Home – Home – Jornals – Russian Geology and Geophysics 2003 number 8

The sections existing in the Permian-Triassic effusive traps of the Siberian Platform (SP) are of three types: (1) monotonous, with slightly altered composition of tholeiites within several successive formations in the same section; (2) with lava composition regularly changed from trachybasalts to picritoids, with a certain petrochemical trend in particular

Many fractures and lithophysae in rhyolite tuffs making up Yucca Mountain in Southern Nevada, USA, are lined with epigenetic minerals (calcite, quartz, chalcedony, opal, fluorite, zeolites, and strontianite) indicative of the migration of mineralized waters within the mountain vadose zone in the geologic past. To evaluate the possibility of formation of these minerals as a result of meteoric water-rhyolite interaction, we performed a thermodynamic modeling of the system rhyolite-carbonate-aqueous solution-gas (CO₂, O₂) open to oxygen (p_O2 = 0.2 bar) and carbon dioxide (p_CO2 = 0.0003 bar). The compositions of produced ground waters and equilibrated mineral associations were calculated using the

The Oceanic Anoxic Event that occurred at the transition from Cenomanian to Turonian (OAE2) is a bright episode in the Cretaceous history. The section of the Cenomanian-Turonian terrigene deposits on the Nizhnyaya Agapa River (Ust'-Yenisei region) with vestiges of OAE2 has been analyzed in sedimentology and palynology. Textural features of deposits as well as their facies interactions in the section have been studied in detail. Stagnation and normal-marine settings alternate, which is typical of black-shale facies. This is well expressed in the replacement of shallow-marine sand rocks containing benthic fauna by black and brownish-gray leaf clays with signs of pyritization, without macrofauna, reflecting settings of stagnation. Qualitative and quantitative changes in the composition of organic-wall microphytoplankton have been established, and the facies-dependent dynamics of taxonomic variety of dinocysts has been traced. Comparative analysis of distribution of palynomorphs has been performed in deposits related to the Cenomanian-Turonian Oceanic Anoxic Event in different regions.

The architecture of the sedimentary cover of the West Siberian plate is analyzed within the limits of a kinematic model implying cyclicity of deposition controlled by tectonic subsidence and uplift. The model accounts for the formation mechanism of large- and smaller-scale lithostratigraphic units (formations, members, sand beds). The vertical distribution of sedimentary units and markers in the section is controlled by the dominant 18 myr periodicity of tectonism. The paleogeography of the region is reconstructed for different periods in the context of an oscillatory process. Correlation of the theoretical lithostratigraphic units with the actual sedimentary section shows that the proposed kinematic model may provide a basis for detailed stratigraphic division of the platform cover of the West Siberian plate.

The structure, composition, and deposition environments of Callovian-Oxfordian strata in the Nadym-Taz interfluve are analyzed on the basis of deep drilling data from the Tyumen' SDB-6. Depositional and sequence stratigraphy modeling reconstructs coastal and shelf conditions of the Vasyugan deposition that records sea-level fluctuations. The Vasyugan sequence revealed by sequence stratigraphy analysis of the Callovian-Oxfordian section is bounded by unconformities and includes a transgressive and a highstand systems tracts separated by a maximum flooding surface. The highstand systems tract is divided into a lower (early HST) and an upper (late HST) units corresponding to progradation-aggradational and progradational deposition, respectively.

On the basis of 53 radiocarbon data from the range 15-9.7 Ka, we have reconstructed the dynamics of mammoth populations in Northern Asia in the Late Glacial and early Holocene. The latest data (12-9.7 Ka) have been calibrated. Within continental Asia, the last mammoths died out on Taimyr around 11,000 cal. yr BP (9.7 kyr BCE). Using paleogeographic data, we tentatively reconstructed the natural habitat of mammoths in Siberia in the Late Glacial. The Volch'ya Griva occurrence in southern West Siberia, for which a radiocarbon date of around 11.1 Ka was obtained, could play a key role in the revision of the dynamics of mammoth distribution after 12 Ka.

Weak point scatterers and diffractors in a vicinity of regular strong reflectors are separated by backward wavefield continuation along Gaussian beams, or solutions concentrated around a specific ray. Gaussian beams are used to construct the so-called selective images which highlight the elements that return incident wave energy at certain angles.
The approach has been tested in terms of resolution and sensitivity to errors in a medium with a user-specified velocity structure. The reported results of numerical experiments demonstrate the selectivity of the method as to dip angle and curvature of regular reflectors neighbored by point scatterers.

Long-term pressure monitoring in the tubing of a 760 m deep mine in the Altai-Sayan region shows that the geodynamic component of crustal stress is strongly influenced by earthquake nucleation.

Water-level fluctuations in wells open to artesian aquifers record pressure head disturbances caused by dilatation of the aquifer and reflect tide, barometric, and tectonic effects. Monitoring of atmospheric pressure and water level in a deep well 300 km far from the Russian Pacific coast (Bychikha site, near Khabarovsk) was used to study the response of the well-aquifer system to periodic loading. The monitoring data were processed to obtain the elastic constants and porosity of the aquifer using analysis of several static models. The experiment confirmed the possibility to monitor these parameters to an accuracy of 1 % and to separate the tectonic component of the pressure head. Anomalous water level changes were analyzed in relation to earthquake nucleation.