Стратиграфия

The Precambrian continental block in the northwestern part of the Yenisei Ridge is called the Kiselikha terrane. It is separated from the main (cratonic) part of the ridge by the Isakovka ophiolite belt. The tectonic nature of this block is debatable due to incomplete and contradictory information about rocks that make it up, their age and geodynamic setting. The paper presents new data on the geology of northern segment of the terrane observed on the Yenisei River coast between the Porozhnaya and Osinovka granite massifs. Three stratigraphic units have been identified. (1) The Kiselikha Formation comprises almost the entire studied area. It is represented by foliated retrograde metamorphic rocks, mostly with an unobvious protolith. Zircons from seven samples revealed a similar detrital age distribution with a predominance of Neoproterozoic dates in the range of 1000–800 Ma along with Archean and Paleoproterozoic clusters. The previously assumed products of synchronous acidic volcanism are absent. (2) The Ust-Kutukas Formation, composed of pillow basalts similar to the stratotype, is exposed in the north of the area. (3) The Ust-Porozhnaya unit (identified in this work) was studied in two local outcrops. It is composed of metasedimentary rocks with clastic material eroded from 790–700 Ma granitoids. Taking into account previously published results, we assume that in the first half of Neoproterozoic the Kiselikha terrane was a part of the active margin of the Siberian paleocontinent. The terrane began to separate in the mid-Neoproterozoic due to back-arc continental rifting, which later led to opening of the Isakovka oceanic basin. The synrift sedimentary material of the Kiselikha Formation was washed out from the pre-Neoproterozoic gneisses, intruded by the early Neoproterozoic granites. Rifting was accompanied by intraplate basaltic magmatism, which was replaced by the pillow-basalts of the Ust-Kutukas Formation. Serpentinite bodies, widespread in this part of the terrane, are not associated with crustal oceanic rocks and may have been exhumed during hyperextension of the continental crust. Subduction under the outer edge of the Kiselikha block continued in the second half of the Neoproterozoic (790–620 Ma). At the beginning of the Vendian, the Isakovka oceanic basin was squashed up between the Siberian craton and the Kiselikha terrane, which again became a part of the parent continent. The rocks of the Kiselikha and Ust-Kutukas formations, together with serpentinite bodies, were thrust onto the terrane from the east.

New results of U–Th–Pb SIMS dating of zircons from the Lower–Middle Permian boundary deposits of the Omolon massif and its southeastern framing are given, a U–Pb age (275 ± 2 Ma) of the Lower–Middle Permian boundary of the Regional Stratigraphic Scale of the Kolyma-Omolon region was obtained for the first time. Stratigraphic sequences of the Lower–Middle Permian boundary deposits in the reference sections of the Omolon massif and its southeastern framing are considered. The characteristics of the benthic ecosystems of the Kolyma-Omolon region in the interval of the end of the Early–the beginning of the Middle Permian are given. A large negative excursion of δ¹³С_carb. value was established in the upper part of the Kungurian stage of the Lower Permian, coinciding with the previously identified negative excursion of δ¹³С_org. in the southern part of the Omulevka block and a similar negative excursion in the South China and the continental deposits of Northern Italy. The paleontological characteristics of the corresponding regional units are presented, illustrated by images of the main representatives of the biota of the lower zone of the middle series, including the characteristic Roadian ammonoid Sverdrupites harkeri (Ruzhencev, 1961) described and depicted for the first time. A new species of productid brachiopod Terrakea koragoi, sp. nov. is described.

The bottom sediments of Lake Tere-Khol, located in the southeast of the Sayano-Tuvinian Highlands, contain a detailed archive of landscape and climatic changes throughout the Holocene. Situated at the boundary between South Siberia and Central Asia, this region marks a transitional zone in terms of the factors driving hydroclimatic changes during the Holocene. To the north and west, the western transport of Atlantic air masses exerted a significant influence, while to the south and east, the Asian-Pacific monsoon circulation prevailed. Reconstructing Holocene climate changes in this region is crucial for understanding atmospheric circulation shifts across the Eurasian continent’s interior. Stable isotope analyses in lake carbonates were conducted to evaluate Holocene moisture fluctuations. The water of Lake Tere-Khol is notably enriched in ¹⁸O by 6–8‰ and ²H by 50–60‰ compared to its inflowing streams and rivers, indicating a strongly evaporative water body. This suggests that variations in δ¹⁸O (14.1 to 20.0‰ SMOW) and their positive correlation with δ¹³C variations (–5.8 to 4.2‰ PDB) in the dispersed carbonate material of Holocene lake sediments primarily reflect shifts in the hydrological regime. Positive δ¹⁸O and δ¹³C excursions correspond to periods of aridification, while negative excursions indicate phases of relative humidity. Three primary humidity epochs were identified in the Holocene. These include a relatively dry phase from the onset of the Holocene until 9.8 ka BP and from 4.4 ka BP to the present, with a humid phase in between, spanning from 9.8 to 4.4 ka BP. Superimposed on these major trends were second-order humidity changes, with variability and the amplitude of fluctuations notably intensifying in the latter half of the Holocene, after approximately 6 ka BP. The wettest interval occurred between 5.2 and 4.4 ka BP, while a sharp and substantial shift towards arid conditions around 4.4 ka BP stands out as the most significant hydroclimatic event of the Holocene. The driest periods were observed between 4.2 and 3.1 ka BP and from 1.9 to 0.1 ka BP. At the turn of the eras around 2 ka BP and in the past century, short episodes of relative wetting interrupted these dry conditions. This late Holocene aridification trend points to a weakening of the Pacific monsoon and a reduction in its reach into Eurasia’s interior, which aligns with cooling trends observed in the latter part of the Holocene.