pdf № 343, July

DOI: 10.19110/geov.2023.7.1

The article presents a brief history of Mississippian brachiopod studies in the North of the Urals during the last 175 years. The regional stratigraphic chart of the Western Urals subregion was originally developed on the basis of brachiopod biostratigraphy. The works of A. Keyserling, G. N. Fredericks, N. V. Kalashnikov, N. V. Enokyan, N. N. Fotieva, D. V. Nalivkin, and other researchers have greatly contributed to the study of the Mississippian brachiopods of the region. The most active use of the North Urals brachiopods for the biostratigraphy of the region falls in the 60—70s of the last century and is mainly connected with the works of N. V. Kalashnikov. However, brachiopods are hardly studied in the southern part of the Tchernyshev Swell and in basins of the Vuktyl, Bolshaya Nadota, Kosyu, and Kharuta rivers. The modern techniques and recent advances in brachiopod research both in Russia and in the world are considered. Considering the changes in brachiopod systematics and Mississippian stratigraphy during the last 30 years, it is concluded that it is necessary to update the data on the taxonomic composition, paleogeographic and stratigraphic distribution of brachiopods in the North Urals region.

Keywords: brachiopods, Northern Urals, Mississippian, history of science.

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DOI: 10.19110/geov.2023.7.2

Monazite from rare-metal-rare-earth alkaline metasomatites of the Lower Mezen occurrence of the Middle Timan has a predominantly cerium composition (Ce₂O₃ > La₂O₃). The weighted average value of the point (Th-U-Pb, CHIME) age of monazites was (564 ± 27) Ma, the age of monazites, obtained from the ThO₂*–PbO isochrone, is (566 ± 127) Ma. The age of the monazite is close in time to the monazites of the Novobobrovsky ore field and the Vendian igneous formations (ultramafic rocks and carbonatites of the Chetlas Complex) formed as a result of a plume pulse.

Keywords: monazite, Th-U-Pb-method, Middle Timan.

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DOI: 10.19110/geov.2023.7.3

Facies associations of Upper Triassic Dalnegorsk limestones of the Taukhinsky terrane (Sakharnaya mountain, Sikhote-Alin) are integrated for the first time in this work. A detailed lithological and genetic description of the organogenic construction of the Sakharnaya Mountain is given. A lithological and paleoecological study of the limestones composing the structure was carried out, lithotypes corresponding to three genetic types were identified: biogenic, biochemogenic and mechanogenic. The sequence of formation of an organogenic structure consisting of four stages was determined: bank, biostrom, biogerm, reef. A detailed description of each stage of development is given. The presented results are of great importance for a better understanding of the development and spread of carbonation accumulation in the Pantalassa Ocean in the Late Triassic. Our research provides new data that can be combined into paleogeographic and geodynamic reconstructions of the Pantalassa Ocean.

Keywords: lithotypes, organogenic structure, paleosociety.

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DOI: 10.19110/geov.2023.7.4

Conodont elements are used as a geochemical archive of seawater. Some compositional features of conodont elements reflect conodont ecology and trophic structure of Palaeozoic pelagic ecosystems. However, the screening of conodont elements prior to geochemical and/or isotopic studies is a real problem. This study evaluates SEM cathodoluminescence (SEM-CL), which is very sensitive to the REE and Mn content of apatite, for the detection of traces of secondary transformation in the composition of conodont bioapatite. The SEM-CL of conodont elements is similar to that of unaltered shark teeth (blue-violet), but differs significantly from that of fossil vertebrate teeth (orange-red). Thermal alteration has little effect on the SEM-CL. Elements with a CAI of 1—1.5 show a redder and more intense CL than elements with a CAI of 5. In the case of corrosion of the conodont element surface in carbonate host rocks, the CL of the outer parts of the conodont element become reddish due to invasion of the carbonate material. Conodont elements from the clay host rock show deep purple SEM-CL. Thus, SEM-CL allows detection of the results of secondary processes in conodont mineralised tissues, including enrichment by REE and/or Mn, corrosion and contamination by carbonate material. This method can be used to screen significantly altered samples prior to chemical and isotopic analyses.

Keywords: conodonts, cathodoluminescence, mineralised tissues, taphonomy.

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DOI: 10.19110/geov.2023.7.5

The paper presents the results of the modelling of the transport of ²¹⁰Pb down the peat core for the Ilassky natural bog massif. The activities of radionuclides ²¹⁰Pb and ¹³⁷Cs were investigated in the core layer by layer. Due to the nonexponential decrease of ²¹⁰Pb activity, significant errors are possible in determining the age and accumulation rates of peat when applying classical dating models. The article presents ²¹⁰Pb dating data obtained using the IP-CRS model. Data analysis showed that the vertical distribution of ¹³⁷Cs and ²¹⁰Pb is related to peat parameters: ash content and bulk density. The ²¹⁰Pb dating of the core showed an age of 1963 for a depth of 17—19 cm, which is consistent with the ¹³⁷Cs data. The dating of this horizon is indirectly confirmed by the data of the ash content and bulk density of peat. The above mentioned confirms the correctness and adequacy of the chosen model of dating of the natural peat bog. The peak of anthropogenic radionuclide content in peat deposits corresponds to 1963, the date of signing the Partial Nuclear Test Ban Treaty, being a reference point for geochronological studies. The results of linear accumulation rate, peat deposit mass accumulation rate and the magnitude of the atmospheric flux of ²¹⁰Pb presented in the study are in good agreement with the data on peatlands of Northern Europe and the European Subarctic of Russia.

Keywords: peat core, accumulation rate, ²¹⁰Pb, ¹³⁷Cs, radionuclides migration, IP-CRS model, Ilassky swamp massif.

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