Home – Home – Jornals – Russian Geology and Geophysics 2021 number 11

The basic software package of a JXA-8230 microanalyzer, like its predecessor JXA-8100, uses the long-established ZAF correction method (with some differences) for a quantitative analysis: Calculation of mass absorption coefficients is based on Chantler’s theoretical data. The core of this method is quantum-mechanical calculation of the cross section of the interaction between an X-ray photon and atomic electrons. This innovation has had a positive influence on the trueness of X-ray microanalysis. Control tests on specimens where the absorption effect is dominant have demonstrated that the results of this analysis are slightly lower (by less than 2%) independently of the matrix absorption interval in which the analytical line is located. As a consequence, the selection of comparison specimens becomes easier: It is sufficient that the specimen under study and the comparison specimen belong to the same isomorphic series and that the intensity of the analytical line of the comparison specimen allows for the measurement with the required accuracy.

We study the relations between several selected elements present in the Sin Quyen IOCG deposit, Lào Cai, North Vietnam, and interpret the obtained correlations, especially with a coefficient higher than 0.7. The correlations with high coefficients are mainly observed for the elements belonging to the chalcophile group (Cu, Ag, Au, Te, and Bi) and for the relation between uranium and Ag, Au, Cu, Pb, and Bi. Although the S-, Fe-, and REE-bearing minerals are predominant in the studied deposit, no strong correlation between them and the other elements was observed, even with Cu. The phenomena are primarily explained based on the geochemical properties of the mentioned elements and the characteristics of IOCG deposits.

The composition of the organic matter (OM) of coals in the Ulug-Khem Basin has been studied. According to Rock-Eval pyrolysis data, this OM is a type III kerogen, sometimes significantly oxidized. The coal of low-grade metamorphism has a high hydrocarbon potential. Based on the gas-liquid chromatography and chromatography-mass spectrometry data on the distribution of n-alkanes, isoprenoids, polycyclic biomarkers, and aromatic hydrocarbons, we have established the composition of the primary OM of the coals and the grade of OM metamorphism. The primary OM of the coals consists mainly of remains of aquatic vegetation and terrigenous OM. The latter includes conifer remains, which are identified from the presence of 4β(H)-19-norisopimarane in the aliphatic fraction of bitumen and from the domination of retene over cadalene and 6-isopropyl-1-isohexyl-2-methylnaphthalene in the aromatic fraction.

This paper presents an alternative variant of stratigraphic subdivision of the upper Riphean-Vendian deposits (approximately corresponding to the Neoproterozoic Erathem of the International Chronostratigraphic Chart (ICC)) of the Bashkir Mega-Anticlinorium (BMA), based on the modern concepts of changes in the biosphere, atmosphere, and hydrosphere (and, accordingly, in the climate and the course of sedimentary processes) in the period ~1000-540 Ma, with regard to the results of recent studies and the proposals for improving the Russian General Stratigraphic Scale (GSS) of the Precambrian. The strict necessity of subdividing chronostratigraphic and lithostratigraphic units is shown. The presented data give grounds to regard the Zil’merdak Formation as a Group and assign three of its lower formations to the Middle Riphean (Yurmatinian). It is shown that the period of accumulation of the Karatau Group (comprising the Katav, Inzer, Min’yar, and Uk formations) was significantly shorter than the duration of the Late Riphean (Karatavian). Special attention is focused on the age of the Uk Formation (analysis of the existing data made it possible to refine its stratigraphic position): It formed, most likely, in the period 780-740 Ma. A high uncertainty of the stratigraphic position of the overlying geologic bodies is demonstrated. The Bakeevo, Tolparovo, Suirovo, and Kurgashla formations must be regarded as part of the Terminal Riphean (Arshinian). The Arsha Group, comprising the Bainas, Makhmutovo, Igonino, and Shum formations in the Tirlyan trough and represented by the Krivaya Luka Formation in the Krivaya Luka syncline, should not be totally assigned to the Terminal Riphean. It is proposed to exclude the Bakeevo Formation and the Tolparovo-Suirovo sequence from the Asha Group, because these deposits are, most likely, a modification of the Arsha Group located on the western flank of the BMA. The Uryuk Formation is probably of pre-Vendian age. The current contradictory data on the stratigraphy of the supra-Uryuk unit of the Asha Group are interpreted. The ages of the geologic bodies composing the Group and its stratigraphic interval still call for a thorough study.

We studied the middle Cambrian unit of the Kuonamka Formation section on the Kyulenke River (Siberian Platform) and performed its biostratigraphic subdivision based on trilobites. The middle Cambrian section has intervals corresponding to the regional zones of the Amginian Stage. Six levels with mass accumulation of fauna remains have been revealed: Two levels are located within the Ovatoryctocara Zone; the third level is at the boundary between the Ovatoryctocara and Kounamkites zones; the fourth layer is confined to the roof of the Triplagnostus gibbus Zone; and the fifth and sixth levels are located within the Tomagnostus fissus-Paradoxides sacheri Zone. The composition of rocks and bitumens of their organic matter (OM) has been studied, including the geochemical specifics of the mineral components of rocks (iron, sulfur, and СО2) and of saturated hydrocarbons of bitumens as well as noncarbonate carbon isotopes in the OM. It has been established that the OM sedimentation took place under normal aeration of the sea basin waters, without hydrogen sulfide contamination of the bottom waters. The intensity of chemical and biochemical transformations of mineral and organic components during diagenesis was controlled by the contents of organic carbon and sulfate ion, the activity of the anaerobic prokaryote community, and the rate of sediment mineralization. We have also established relationships between the content of organic carbon in potentially oil source rocks and the contents of iron oxide, total sulfur, and sulfide and sulfate sulfur as well as the ratios of saturated hydrocarbons. The alternation of highly carbonaceous black shales and carbonaceous rocks is apparently due to a change in the composition of biologic communities of microorganisms (sources of hydrocarbon biomarkers) and in the intensity of OM transformation during diagenesis. We assume that the OM transformation included sulfate reduction and dealkylation of high-molecular steroids in the unconsolidated OM-enriched marine sediments with the participation of bacteria. The intensity of these processes depended on the mass of the primary OM, the amount of sulfate ion, and, hence, the pH and Eh of the medium.

The paper presents new data on the upper Silurian litho- and biostratigraphy of the Gorny Altai area. Sediments within this interval store a succession of taxonomically representative middle-upper Ludfordian, lower Pridoli, and Lower Devonian (Lochkovian-Pragian) conodont assemblages. The new fauna constraints made a basis for updated correlations of the local and regional stratigraphic units at the Silurian/Devonian boundary of Gorny Altai with the stages of the International Stratigraphic Chart. The correlation results reveal a mismatch between the boundaries of the local and regional Silurian units and the respective boundaries of stages in the International Stratigraphic Chart.

In order to determine the seismotectonic activity of faults in the Holocene, we performed trench studies of the ruptures produced by the catastrophic Mogod earthquake (5 January 1967, М = 7.5-7.8, I ₀ = 9-10) in the junction zone of the N-S striking Hulzhin Gol fault and the NW striking Tullet fault. Paleoseismic interpretation of seismic-deformation sections and radiocarbon dating of the samples allowed determining the kinematics and obtaining, for the first time, the absolute ages of paleoevents preceding the Mogod earthquake. Analysis of the tectonic conditions for realization of earthquake sources has shed light on the complex structure of ruptures in the area of the Mogod earthquake epicenter, within which three segments differing in the displacement amplitudes and kinematics have been identified. The research data indicate the repeated activation of the Tulet and Hulzhin Gol faults in the Late Pleistocene-Holocene. The absolute age of the latest activation is 596-994 AD for the Tulet fault and 11,379-6235 BC for the Hulzhin Gol fault. The cumulative deformation from paleoearthquakes in the trench sections in the Tulet fault zone points to at least two displacements of thrust kinematics, with the latest of them having an amplitude of 2.8 m. The paleoearthquake in the Hulzhin Gol fault zone is characterized by the presence of lateral slip. The amplitudes of deformations attest to earlier earthquakes similar in energy to the 1967 Mogod event or even stronger in the fault node. The obtained data on the timing of these earthquakes and the amplitudes of the accompanying displacements made it possible to estimate slip rates along the faults: 0.2-0.3 m/kyr horizontal-slip rates on the Hulzhin Gol fault and 0.5-0.7 m/kyr vertical-slip rates on the Tulet fault.

The Gobi-Altai, Mongolia, includes high mountain ranges that have accommodated the compressional stresses derived from the collision between the Eurasian and Indian Plates. The Gurvan Bogd, which is one of the main mountain ranges in the Gobi-Altai, is a restraining bend along the Bogd sinistral fault. Although surface ruptures did not form near the Artz Bogd during the M_w = 8.1 Gobi-Altai earthquake of 1957, it is still active, as evidenced by a growing topography (i.e., forebergs). Six foreberg ridges have formed in the foreland of the Artz Bogd, which are considered to be the result of surface deformation of alluvial fans due to thrusting. One stream has cut down to expose a foreberg tip, providing the opportunity to explore the slip evolution of the region. Here we map a growing fault structure related to blind thrusting. We identify five faulting events from an analysis of the outcrop and apply optically stimulated luminescence dating to the faulted sedimentary layers, yielding an average slip rate of 0.045 ± 0.007 m/kyr and an earthquake recurrence interval of 5.8 ± 0.5 kyr over the last ~32 kyr. Furthermore, the long-term (~600 kyr) uplift rate of the foreberg is 0.067 ± 0.007 m/kyr, as deduced by dividing the vertical displacement of the alluvial fan surface by the ¹⁰Be surface exposure ages of boulders on the fan. The discrepancy (20-30 %) between these two deformation rates may be due to the different timescales they cover and an along-strike gradient in slip rate.

Terraces at four hypsometric levels were studied in the Vuoksa River basin (northern part of the Karelian Isthmus, NW Russia). New data on nine sections of late Quaternary-Holocene sediments have been obtained. Their age has been determined (for the first time for surface deposits in the studied region) in the interval from 90 to 2 ka. The terrace sediments are disturbed by deformations (faults, folds, and liquefaction) caused by six strong earthquakes in that period. The relationships among the terrace levels, ages, stratigraphy, and structures of loose sediments point to their formation under the impact of differentiated tectonic motions triggered by the activation of the ancient “Vuoksa” fault zone in the late Neopleistocene and Holocene.