Stratigrafiâ. Geologičeskaâ korrelâciâ

Northwestern segment of the Precambrian Yenisei Ridge contains ophiolite and is known in the literature as the Isakovka terrane or Isakovka domain. We suggest dividing it into two belts: the Kiselikha one (western) and Torzhikha (eastern), which differed in geodynamic regime during the late Neoproterozoic (~750–600 Ma). It is believed that the Kiselikha belt is composed mostly of volcanic rocks erupted at island arc setting in the second half of the Neoproterozoic, and that collision of this arc with Siberian continent formed the Yenisei Ridge orogen. This idea has not been sufficiently confirmed by geological and geochronological data. Dating of four detrital zircon samples extracted from sedimentary and volcanic-sedimentary rocks in the southern part of the belt revealed that the sampled strata belong to three different Precambrian levels: the Mesoproterozoic, the mid Neoproterozoic (800–750 Ma), and the end of the Neoproterozoic (620–600 Ma). Thus the authorized stratigraphic layout of the belt as well as its proposed island-arc origin require revision. By this paper we announce the identification of the Kiselikha terrane, which was a part of active margin of the Siberia paleocontinent at the beginning of the Neoproterozoic. Approximately in the middle of the Neoproterozoic, this block was rifted off Siberia and further evolved as a microcontinent bounded by an active margin from the outer side.

On the basis of the mineralogical-petrographic and geochemical study of igneous rocks of intermediate composition of the Pripyat complex of the Late Devonian Pripyat-Dnieper magmatism area, two subphases of the Pripyat magmatism phase have been definished: late Fransian (Chernin or Skolodino-Chernin) with potassium specification and early Famennian (Turov-Drozdov (Yelets)) with potassium-sodium specification. For the first time, the U–Pb age of titanite from trachyandesites of the parametric borehole Pribor was obtained for rocks of Pripyat-Dnieper magmatism area at 364 ± 1 Ma. These data confirm the Famennian age of the Yelets subphase of the Pripyat phase of magmatism and make it possible to determine the duration of the magmatic activity of this structure at least 14 Ma taking into account the isotope dating of the earliest phases of magmatism in the Pripyat-Dnieper-Donetsk Paleorift Structure as 388 ± 12 Ma, 383.3 ± 3.8, 384.7 ± 3.9 and 383.6 ± 4.4 Ma in the Azov Region and 381 ± 2 Ma in the Pripyat-Dnieper magmatism area.

The Zaisan Basin is a unique geological structure that preserves a most complete sedimentary archive of late Mesozoic through most of the Cenozoic era. A century of studies brought very rich data on the stratigraphy and geological history of the Zaisan sedimentary sequence. The recent scientific progress made it possible to refine the Cenozoic stratigraphic scale of the region. Here we present the results of a detailed morphostructural analysis and propose morphotectonic zoning of the Zaisan Basin based on the modern tectonic and geodynamic concepts. New results of magnetostratigraphic dissection of the Kalmakpai stratotypic section, the most complete section of the sedimentary strata of the depression, and its comparison with the drilling data of its axial part are presented. By the synthesis of magnetostratigraphy with the rich regional biostratigraphic record we refine the stratigraphy of the region and clarify dynamics of tectonic movements and the development of relief and landscapes of the Zaisan Depression and the bounding alpine structures of Saur-Tarbagatai during the Cenozoic.

The results of the palinologycal study of the Kurshskaya Formation stratotype (Primorsky quarry, Kaliningrad Oblast) are presented. In the lower part of the Kurshskaya Formation (Member of “chocolate” clays), a complex of dinoflagellate cysts with Areosphaeridium diktyoplokum, Glaphyrocysta semitecta, and Cordosphaeridium funiculatum of latest Eocene age was found. The formation is characterized by four spore-pollen assemblages: (1) Pinuspollenites–Inaperturopollenites–Sciadopityspollenites assemblage of latest Eocene age in the “chocolate” clays and in the lower part of brown sands; (2) Sequoiapollenites–Betulaepollenites betuloides assemblage of early Oligocene age in the lower part of the brown sands member; (3) Boehlensipollis hohli–Carpinipites carpinoides assemblage of early Oligocene age in the middle part of brown sands member; (4) Alnipollenites–Corylopollis assemblage of late Oligocene–early Miocene age in the upper part of the brown sands of the Kurshskaya Formation. The general age of the Kurshskaya Formation is terminal Eocene–early Miocene. In the lower parts of the Zamland Formation, which overlies the Kurshskaya Formation, the Pinuspollenites–Tricolporopollenites pseudocingulum–T. euphorii assemblage presumably of middle Miocene age was found. Based on the obtained palynological data, the conditions of sedimentation at the end of the Eocene–Oligocene–early Miocene in the South Baltic region are reconstructed. The regression of the sea basin began as early as the end of the Priabonian, the climate was still quite warm and humid, close to subtropical. Mesophytic mixed coniferous-broad-leaved forests grew along the shores of the strait, low-lying areas of land were occupied by marsh vegetation. Cooling at the Eocene/Oligocene boundary resulted in the appearance of hemlock in plant communities and an increase in the proportion of catkins (alder, birch, hornbeam). Presumably, in the late Oligocene–early Miocene, the proportion of small-leaved trees, especially alder and hazel, sharply increased in mesophytic forests, while the number of pine trees decreased. Wetter and warmer climatic conditions are assumed for the early Miocene: this time is characterized by an increase in the number of walnut, cypress, and Cyrillaceae. In the middle Miocene, the climate was still quite warm, but drier, and such moisture-loving species as podocarpus, spruce, glyptostrobus, and swamp cypress disappeared from plant communities.