Home – Home – Jornals – Russian Geology and Geophysics 2020 number 7

Unique magnesian-aluminous ultrabasic sapphirine-spinel and garnet gedritites of the Aulandzha block were studied. The deepest part of the Archean Stage of the Omolon massif is exposed in this district. The gedritites differ strongly in petrochemistry and geochemistry from other metaultramafites of this block. The difference is in their enrichment in alumina, zirconium, barium, rubidium, hafnium, and uranium and depletion in calcium and heavy rare-earth elements. The oxidizing potential in the sapphirine-spinel assemblage, estimated by physicochemical modeling using the Selector program complex, showed high oxygen fugacity in these rocks, close to the value of the magnetite-hematite buffer, which was never observed in ancient granulite complexes. The above petro- and geochemical features of the gedritites of the Aulandzha block, having an isotopic age of 1.9 Ga, are explained by the fact that these rocks are probably the crust of weathering of the enclosing metaultramafites. If this hypothesis is true, then the above rocks may indicate that the oxygen potential on the Earth’s surface corresponded to the magnetite-hematite buffer as early as the Paleoproterozoic.

We report the chemical and isotopic compositions of volcanic rocks of the Akhtang and Kostina mountain massifs in the Sredinny Range, Kamchatka. The analyzed rocks are similar in composition to the earlier studied volcanics on the eastern flank of the southern part of the Sredinny Range. Results of K-Ar isotope dating reveal three stages of volcanic activity in the two massifs. These stages are divided by long (1.4 and 2.4 Ma) periods of quiescence. In the Akhtang massif, the eruptive activity was at 4.9-4.0, 1.9-1.7, and 0.3-0.2 Ma, and in the Kostina massif, at ~8.0, 5.6-4.9, and ~3.5 Ma. Two early stages of both massifs are characterized by the eruption of island arc type rocks, and the late stage, by the eruption of rocks of hybrid geochemical type. The Mio-Pliocene (N₁-N₂¹) rocks of the Mt. Kostina massif are similar in geochemical features to the early Pliocene (N₂¹) rocks of the Akhtang massif, and the late Pliocene (N₂²) lavas of the former massif are similar to the middle Quaternary (Q₂) rocks of the superimposed monogenetic volcanism zone of the latter massif. For the Akhtang massif it has been first discovered that the volcanic reactivation after the long quiescence periods was accompanied by a change in the composition of rocks and in the type of eruptive activity (from the eruption of plateau-effusive rocks to the formation of stratovolcano and monogenetic volcanism zones). The obtained data on the age and composition of rocks as well as some morphological features of the studied massifs suggest that the plateau-effusive rocks of the Sredinny Range might be related to central-type eruptions.

Supplementary materials

Ground Penetrating Radar (GPR) surveys in the area between Sherashevo and Inkino villages provide insights into the structure of the Delta Fault and allow estimating the amount of vertical slip caused by the M = 7.5 Tsagan earthquake of 12 January 1862. The surveys with shielded AB-90 and AB 250-M antennas of an OKO-2 georadar along five profiles spaced at 25 m reveal normal slip from 2.6 to 4.5 m in different segments of the main seismogenic fault. The surface rupture caused by the 1862 event is traceable in interpreted radar images together with subsidiary faults; some possibly resulted from the 1959 Middle Baikal earthquake (M = 6.8). The GPR data are used to construct a 3D model of the area, which illustrates the evolution of the Delta Fault scarp since the Tsagan earthquake. Much of surface rupture during the Tsagan event is due to gravity sliding, judging by the amount of displacement estimated from GPR, structural, and field data of different years. Comprehensive understanding of the displacement pattern along the seismogenic fault requires further study extended to other segments of the fault zone.

Variations in the magnetic and geochemical properties of the sediments of Lake Bol’shoe Yarovoe (Altai Territory) were studied. The data were derived from five core columns (up to 4.5 m long) covering a time interval of more than 8000 years. In addition, coercive spectra were obtained for 792 samples taken every 2 cm. Coercive spectra were then used to identify soft magnetic (10-15 mT) and hard magnetic (35-50 mT) components. The soft magnetic component is detrital, and the hard magnetic component is biogenic, which is confirmed by microscopic studies. Moreover, the samples contain micrometeorite particles. Variations in geochemical properties allow reconstruction of the environmental history of the lake. Sediments in the lower part of the section vary both in the content of ferrimagnetic components and in geochemical properties, which indicates a sharp environmental change between ~6100 and ~7600 years ago. Alternation of warm (dry) and cool (humid) periods is observed higher in the section, between ~4100 and ~6100 years ago. It has been found that the variations in the magnetic components are consistent with the environmental changes and can be used for the historical reconstruction.

We present results of a study of carbonate coatings formed at the lower surfaces of pebble inclusions in Holocene-Upper Pleistocene deposits of the Irkutsk-Cheremkhovo plain. The coating resulted from the leaching of carbonates from the overlying deposits during pedogenesis. They are composed of pure and magnesian calcite with minor inclusions of quartz and feldspars. The coatings form morphologically and compositionally different microlayers, which reflect a successive change in the pedogenesis conditions during their formation. The stable-isotope composition is as follows: δ13С is -6.80 to -2.05 ‰, and δ18О is -16.33 to -10.15 ‰. It reflects the precipitation of carbonates during the degassing of soil solutions in the course of periodic freezing-thawing processes, dynamic increase and decrease in the biological activity of soils, and alternating moistening of soil with meltwater and rain water in spring and its subsequent freezing, which could occur in the region in spring and autumn. A joint analysis of the carbon isotope composition of the organic matter of soils and carbonate coatings shows that the latter formed in the phytocoenosis environment with predominant C3 vegetation. The difference in the morphology and occurrence of the coatings permit them to be divided into three groups. The coatings of the first group formed in the Middle Holocene (3.6-3.3 cal. ka BP), and the pendants of the second and third groups, in the second half of MIS-3 (24.1-23.3 and ~34-35 cal. ka BP, respectively). The paleoecologic conditions reconstructed for the established stages of carbonate coating formation are in satisfactory correlation with the general course of climatic fluctuations in the region and in the Northern Hemisphere as a whole. They reflect the influence of temperature and humidity fluctuations on the dynamics of pedogenesis processes. Comparison of the age of the coatings with the age of recent and buried soils shows that the pedogenic carbonate coatings in the Upper Angara soils are a relict feature of the previous epochs of pedogenesis (MIS 3) and the first stages of recent soil formation, which began, most likely, in the Middle Holocene. Based on analysis of the rates of coating formation and comparison of the isotope composition of the coatings of different age groups, we assume that the climate in the Karga (MIS 3) megainterstadial was more humid than that in the Middle Holocene, with the temperatures of pedogenesis being the same. Both in the Middle Holocene and in the Karga epoch, the regional soils received little heat and were seasonally frozen for a long time.

Well logs and core analyses of Berriasian-Aptian petroliferous sediments from the western Gydan Peninsula are used to model their biofacies and lithofacies and to reconstruct the respective sequence stratigraphy. The sedimentological and palynological data, along with logging results, provide constraints on the history of transgressive and regressive events and make a basis for paleogeographic reconstructions for the Lower Cretaceous deposition of reservoir sand beds.

The gradual transition of the Bazhenov Formation top into the overlying deposits makes it difficult to establish its upper boundary. The problem is aggravated by the lack of core material when the formation is recognized according to the results of geophysical well studies. A comprehensive analysis of geochemical and lithological data and the results of geophysical well surveys enabled us not only to determine the specific structure of the transitional zone from the Bazhenov Formation top to the rocks of the sub-Achimov member but also to propose the criteria for delineating the upper boundary in the central regions of the West Siberian sedimentary basin within the Khentei hemianteclise, the South Nadym megamonoclise, and the Mansi syneclise. Four members are distinguished in the transitional zone of the studied well sections (from bottom to top), which differ in lithological, geochemical and geophysical characteristics: (I) the “coccolithic” upper part of the Bazhenov Formation; (II) transitional member from the upper part of the Bazhenov Formation to the bottom of the sub-Achimov member; (III) transitional member from the bottom of the sub-Achimov member to its lower part; and (IV) the lower part of the sub-Achimov member. Member II is virtually not distinguished within the Khentei hemianteclise and the South Nadym megamonoclise. Member III contains the boundary between zones with different redox conditions. In case member II is distinguished, the upper boundary of the Bazhenov Formation corresponds to its top. To recognize the Bazhenov Formation top, it is necessary to use the integrated-analysis results of the lithological and geochemical studies of the well core and logging data (gamma-ray logging, neutron gamma-ray logging and its variations, lateral logging, and induction logging) when focusing attention mainly on radioactivity.

Petroleum geochemical characteristics are a major parameter of hydrocarbon field development. The present study is an attempt to decipher the unusual Bangestan (Cretaceous age) oil distribution throughout the Ahvaz oil field, SW Iran, based on SARA test and GC-MS analysis of selected oil samples. The results indicated that all analyzed oils belong to the paraffinic group. Hydrocarbon indicators, such as tricyclic triterpane C₂₂/C₂₁ (high), C₂₄/C₂₃, and C₂₆/C₂₅ (low) and hopane C₃₁R/C₃₀ (high) ratios and C₂₅/C₂₆ ratio vs C₂₅/C₂₆ tet ratio, show that these oils sourced from carbonate-marl rocks. The high saturated/aromatic hydrocarbon ratio in these oil samples may be related to long migration or high maturity. The biomarker variation exhibits a marine environment for the source rocks deposition. The observed oil maturity trend is showing a good correlation with the prevailing geothermal gradient and possible basement faults and fractured system. The increasing oil maturity in the eastern part of the field may be related to a heat flow anomaly. It seems that the area around well C (the area of wells C, D, or E) can be considered a petrochemical separator of fluids for two sides of the field. Therefore, it can be concluded that the observed petrochemical pattern is a complicated response of several factors: the presence of a palaeohigh, basement-controlled faults, petroleum source rocks, fractured system, and geothermal gradient variation in this deep reservoir.

The paper considers the manifestation of electromagnetic (EM) signal over the conducting polarizable ground on the measuring lines located on the axis of the source and inside it (for the symmetric Schlumberger installation). The research is based on a numerical experiment. Calculations of the EM response from one-dimensional models were carried out. The polarizability is taken into account by the frequency-dependent resistivity, using the Cole-Cole model. We describe the results of a numerical experiment on calculation of the induction signal over a conductive polarizable medium on grounded 2- and 3-electrode measuring lines located in the axial and equatorial zones of the source. It is shown that the induced polarization and the polarization associated with galvanic and eddy current are manifested in different ways.