Abstract
For the first time we carried out a detailed comprehensive study of Frasnian stage in the lower сurrent of the Malaya Usa River located within the Eletskaya structural-formational zone of the Polar Urals. The Middle-Late Frasnian age of these rocks was determined by conodonts in the sequence from hassi-jamieae to Late rhenana of standard conodont zones. We analyzed the distribution of conodont biofacies in the section. Polygnathus-Ancyrodella biofacies is most developed in the interval of the hassi-jamieae zones. We found increasing biodiversity of conodonts and prevalence of deep-water Palmatolepis biofacies in the interval of the Early rhenana-Late rhenana subzones. We studied the material composition and secondary changes in the carbonate rocks of the section, which allowed reconstructing primary sedimentary structures and sedimentation environments. In general, the deposits were formed within a narrow intra-shelf depression, to which intra-basin clastic material actively entered. At some intervals of time, more deep-water clay-carbonate sedimentation conditions, which were associated with the global eustatic events at the jamieae-Early rhenana (Semichatovae transgression) and Early rhenana-Late rhenana (Lower Kellwasser transgression) are noted. The analysis of isotope-geochemical data presented that the early stage of the Semichatovae transgression corresponded to a slight shift in the isotope ratio of carbon from 1.5 to 1.8‰, after which the isotope composition lowered to 0.4‰. The subsequent variation changes from 0.8 to 1.7‰ gradually decreased, reaching the value of 0.9-1.0‰. At the level of the Lower Kellwasser transgression, a slight positive shift in the isotope ratio of carbon from 0.1 to 0.7 with subsequent variations in the range of 0.5-0.7‰ is noted. Similar changes in the isotope composition of carbon at the event levels of Semichatovae and Lower Kellwasser are observed world-wide and mostly developed in the sections of Central and Southern Europe, North America and North Africa.
Highlights
Впервые проведено детальное всестороннее исследование разреза франского яруса, расположенного в нижнем течении р
For the first time we carried out a detailed comprehensive study of Frasnian stage in the lower сurrent of the Malaya Usa River located within the Eletskaya structural-formational zone of the Polar Urals
The Middle-Late Frasnian age of these rocks was determined by conodonts in the sequence from hassi–jamieae to Late rhenana of standard conodont zones
Summary
Изучением верхнедевонских отложений данного района занимались многочисленные исследователи, такие как Н.Н. Fransnian events in the section of the western slope Polar Urals (D1-2sv) до отложений доломитово-известняковой толщи (D3di) [Шишкин, 2002], что соответствует данным Г.А. Согласно работе [Шишкин и др., 2005], шервожская свита выделена в объеме верхней части эмского яруса (койвенский, бийский горизонты), эйфельского и живетского ярусов, а доломитово-известняковая толща – в объеме всего франского яруса. Проведенный биофациальный анализ по конодонтам и сопутствующей фауне, а также первично-осадочные структуры и характер цемента дают нам основание говорить о том, что формирование этих отложений проходило в пределах узкой внутришельфовой впадины Однако в последние годы появляются новые данные об их широком распространении как на поверхности в обрамлении Енганэпэйской антиклинали [Шишкин, 2002б; Цыганко, 2011], так и (по данным сейсморазведки и бурения) в поднадвиговой зоне С-СВ Большенадотинского рифа [Богданов и др., 2015; Жукова, Петренко, 2015].
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