Home – Home – Jornals – Russian Geology and Geophysics 2013 number 9

Based on geological data and the geochemical and isotopic (Sr, Nd) parameters of the Devonian volcanic associations of the Minusa basin, the main regularities of volcanism development are considered, the composition of magmatic sources is studied, and the geodynamic mechanisms of their involvement in rifting are reconstructed. The early stage of formation of the Minusa basin was characterized by intense volcanism, which resulted in differentiated and, more seldom, bimodal volcanic complexes composed of pyroclastic rocks and dolerite sills. At the late stage, only terrigenous deposits accumulated in the basin. It has been established that the basites are similar in composition and are intermediate in geochemical characteristics between intraplate rocks (OIB) and continent-marginal ones (IAB). The basites, like OIB, have high contents of all lithophile elements, which is typical of enriched mantle sources, and, like IAB, show negative anomalies of Nb, Ta, Ti, and, to a smaller extent, Rb, Th, Zr, and Hf, selective enrichment in Pb and Ba (and, sometimes, Sr), and a weak REE differentiation (7 < (La/Yb) N < 17). In contrast to the basins in other segments of the Devonian Altai–Sayan rift area, the igneous-rock associations in the Minusa basin are characterized by a worse expressed geochemical inhomogeneity of rocks and lack of high-Ti (>2 wt.% TiO2) basites. The Sr and Nd isotope compositions of the Minusa basites deviate from the mantle rock series toward the compositions with high radiogenic-strontium and low REE contents. This points to the melting of a mantle substratum (PREMA-type) and carbonate-rich sedimentary rocks, which were probably assimilated by basaltic magma. The correlations between the contents of trace incompatible elements in rocks with SiO2 = 53–77 wt.% testify to the assimilation of crustal substrata by parental basaltic melts and the subsequent differentiation of contaminated magmas (AFC model). We propose a model for the formation of primary melts with the simultaneous participation of magmatic sources of two types: plume and fluid-saturated suprasubductional, localized beneath the active continental margin.

This paper presents mineral reactions in metapelites and estimations of PT -conditions of metamorphism for paragranulites of the Irkut block of the Sharyzhalgai uplift (Siberian Platform). The geothermometeric data based on exchange reactions and the contents of Na in cordierite and Ti in zircon indicate that the peak temperature of metamorphism reached ca. 800 °C at 6–7 kbar. The observed mineral reactions suggest that the pressure decreased to ca. 3 kbar at the retrograde stage of metamorphism.

We performed a thermomagnetic analysis of 91 samples and a probe microanalysis of five samples of sedimentary rocks from the lower zone of the borehole BDP-98 drilled at the bottom of Lake Baikal. The results show the scarcity of native iron: It was found only in five samples. Its concentration varies from ~10–5 to 7·10–4 %. The distribution of native-iron concentrations is bimodal, with a distinct “zero” mode. This scarcity of native iron in the Baikal sediments distinguishes them from continental (Eurasia) and oceanic (Atlantic) sediments of different ages. It is due to the high rate of sedimentation in the studied interval of BDP-98.

The chemical composition of bottom sediments in the Chukchi and, partly, East Siberian Seas was studied. In the south and west of the Chukchi Sea, a zone has been detected with the accumulation of sediments rich in organic carbon, an increased background content and anomalies of sulfophile metals (Mo, Zn, Hg, Ag, Au), iron group metals (V, Ni, Co), and some PGE (Ru, Pt). This zone is confined to the neotectonic active system of rift troughs extending fr om Bering Strait and the eastern Chukchi Peninsula to the continental slope, wh ere it is bounded by the Cenozoic Charlie rift basin of the Canadian hollow. The geochemical features of the carbon-enriched sediments evidence that they formed under oxygen-deficient conditions and, sometimes, in suboxic and anoxic environments near endogenic water and gas sources. The high carbon and metal contents suggest that the fine-grained sediments in the rift troughs of the Chukchi Sea are a possible analog of some types of ancient highly carbonaceous sediments belonging to black shales.

In geologic objects of different characters and scales, the fractal properties of topography are related to the intensity of endogenic energy flows and the composition of geologic complexes. A good correlation between the topographic differentiation of the Khibiny pluton and the variables of different levels of its structural and compositional organization (mineral and chemical compositions of the rocks and minerals, rock texture, etc.) suggests that topography formation is an element of the self-organization of the Khibiny pluton. Analysis of the fractal dimensionality of topography in the Khibiny pluton, Primorye, and detailed areas in Transbaikalia revealed a coincidence of its maxima with the position of ore clusters, fields, and deposits, i.e., areas with the contents of elements significantly higher than their clarkes. All the above data suggest that the fractal properties of topography can be used as a prospecting criterion.

The Middle Oxfordian to lowermost Upper Kimmeridgian ammonite faunas from northern Central Siberia (Nordvik, Chernokhrebentnaya, and Levaya Boyarka sections) are discussed, giving the basis for distinguishing the ammonite zones based on cardioceratid ammonites of the genus Amoeboceras (Boreal zonation), and, within the Kimmeridgian Stage, for distinguishing zones based on the aulacostephanid ammonites (Subboreal zonation). The succession of Boreal ammonites is essentially the same as in other areas of the Arctic and NW Europe, but the Subboreal ammonites differ somewhat from those known from NW Europe and Greenland. The Siberian aulacostephanid zones – the Involuta Zone and the Evoluta Zone — are correlated with the Baylei Zone (without its lowermost portion) and the Cymodoce Zone/lowermost part of the Mutabilis Zone (the Askepta Subzone) from NW Europe. The uniform character of the Boreal ammonite faunas in the Arctic makes possible a discussion on their phylogeny during the Late Oxfordian and Kimmeridgian: The succession of particular groups of Amoeboceras species referred to successive subgenera is revealed by the occurrence of well-differentiated assemblages of typical normal-sized macro- and microconchs, intermittently marked by the occurrence of assemblages of paedomorphic “small-sized microconchs” appearing at some levels preceding marked evolutionary modifications. Some comments on the paleontology of separate groups of ammonites are also given. These include a discussion on the occurrence of Middle Oxfordian ammonites of the genus Cardioceras in the Nordvik section in relation to the critical review of our previous paper. The discussion shows that the oldest deposits in the section belong to the Middle Oxfordian, which results in the necessity for some changes in the foraminiferal zonal scheme of Nikitenko et al. The ammonites of the Pictonia involuta group are distinguished as the new subgenus Mesezhnikovia Wierzbowski and Rogov.

A theoretical study has been performed to check the possibility of using ultrabroadband nanosecond electromagnetic pulses as a geosteering tool for horizontal drilling to estimate the distance to the oil–water contact (OWC) in a floating oil accumulation. The voltage of a microwave-bandwidth pulse at the dipole receiver of a downhole radar was modeled for the case of a horizontal borehole near OWC in a formation saturated with oil and water. Numerical solutions to the boundary problem formulated on the basis of the Maxwell equations were obtained with the Microwave Studio software (www.cst.com). The frequency-dependent dielectric constants of the layered saturated formation and the drilling fluid were assumed according to experimentally tested models. The modeling has demonstrated that nanosecond electromagnetic pulses arriving from a layered oil–water contact can in principle be acquired and the distance from the wellbore to the OWC median can be inferred from the respective time delays recorded by a downhole radar. Additionally, the possible dynamic range and accuracy of sensing have been estimated.

The upper-mantle structure was studied from first-arrival data along the Meteorite profile, run using underground nuclear bursts. Unlike the layered, slightly inhomogeneous models in the previous works, emphasis was laid on lateral inhomogeneity at the minimum possible number of abrupt seismic boundaries. We used forward ray tracing of the traveltimes of refracted and overcritical reflected waves. The model obtained is characterized by considerable velocity variations, from 7.7 km/s in the Baikal Rift Zone to 8.0–8.45 km/s beneath the Tunguska syneclise. A layer of increased velocity (up to 8.5–8.6 km/s), 30–80 km thick, is distinguished at the base of seismic lithosphere. The depth of the layer top varies from 120 km in the northern Siberian craton to 210 km in its southeastern framing. It has been shown that, with crustal density anomalies excluded, the reduced gravity field is consistent with the upper-mantle velocity model.

An important but problematic component of nuclear geophysical technologies is computer inversion of measurement data based on the equation of particle transport. In this paper, we propose an approach and iterative methods for this inversion that are applicable to many problems of evaluation formation characteristics, including elemental composition, from data of corresponding logging methods. This approach is based on defining characteristic elements, interactions, and trajectories and using the superposition principle for transport processes, and linear a priori constraints on unknowns are used. In comparison with the previous publications of the author, this paper deals with a broader type of measurement data, including the reading ratio of detectors and normalized measurements. The iterative methods are specified for problems of determining the porosity and density from data of neutron–neutron and gamma–gamma logs, respectively. For the first of them, the results of numerical experiments are presented.