Home – Home – Jornals – Russian Geology and Geophysics 2009 number 5

The Barents Sea shelf is an attractive target as a prospective large petroleum province. Further development of geological and geophysical exploration in the area requires high-resolution biostratigraphic constraints and update stratigraphic charts. The zonal succession of Lower and Middle Jurassic assemblages of foraminifers and ostracodes of the Barents Sea fits well the division for northern Siberia based on correlated independent Jurassic and Cretaceous zonal scales on all main microfossil groups, of which some scales were suggested as the Boreal Zonal Standard. The stratigraphic range of the Barents Sea microfossil assemblages has been updated through correlation with their counterparts from northern Siberia constrained by ammonite and bivalve data. Joint analysis of foraminiferal and ostracode biostratigraphy and lithostratigraphy of the sections allowed a revision to the stratigraphic position and extent of lithological and seismic units. The discovered similarity in the Lower and Middle Jurassic lithostratigraphy in the sections of the Barents Sea shelf and northern Siberia, along with their almost identical microfossil taxonomy, prompts similarity in the Early and Middle Jurassic deposition and geological histories of the two areas.

Devonian and Lower Carboniferous continental deposits in the south of the Siberian Platform formed in eight sedimentary basins localized within stable ancient land Angarida (Angaraland). Interbasin correlation of the stratigraphic units of these deposits was based mainly on analysis of paleontological data and marker horizons. It has been established that: (1) the Early and Middle Devonian flora of Angarida grew locally and only in its western part (Minusa and Rybinsk paleobasins); (2) in the Late Devonian, vegetation was dispersed over a vast area, as evidenced from the findings of its imprints in all existed sedimentary basins of continental lithogenesis; and (3) the Early Carboniferous in Angarida was the time of the flourishing of higher plants, which were abundant but poor in species. Lithologic benchmarks at different age levels of paleobasin sediments have been recognized. The most important marker horizons are composed of porcelain-like silicified tuffs, limestones with inclusions of brown chalcedony, calcilutites, salt rock, wax-like bentonitic mudstones, flint clays, tripolis, etc.

The generalized eustatic and tectonoeustatic models developed by the author are tested on Lower Cretaceous deposits of the eastern part of the Russian Plate. The models are applicable to facies analysis of sections of epicontinental basins with mainly slope sedimentation. They demonstrate possible variations in section lithology depending on the rate of eustatic changes and the intensity and direction of epeirogenic movements. It has been revealed that the Lower Cretaceous sections in the east of the Russian Platform formed as a result of the synchronous global eustasy and regional epeirogeny. Superposition of the global eustatic curve onto the Lower Cretaceous chronostratigraphic chart of the eastern part of the platform showed that global eustasy, periodically concealed by regional epeirogeny, played a crucial role in the Early Cretaceous history of the study area. Regional epeirogenic and eustatic curves were constructed. The epeirogenic curve demonstrates the contribution of vertical tectonic movements to the overall eustatic-epeirogenic result recorded on a regional eustatic curve. The latter was constructed from the analysis of the spatial and temporal changes in the stratigraphic position of formations and strata and transgressive surfaces ranking. Eustatic cycles of different ranks, from elementary (systems tracts) to regional scale, have been recognized. In the rank of largest lithostratigraphic units, three sequences are revealed: Valanginian (RP-1K), Upper Hauterivian-Upper Aptian (RP-2K), and Albian (RP-3K), which reflect the crucial stages of the Early Cretaceous evolution of the eastern Russian Plate. The eustatic-epeirogenic processes during accumulation of formations and strata from Early Berriasian to Late Albian (145.5-99.6 Ma) are considered. It is shown that the division of the studied composite section into sequences permits precise prediction of diverse solid minerals.

Results of complex geological, petrological, geochemical, and isotope-geochronological studies of Neoproterozoic postcollisional A -type granites of the Glushikha complex, Yenisei Ridge, are presented, as well as results of mathematical modeling. The localization, sizes, and depths of formation of magmatic bodies and the physicochemical conditions of the formation of granite intrusions and their magmatic sources are considered. The research is focused on the modeling of the thermal history of the formation and subsequent cooling of granites in the collision orogen. The modeling of heat transfer during the acid-magma intrusion was performed by the example of the Lendakha and Glushikha leucogranite plutons. With the combined methods of the modeling of heat transfer and the behavior of K/Ar isotopic system, the models for leucogranite pluton cooling have been verified for the first time. The time of formation and cooling of granite bodies is estimated, and a generalized geodynamic model for the formation of the Glushikha postcollisional A -type leucogranites is proposed.

The petrographic, petrochemical, and mineralogical compositions of the Kuvalorog Ni-bearing cortlandite-norite intrusion and endocontact leucodiorites hosting pyrope-almandine garnet are considered. Lamprophyre dikes with scarce pyrope-almandine garnet, first discovered in the massif, and plagioclase-pyroxene symplectites in garnet porphyroblast areas are studied. Judging from the petrography of rocks and the composition of inclusions rich in incompatible elements in the garnet, the mineral was produced by the reaction of orthopyroxene with the anorthite component of plagioclase at the subsolidus stage of formation of endocontact diorites. This reaction was probably favored by the fluid phase abundant in the parental magma of the Kuvalorog intrusion and, especially, in the zones near its contact with relics of terrigenous rocks, where it was produced as a result of the rock dehydration under the thermal effect of the intrusion.

We have measured the dielectric spectra of sand and bentonite mixtures with oil and saline water (NaCl solution) in the frequency range from 50 MHz to 16 GHz at 22 °C. The measured spectra were compared with predictions in terms of the generalized refractive mixing dielectric model (GRMDM), and the respective error was estimated as a check of the model applicability to describe oil-bearing formations. The results may be useful in developing new methods of electromagnetic logging.

Water salinity at shallow sea depths in shelf areas changes with depth, which causes respective conductivity changes. We discuss algorithms for computing monochromatic electric fields using the vector finite-element method with different realizations of depth dependence of conductivity. The algorithms have been applied to compute vertical conductivity patterns and to explore its influence on the electric field measured on the surface.

We investigate the 2D thermal structure of lithosphere in the Central Asian and Pacific tectonic belts and adjacent cratonic areas of Siberia and North China using a synthesis of geothermal data from six geoscience transects covered by seismic, resistivity, and gravity surveys. The patterns of rock density, radiogenic heat production derived from U and Th abundances, thermal conductivity, temperatures, and respective heat flows reveal a layered structure. The model with layers distinguished according to density and thermal parameters includes well pronounced dome-shaped features in the crust which correlate with upwarps of the asthenospheric top. The domes are marked by high heat flows of 60-90 mW/m² with a mantle component higher than the crustal one (30-60 mW/m² against 20-30 mW/m²) and temperatures as high as 800-1100 °C at the Moho. Many of these features correspond to known and potential petroleum basins.