the middle reaches of the Lena River, Siberian Platform. It has been found that the trilobite assemblage fr om the Bergeroniellus ketemensis Zone (bottom of the Toyonian) is very much the same as that from the underlying Bergeroniaspis ornata Zone (terminal zone of the Botomian). It has been suggested to combine these zones into one with the index species Bergeroniaspis ornata . The greatest changes in the trilobite assemblages are observed at a higher stratigraphic level, at the bottom of the Lermontovia grandis Zone (second Toyonian zone). This suggests that the lower boundary of the Toyonian should be placed at the bottom of this zone. This new boundary shows substantial changes in the trilobite abundance and diversity and is well pronounced not only in the stratotype sections of the Lena River but also elsewh ere in the Siberian Platform (the northeast and northwest of the region, the Anabar area, and others).

The article discusses numerical simulation of the transient electromagnetic field. The field source is an induction coil. We consider the situation when a logging tool is in a horizontal well in a medium with horizontal and vertical boundaries. The specific features of this problem are the metallic mandrel of the tool, 3D geometry, and distant boundaries. The method of separate computation of normal and anomalous fields is proposed. The finite-element method is used for spatial approximation of the field, and the implicit finite-difference scheme is used for the approximation of the field in time. The correctness and advantages of the method are shown. Some numerical results are demonstrated. The method proposed can be used when designing tools for geosteering.

The paper is devoted to mistakes and omissions in earthquake catalogs for East Siberia during the historical observation period. The earthquakes of 8 March 1866 and 18 May 1904 are used as examples of misinterpretations or underuse of historical evidence. The main parameters of these events have been redetermined from primary data. The paper also analyzes the "forgotten" earthquake of 1 August 1910 and the previously unknown event of 6 August 1910, which are missing from the regional parametric catalogs. It is concluded that the existing macroseismic and parametric earthquake catalogs of the study area need a thorough revision, correction, and supplementation.

Risk analyses made in the area of seismic activity are going to be of great importance in determining the earthquake interoccurrence times. Several statistical methods have been developed for this purpose. Recently, Exponential, Gamma and Weibull distributions are the frequently used methods in this regard. In this study, we investigate the interoccurrence time statistics of earthquakes which occurred in the area coordinated 39°-42°N and 30°-40°E in the North Anatolian Fault Zone (NAFZ) between the years 1960-2008, with a mixture of two different distributions of Exponential, Gamma, and Weibull and a mixture of the same kind of distribution. We found that the mixture distributions are more suitable than the other examined distribution models for small magnitudes ( m_c ≥ 3). Also, Weibull-Gamma and Weibull-Exponential distributions are agreeable for large magnitudes ( m_c ≥ 5).

A physically simple method is suggested to measure the permeability of porous reservoirs on the basis of resonance radial oscillations of borehole fluid. The interfacial velocity difference between the porous solid and the fluid is highly sensitive to the permeability of the formation outside the borehole at the resonance frequency. Thus, the permeability can be inferred from the instrumentally measured velocity difference. The instrumental measurements have technological limitations due to the finite size of the source. The two-velocity acoustic data from saturated porous media can be processed using the high-performance and high-order accurate WENO/Runge-Kutta 2D finite-difference scheme.