Home – Home – Jornals – Russian Geology and Geophysics 2002 number 8

The basement structures of the southwestern Siberian craton include the cis-Sayan and Angara-Kan marginal uplifts that are parts of the Yenisei Ridge. The postcollisional granitoids abundant within the cis-Sayan uplift were earlier assigned to the Sayan and Shumikha complexes, but there was no unanimity in interpretation of their geodynamic environments. Geochemical studies of the Sayan granitoids at the junction between the Biryusa block and the Urik-Iya graben fill and the Shumikha granitoids at the junction of the Onot greenstone belt and the Sharyzhalgai block of mafic gneisses have confirmed the postcollisional nature of both complexes. The Shumikha granitoids share some features of A-type granites, including those typical of the Primorsky rapakivi-like complex in the western Baikal region. The Sayan and Shumikha granitoids were dated by the U-Pb zircon age method as 1858 20 and 187117 Ma, respectively. Interpreted in the context of 1.8÷1.9 Ga granite magmatism on the craton periphery, the intrusion of the two complexes most likely postdated the collision responsible for the formation of an Early Proterozoic supercontinent between 2.0 and 1.9 Ga. Together with the Primorsky and Taraka complexes and the Akitkan volcanoplutonic belt, the Sayan and Shumikha intrusions record rather the onset of postcollisional extension which failed to cause continent break-up and oceanic crust production than the formation of the Proterozoic supercontinent.

The location and water chemistry in lakes and springs in the Olkhon region are structurally controlled by Cenozoic faults of two types: i) inherited NE faults that follow the Early Paleozoic syncollisional (late stage) dextral strike-slip faults; ii) NEN pull-apart faults produced by sinistral strike-slip movements at the early stage of Baikal rifting.
Water in springs has a generally low salinity (0.2 ÷0.5 g/l) and compositions with invariable cations (Ca-Mg) and variable anions (from HCO₃ to SO₄-HCO₃). The lakes we studied are fresh-water (up to 1 g/l), brackish (1 ÷10 g/l), and saline (14 ÷45 g/l). Fresh-water lakes have mostly Mg-Ca-Na or Mg-Na-Ca bicarbonate chemistry, the composition of brackish lakes is either HCO₃-SO₄ Na-Mg (Mg-Na) or SO₄-HCO₃-Cl, Na-Mg (Mg-Na), and that of saline lakes is dominated by SO₄, Cl, Mg, and Na. The composition of limnic water is controlled by the chemistry of feeder groundwater and by the grade of water metamorphism.
Springs and lakes of different salinities show regular association with the two types of faults: Low-mineralized springs and fresh-water lakes are located within inherited faults, whereas most of brackish and saline lakes are attributed to pull aparts. There are two possible explanations for this regularity. (1) The water chemistry of springs and fresh-water lakes in inherited faults is closer to the meteoric water as the depth of their feeder channels is constrained by complex thrust and strike-slip geometry of older faults with shallow-dipping planes. Pull-apart faults associated with synrift sinistral strike-slip motions are discordant with pre-Cenozoic structures and drain deeper aquifers with compositions (Cl-Na) strongly different from meteoric water. Mixing of various ground waters and their metamorphism may be responsible for the observed variations in total mineralization and chemistry of lake water. (2) The diversity of water compositions in lakes is rather controlled by metamorphism (most often cryogenic) of the feeder meteoric water, and pull-apart faults maintain long existence of relatively small lakes in which water evolution reaches a high grade.

Native gold samples (Au-Ag system) in the range of fineness from 200 to 1000 received complex comparative study. It has been established that HNO₃ etched samples with gold fineness of about 700 and less have a two-phase structure. Atomic absorption and microprobe analyses show that the phases are pure Au and Ag. Gold and silver particles are a few hundred nanometers in size. Distribution of particles is uniform, i.e., this is an ultradispersive statistically homogeneous mixture composed of native gold and native silver.
We have observed no obvious two-phase structure in the HNO₃ etched samples with gold fineness in the range of 700 to 850 . The fineness of gold increases in the surface layer, and traces of silver are observed in the solution. At some points, the microprobe analysis of fineness (without etching) reveals a drastic change in fineness, which either increases or decreases relative to the average value (deviations up to 250). We suggest that the samples of this group are similar to those from the previous group, but predominant gold prevents silver particles from contacts with HNO₃. Therefore, the samples of this composition do not dissolve in HNO₃.
There is no evidence of phase heterogeneity in samples with a gold fineness of 850 to 1000 . We suppose that Au and Ag substitute isomorphically for each other in the crystal lattice and form a homogeneous solid solution.
Possible reasons for the phase heterogeneity of native gold are discussed. We suggest that it is the result of micro block crystallization associated with high over saturation of solutions.

The Ural-Siberian young platform is reconsidered in terms of Yanshin's criteria as formed through the early and late Neogean evolution stages.
The early Neogean stage (Riphean-Paleozoic, 1600÷250 Ma) produced the northwestern segment of the Ural-Mongolia fold belt with new continental crust. At the late Neogean stage (Mesozoic-Cenozoic, since 250 Ma), the young platform formed by amalgamation of the West Siberian and South Kara plates, the Urals, the Kazakhstan and Altai-Sayan shields, and the Yenisei Ridge in response to Arctic-Atlantic rifting and orogeny in southern Asia.
Young platforms differ in structural inheritance of their sedimentary cover from the basement and the existence of an intermediate Paleozoic structural stage.

The Callovian and Upper Jurassic section in the Tyumen' superdeep well SD-6 (West Siberia) drilled with almost continuous recovery of core (about 200 m) and characterized by rich assemblages of microfossils is a unique object for development and improvement of Jurassic regional biostratigraphic schemes of West Siberia based on microfossils. This paper presents results of micropaleontological and palynological research (foraminifers, microforaminifers, ostracods, dinoflagellate cysts, acritarchs, prasinophytes as well as spores and pollen grains of terrestrial plants) and bio- and lithostratigraphic features of the uppermost Middle and Upper Jurassic part of the reference section from the Tyumen' superdeep well. The Vasyugan, Georgievka, and Bazhenov Formations are described in detail, taking into consideration their micropaleontological and palynological characteristics. The strucural features of sand productive beds of J^o₂ and J₁ groups in the Vasyugan Formation and J^o₁ group at the base of the Georgievka Formation are discussed. Comprehensive lithostratigraphic analysis of Callovian-Upper Jurassic part of the section SD-6 and biostratigraphic data allow us to localize the boundaries of some formations, subformations, and beds in this well. The main regularities of stratigraphic and lateral distribution of Callovian and Upper Jurassic formations as well as their thickness in the Urengoi district and adjacent territories are revealed.
Analysis of stratigraphic distribution of microfossil species allow a detailed zonal subdivision of reference section SD-6, with this section being the most complete Jurassic succession in northern West Siberia. The Jurassic regional-scale biostratons were revealed in Siberia according to foraminifers (f-zones), some of them being proposed for West Siberia for the first time. Also for the first time, an almost continuous sequence of dinocyst-based biostratons (dinozones) for Callovian-Middle Volgian has been established in a single section. It may be the basis for the development of Jurassic zonal scale according to dinocysts for West Siberia.
Distinctive features of microbenthos communities and microphytoplankton associations reflecting biofacies have been studied. The main regularities of distribution of microbenthic communities and microphytoplankton associations depending on changes of transgressive-regressive events and paleoenvironments in the Callovian-Late Jurassic have been established.

Active faults and faults reactivated in Cenozoic or, less often, Mesozoic time are considered in relation to regional heat flows in the Baikal Rift and its surroundings. Abnormal heat flow and manifestations of thermal activity (hot springs, Cenozoic volcanoes and flood basalts) are most often associated with faults. Thermal activity of faults, controlled by faulting mechanism and by the interplay of conductive and convective transport of deep heat, is the highest in strike-slip and normal faults and the lowest in thrusts. Geothermal parameters as indicators of regional tectonothermal activity can thus serve as a proxy in tectonic implications.