pdf № 323, November

DOI: 10.19110/geov.2021.11.1

We studied Upper Paleozoic (P₁pt-ng) carbonaceous shales and siltstones from the area of the Ust'-Kara astrobleme (Pay-Khoy). We analyzed mineralogical and geochemical features of carbonaceous rocks of the target in the vicinity of the Ust'-Kara astrobleme event using a complex of modern methods to identify possible mobilization, redeposition and concentration of ore substance under intensive post-impact hydrothermal activity. Geochemical features of carbon deposits, altered by post-impact hydrothermal processes in the vicinity of the Ust'-Kara impact structure, have been determined. We found anomalous contents of Ti, Mn, Cr, Zr, Ni, Li, Co, Sc and REE. Inherent rare metal and rare earth minerals (monazite, florensite), sulfides (pyrite, chalcopyrite, marcasite, sphalerite), apatite, barite, anatase, chrome spinels were diagnosed.

Keywords: carbonaceous rocks, metal content, hydrothermal processes, post-impact transformations, ore potential.

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DOI: 10.19110/geov.2021.11.2

The article presents results of calculations of the coefficients of discreteness and energy consumption of megablocks, transblock zones and intersection nodes in the territory of the European North of Russia (the Baltic Shield and the North of the Russian Plate). Calculations of fault surface density and the crushing («blockiness») function are carried out. Analysis of the obtained parameters showed that seismically active zones and nodes of their intersection are characterized by high values of the studied parameters comparable to the values typical for the Arctic seismic belt. Thus, we can talk about the spatial connection of blocks with increased discreteness, energy saturation and surface density of the fault with the topografic position of earthquakes.

Keywords: the European North of Russia, blocks of the Earth's crust, Energy consumption, seismicity.

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DOI: 10.19110/geov.2021.11.3

The isotopic composition of carbon and oxygen of 165 samples of carbonate rocks of genetically different organogenic structures: the Upper Ordovician Bol'shaya Kos'yu reef, the Upper Devonian Shar'yu microbial mound, the Kozhym skeletal mound, the Upper Miocene Kazantip bioherm complex and deep-water organogenic-carbonate structures of the north of the Mid-Atlantic Ridge was studied. The reef and microbial mound are characterized by d¹³С_PDB values within the limits close to normal-sedimentary marine carbonates (–0.33—3.13 ‰) and (0.8—3.0 ‰), but with significant variations in d¹⁸О_SMOW — (22.24—30.0 ‰) and (20.4—26.3 ‰), respectively. The most isotopically-heavy carbon composition (5.1—7.3 ‰) in combination with varying d¹⁸О_SMOW values (22.4—30.0 ‰) is characteristic of limestones and brachiopods of the skeletal mound. Fluctuations of d¹³С_PDB and d¹⁸О_SMOW values in carbonate rocks of the Kazantip bioherm complex are the most expressive (–2.76—7.17 ‰) and (24.20—33.1 ‰), respectively. Deep-water organogenic structures from the bottom of the axial zone with active volcanism, in contrast to others, showed stable d¹³С_PDB values (–0.98—0.83 ‰) within the area of normal-sedimentary marine carbonates, whereas for oxygen the values are istopic-heavy (32.27—39.75 ‰). As a result, the specificity of the development of the objects under study, established by lithological-paleoecological and chemical-physical methods, found its substantiation in isotopic values, paleosalinity and paleotemperatures caused by paleogeographic and climatic conditions.

Keywords: reef, microbial and skeletal mounds, bioherm complex, deep-water organogenic-carbonate buildups, isotopic composition of carbon and oxygen in carbonates.

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DOI: 10.19110/geov.2021.11.4

The Upper Silurian is widely developed in the Northern Urals, where it is represented by various facies. The continental slope facies and the reef shoal facies are well documented in the geological literature, while as back-reef lagoon facies have not yet been characterized in detail. The studied objects included two sections of the back-reef deposits in the Ilych River basin. In this work, we characterize rock types the make up the sections, and their isotopic characteristics. The Late Silurian back-reef lagoon was characterized by a mosaic sediments distribution, which included the island areas between which there were areas with a quiet hydrodynamic regime. In the latter, mainly muddy carbonates were accumulated, represented by clotted fenestral limestones. The area of small islands was characterized by a wide distribution of lime sands, which clastic material was derived from probable paleosoil carbonates. They are characterized by slightly increased d¹³C values, which, probably, reflects isotopic fractionation at the evaporative geochemical barrier. Dolomitization and karstification are described among secondary alterations in the studied sections. Cementstones is interpreted as karst cavities precipitates.

Keywords: Upper Silurian, Northern Urals, lagoon deposits, isotopes of C and O.

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DOI: 10.19110/geov.2021.11.5

Carbonate rocks represented by nodules, lenses, layers of different morphology and length are typical for the black shale formations of different ages. They are of the greatest interest in oil source rocks as indicators of complex and not always unambiguously interpreted geological processes. A special place among these sedimentary bodies is occupied by microbialites, which indicate suppression of development of marine organic biocenoses, and often reflect emanation processes in ancient strata. Proof of these phenomena is fundamentally important for predicting and assessing the oil and gas potential of unconventional reservoirs.

On the example of carbonate solids of Triassic and Jurassic black shale formations, we present a complex analytical method to determine the microbial biochemical genesis of rocks on the base of the isotopic composition of carbon and oxygen, together with the hydrocarbon molecular markers of organic matter. The geochemical features of the isolated microbialites suggest that they are resulted from a complex history of black shale formations, which reflects both background lithogenetic transformations and superimposed processes, including high-temperature hydrothermal ones.

Keywords: Black shale formations, microbialites, isotopic composition of carbon and oxygen, biomarkers.

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