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Том 25, № 2 (2017)
- Год: 2017
- Статей: 5
- URL: https://ogarev-online.ru/0869-5938/issue/view/11002
Article
The Sarmatia megablock as a fragment of the Vaalbara supercontinent: Correlation of geological events at the Archean‒Paleoproterozoic transition
Аннотация
The results of correlation between geological events in the period of 2.8‒2.0 Ga provide grounds to assume that the Sarmatia lithospheric megablock definable in the southern part of the East European Craton belonged to the ancient Vaalbara supercontinent consisting of the Pilbara and Kaapvaal cratons. In the period of 2.8‒2.6 Ga, all of them represented fragments of the continental crust consolidated at approximately 2.8 Ga and subjected to continental rifting, which was accompanied by intense basite volcanism. In the period of 2.50‒2.45 Ga, these three cratons were characterized by similar tectonic settings and accumulation of banded iron formations. Precisely these banded iron formations of the largest Transvaal, Hamersley, Kursk, and Kremenchug‒Krivoi Rog iron ore basins accumulated in the period of 2.50‒2.45 Ga in a single oceanic basin serve as a basis for adequate paleotectonic reconstructions of the Vaalbara supercontinent. In the period of 2.45‒2.20 Ga, all three cratons were subjected to a long-lasting break in sedimentation followed by activation of continental rifting with terrigenous sediment deposition, which terminated with basite volcanism ca. 2.2 Ga. These events gave start to the Vaalbara breakup, which represented a multistage process with alternating divergence and convergence phases of supercontinent fragments until the Kaapvaal and Zimbabwe, Pilbara and Yilgarn, and Sarmatia and Volgo-Uralia cratons, respectively, became eventually united.
123-145
Geochemical features and sources of clastic material in Paleozoic terrigenous deposits of the northeastern flank of the South Mongolia–Khingan orogenic belt
Аннотация
The results of geological and geochemical studies of terrigenous rocks of the main stratigraphic subdivisions in the northeastern flank of the South Mongolia–Khingan orogenic belt and also the results of U-Pb (LА-ICP-MS) geochronological studies of detrital zircons from these deposits are presented. It is demonstrated that the studied rocks differ significantly in the nature of distribution of detrital zircon ages and, consequently, they cannot be members of a single sedimentary sequence. The data obtained confirm the standpoint according to which the northeastern flank of the South Mongolia–Khingan orogenic belt represents a “joint” zone separating the Argun and the Bureya-Jiamusi Superterranes. This joint zone was formed as the result of closure of the oceanic basin separating the specified continental massifs in the Paleozoic era. The geochemical features of the studied rocks indicate their formation in the conditions of the island arc or the active continental margin. Lack of zircon generations younger than Ordovician age in the studied samples allows assuming that the sedimentary sequences identified within the northeastern flank of the studied belt as the Necla, Dagmara, siltstone-sandstone, and Gramatukha sequences that formed from the end of the Vendian (?) to the Devonian correspond to the youngest stages of belt formation. These sediments in the current structural plan evidently represent fragments of accretion complexes cropping out in fragments among the Cenozoic sequences of the Amur-Zeya Depression.
146-166
The upper Devonian Sargaevo regional stage in the Tsil’ma River basin, Middle Timan
Аннотация
The results of bio- and lithostratigraphic study of Sargaevo deposits in the Tsil’ma River basin are discussed. The palynological data used for biostratigraphic subdivision of the section provide grounds for defining the miospore assemblage corresponding to the regional miospore Cristatisporites pseudodeliquescens Zone. It is established that deposits of the Sargaevo regional stage are distributed almost through the entire Tsil’ma River basin. They are characterized by distinct lithological features and may serve as a stratigraphic reference unit in field investigations.
167-187
Ammonites and stratigraphy of the terminal part of the Middle Volgian Substage (Upper Jurassic; Epivirgatites nikitini zone and its equivalents) of the Panboreal Realm: 1. Late morphogenesis and systematics of Dorsoplanitinae
Аннотация
Problems of correlation of the upper part of the middle Volgian beds of European Russia and Arctic regions with the Portlandian of England and northern France largely result from existing uncertainties in the taxonomy of the ammonite subfamily Dorsoplanitinae. The poor knowledge of the morphogenesis of this group precludes effective and stable taxonomy and recognition of parallelisms, thereby decreasing the group’s correlative-biostratigraphic potential. In Dorsoplanitinae, dimorphism is revealed in the differences in the terminal shell diameter and, to a lesser extent, in the sculpture, whereas apertural modifications characteristic of many Perisphinctoidea are not observed in members of this group. The middle Volgian Dorsoplanitinae are represented by minimorphs and macromorphs, taxonomically different at the species or genus level. The Volgian minimorphs are mainly represented by the genera Epivirgatites and Lomonossovella, whereas macromorphs are represented by Titanites s.l.; in the genus Taimyrosphinctes, micro- and macromorphs belong to different subgenera. All morphological types of Volgian Dorsoplanitinae have vicariate equivalents in the Portlandian, which can be used for the interregional correlations.
188-228
The climate-forming role of Early Paleogene marine currents in high latitudes of Eurasia
Аннотация
Paleobotanical evidence for the climate-forming significance of marine currents in the Early Paleogene of the Arctic is reported. It is based on the analysis of mega- and microfossil plant assemblages derived from the outcrops located along the Arctic coastline of Eurasia and on the islands within the Arctic shelf. The occurrence of warm marine water masses is inferred from findings of Lower Paleogene microplankton and megafossils and palynomorphs of higher plants including Lauraceae, Magnoliaceae, Araliaceae, Loranthaceae, Palmaceae, and members of other thermophilic groups along the Arctic coast. The Early Paleogene climate in this realm was milder and was influenced by currents washing the continent and by marine heat transfer via the Trans-Siberian strait together with the atmospheric heat transfer.
229-239