Home – Home – Jornals – Russian Geology and Geophysics 2004 number 3

New paleontological findings, analysis of section structure, and literature data on the Neoproterozoic stratigraphy of the Altai-Sayan Folded Area, ASFA, provide support to the recognition of three heterochronous polyfacies complexes of deposits: Vendian, Baikalian, and Mayaian (pre-Baikalian Late Riphean).
The Vendian-Cambrian boundary is recommended to be drawn according to the change of complexes (associations) of calcareous algae and small shelly fossils at the base of Member 4 of the Anastas'ino Formation, the main phosphorite-bearing horizon of Member 2 of the Zabit Formation in the East Sayans and its analogs in the Kuznetsk Alatau. The lower Vendian boundary is related to tectonic restructuring expressed in an interruption in sedimentation. The sedimentary bodies between the Vendian-Cambrian boundary and the regional gap are characterized by Ediacaran fauna, small shelly fossils, and Vendian calcareous algae. The Late Riphean complexes are distinguished on the basis of data on geochronology and evolution of geodynamic settings in the region.
The reported materials considerably change our ideas of the age and lateral persistence of composition of the earlier recognized ancient regional horizons: Kabyrza (R₂), West Siberian (R₃), Belka (V), and their analogs on the adjacent territories.
It is shown that the geodynamic development of the Altai-Sayan Folded Area is principally similar to the development of the Baikal-Patom Folded Area.

Experiments show a considerable silica decrease in the water of Baikal through 1995-2001 in the conditions of anomalous local warming in the lake region and growing phytoplankton production. Annual silica loss in those years was over 4 times the typical loss in normal conditions, and ~30 years of river input is required to make up for the total deficit. Silica variations were modeled as fluxes from bottom to surface waters for the three subbasins of the lake. Long-term variations of silica contents in the Baikal water are apparently controlled by climate. Secular warming peaks are marked by 4-6 times greater consumption of silica by diatoms and its correspondingly faster accumulation in bottom sediments of Baikal.

Mineralogical, petrographic, and petrochemical studies of Lower Proterozoic nonmetamorphosed sandstones and tuffites (Udokan complex) of the Ugui trough were carried out. The Dickinson and Bhatia methods were applied to reconstruct the tectonic settings of provenances. The sequence of Lower Proterozoic petrofacies of the Ugui trough is the same as the sequence of petrofacies of the foreland basin bordering a recycled orogen. The basement of the complex is composed of volcanosedimentary and arkosic petrofacies of rift phase. Above are petrofacies of quartzy and subarkosic arenites transported from the continental block. The section of Lower Proterozoic deposits is crowned with lithic and arkosic arenites supplied from the recycled orogen.

Southern Transuralia lies at the junction of three largest geologic structures: the Uralian Mountains, West Siberian Plate, and Turan Plate. In the Cretaceous, the conditions of sedimentation were drastically different in all structures. In southern Transuralia these conditions interfere with each other to provide a mixed lithology of the formations making up the Cretaceous section, which hinders stratigraphic correlation between sections, tracing of formations, understanding of the specific geologic history of the region, and reconstruction of paleogeography and parameters of paleoenvironment. This paper suggests a system of mutually related lithological and geophysical characteristics permitting these difficulties to be overcome. Steady benchmarks of electrical logging, whose origin is linked with a drastic change of lithological characteristics of formations, allows a strict correlation between sections of boreholes drilled without core sampling. Using them we also can trace the character of distribution of formations within the study area, record the features of their occurrence (depths of sole and roof, thickness) and structure, providing much new information, which is very important in search for and prospecting of ores, in drawing of paleogeographical and other schemes. Correlative geophysical schemes illustrate the behavior of some horizons in space, permitting us to evaluate the change of their composition, and degree of homogeneity, which is especially important for geological survey and mapping of the territory and for prediction of ore potential. Our lithological and geophysical investigations allow core-free drilling of the cover rocks, which makes the field works less labor- and time-consuming.

Minerals of platinum group elements (PGE) from gold placers were studied in various regions of the Sayan-Baikal Folded Area (Buryatia). Four types of mineral associations have first been recognized: Ru-Ir-Os, Os-Ir, Pt-Fe, and sperrylite, which differ from each other in a set of platinoids. Primary minerals are dominated by Os-Ir-Ru alloys; Pt-Os-Ir-Ru and Pt-Fe solid solutions are less common; and sperrylite is scarce. Secondary PGE minerals are sulfides, arsenides, sulfoarsenides, sulfoantimonides, and Pt-Cu alloys. Based on the mineral compositions and our own and literature data, conclusions about the source rocks for PGE minerals have been drawn. The Ru-Ir-Os association is supposed to have formed in chromitites of dunite-harzburgite complexes of ophiolite belts; the sperrylite one, in layered gabbro-peridotite massifs with Cu-Ni sulfide mineralization; and the Os-Ir association, in zones of hydrothermal processes in ultrabasic rocks and their metamorphic varieties. The source rock for Pt-Fe association has not been established yet. This association might be genetically related to zonal basic-ultrabasic massifs of the Uralian-Alaskan type. Its occurrences in the Muya zone suggest the existence of commercial gold placers in northern Buryatia.

Detailed petrochemical characteristics of Paleozoic (Cambrian(?)-Silurian and Carboniferous) granitoids of the West Stanovoy structure-formational zone (SFZ) (central Selenga-Stanovoy mountain area (SSMA), Transbaikalia) are presented. The Early Paleozoic granitoids of the Krestovsky and Olekma igneous complexes are shown to be similar in evolution and lithology in both N-S and W-E strikes, which hampers distinction between them. A Carboniferous granite-leucogranite intrusive complex is recognized in the study region, which is consistent with the magmatic history of the adjacent Khilok-Vitim SFZ. In accordance with the known lithological criteria for paleogeodynamic interpretation of granitoids, the Early Paleozoic igneous formations of the West Stanovoy SFZ have all features of palingenic batholiths of active continental margins (ACM), which is confirmed by modern paleogeodynamic reconstructions of the region. Carboniferous granitoids might have formed either during the development of a "transform" margin at the shifts of the Yablonovy terrane along the ACM of the Paleo-Siberian continent or under the effect of within-plate mantle plume, which gave rise to the Angara-Vitim batholith. The results obtained are of paramount importance for petrochemical identification of Paleozoic SSMA granitoids and for solving the problems of the pre-Mesozoic geodynamics of the Mongolo-Okhotsk belt and Central Asian paleo-ocean as a whole.

The total number of oil and gas fields in a province is approximated by the Weibull distribution. The statistical performances of this parameter are related to the parameters of a truncated Pareto distribution. The total number of fields is estimated on the basis of size distribution of several largest fields in a province and the estimator quality is tested through Monte Carlo imitation.

The Uralian orogen includes a group of continental terranes in the west and island-arc terranes in the east. The East Uralian zone consists of two large island-arc terranes of different ages: the Tagil terrane in the northern and central Urals and the Magnitogorsk terrane in the southern Urals. The two terranes produce a strong gravity anomaly in their western part known as the Main Uralian Gravity Supermaximum. The axis of the anomaly is delineated by mafic-ultramafic complexes of the Ural Platinum belt within the Tagil terrane but no such complexes are exposed within the Magnitogorsk terrane in the south. Two-dimensional Bouguer gravity inversion of the gravity step and the supermaximum along seven profiles traversing the Irendyk zone in the western Magnitogorsk terrane indicates that gabbro complexes similar to those in the Platinum belt may be buried at several kilometers beneath basaltic andesites of the Irendyk island arc. The inversion was based on the known geometry of the gravity producing bodies in the Platinum belt, and the results were interpreted taking into account gravity and geological evidence in view of tectonic similarity between the Tagil and Magnitogorsk terranes. The gabbro complexes in the Magnitogorsk terrane may have remained buried because of a lower-angle dip and a shallower base level of erosion of the Irendyk island arc.

Propagation of seismic waves in solids is described by equations including grain interaction rather than the standard equation of state. The new approach allows prediction of some physical properties of solids from seismic data.
The modeling results provide explanations for the difference between the measured static and dynamic elastic moduli and for the density dependence of sound velocity (increases with density in nonmetals and decreases in metals).
The theoretical results were tested against known experimental data and showed good agreement.

Elastic properties of rock-forming minerals and their pressure and temperature derivatives are estimated using inversion of pressure- and temperature-dependent P and S velocities. The elastic constants obtained by the new method are tested against averaged Voigt-Reuss-Hill elastic constants obtained experimentally on single crystals.

It is suggested that synphase convergence of traveltime curves of reflections from low- or high-velocity layers be used as a seismic layering criterion. The results are qualitative and can be used for reference in interpretative modeling.

The concentrations of cosmogenic radionuclide ¹⁰Be in the Miocene section of the BDP-96-1 (100 to 200 m) and BDP-98-2 (200 to 600 m) cores from Lake Baikal were used as independent geochronology to test the previous magnetostratigraphic age models and to time the core base. Only one of three alternative models for the BDP-98 core showed perfect consistency with decay-corrected ¹⁰Be concentrations (with a half-life T^1/2=1.5 myr). The basal age of the 600 m BDP-98 section was estimated at 8.4 myr.