Home – Home – Jornals – Russian Geology and Geophysics 2022 number 5

We have studied an Upper Devonian volcanoterrigenous carbonate complex in the north of the Kuznetsk trough. The northern part of the complex is subducted beneath the Meso-Cenozoic sedimentary cover of the West Siberian geosyneclise. Analysis of sedimentologic processes, the nature and specific composition of pyroclastic impurity, and the petrographic and chemical compositions of sedimentary rocks makes it possible to reconstruct the facies and geodynamic settings. It is shown that the fine-grained pyroclastic material is predominantly of andesite composition, whereas felsic and mafic volcanic components are subordinate. In the transition from the lower Frasnian to the Famennian, the content of felsic pyroclastic component increases. The geodynamic settings of the shelf and continental slope of the Siberian continent up to its base have been established. Facies analysis shows that the slope is west-facing (in the present-day coordinates). The deposits accumulated mainly in the basin; these are fine-grained terrigenous rocks, fine-grained limestones, slope landslide breccias and olistostromes, turbidites, and “conglomerate-like limestones” (paleoseismites). According to the geochemical data, the Late Devonian paleogeodynamic setting was stable and corresponded to the setting of an active continental margin. Arid to semihumid lithogenesis has been established. Marine sediments formed in an oxidizing environment with good water aeration.

Concepts of the stratigraphy of the coastal and shallow-sea sections of the uppermost Middle Jurassic and Lower Cretaceous, exposed on the right bank of the Anabar River, have radically changed many times. The performed study and analysis of the published data are aimed at refining the bio- and lithostratigraphic subdivision of the section, substantiating its stratigraphic completeness, and describing the identified stratigraphic units in detail. Comprehensive biostratigraphic studies of the uppermost Bathonian-lowermost Boreal Berriasian reveal a sequence of nine biostratigraphic units with dinocysts and palynoflora in the ranks of zones and local zones. Some of the palynological biostratigraphic units have been identified for the first time. Foraminifer assemblages of the boreal standard zones are recorded in the sections under study. The obtained biostratigraphic data and analysis of all known ammonite occurrences make it possible to demonstrate the stratigraphic continuity of the section in the Anabar River region, despite the reduced thickness of the stratigraphic units. The stratigraphic position of the boundary between the Sodiemykha and Buolkalakh formations is accurately defined. According to the interpretation proposed, the basal horizon of the Buolkalakh Formation is associated with a beginning of a new major sedimentation stage in the late Oxfordian-early Kimmeridgian, identified as the lower boundary of lithostratigraphic units of different ranks throughout the entire Arctic Region and partially in the Boreal Region. The lithogeochemical parameters of the studied formations are obtained for the first time. The content of C_org in the studied samples does not exceed its Clarke values, and the pyrolytic parameter T _max indicates that the organic matter is immature and the petroleum potential is low. The considered organic matter of the rocks is characterized by a heavy isotope composition of carbon, suggesting its mostly terrigenous genesis.

Structural and lithological comparison of intraformational plicative liquefaction deformations (convolutions) in Late Quaternary lacustrine complexes of the Baltic Shield (sediments of small glacial lakes) and the Tien Shan (sediments of a large stationary basin in the Issyk-Kul depression) is carried out in order to clarify their genesis. These regions have sharply different levels of seismotectonic activity, which makes it possible to most fully determine the role of a seismic factor in the formation of bedding disturbances. Convolutions in the Baltic Shield are represented by load casts, flame structures, pseudonodules, and regular folds with more pronounced anticlinal bends. It is shown that convolutions occur here only in the most liquefied soils (siltstones and fine-grained sands) with low cohesion and fluid permeability in the case of unstable (increased density and coarsening of the sediment from bottom to top) or neutral (homogeneous composition) stratification of weakly consolidated sediments, and the ratio of the widths of the synclinal and anticlinal parts of the folds ( K _syn) for these structures is 1.0-7.5. These signs point to the formation of disturbances because of a spontaneous instability or an instability initiated by weak mechanical influences in the sedimentary stratum under conditions of a normal vertical gradient of the sediment viscosity. Deformations in the second region are structures typical of the Baltic Shield, columnar structures of deep interpenetration of layers, diapirs, and clastic dikes. There are also flexural folds with K _syn ≈ 1, which formed at a higher degree of sediment consolidation than the actual liquefaction structures. The convolutions of the Issyk-Kul depression are developed not only in sand-silty sediments but also in relatively lowly liquefied soils (clay, gravel, and pebbles). These structures formed under both unstable and stable density stratification of the sediment (for example, clay-on-sand), and K _syn = 0.3-2.5. Thus, the formation of convolute structures in the Issyk-Kul depression is impossible without strong dynamic impacts on stratified sediments during their accumulation. The results obtained make it possible to substantiate the predominantly diagenetic (convective and landslide) genesis of convolutions in the lacustrine complexes of the Baltic Shield and the seismogenic genesis in the limnogenic deposits of the Tien Shan. A number of new criteria for identifying seismites are proposed, which determines the practical significance of the study.

Bottom sediments of lakes are regarded as one of the most informative continental facies used to reconstruct environmental changes in lake catchments. They usually contain a high-resolution and continuous record of geologic events, the knowledge of which is important for understanding the evolution of sedimentary processes and landscapes under the influence of climatic changes and the impact of short-lived catastrophic processes. The southwestern Baikal region (the Tunka rift and its East Sayan mountain framing) is replete with lakes of different geneses and morphologies. Recent fieldworks have allowed a multiproxy study of the lacustrine sediments of several lakes of the Oka and Tunka regions. The results demonstrate that there were no glaciers in the foothill plain of the Tunka rift valley earlier than 14,800 cal yr BP. On the Oka plateau, within the East Sayan high-mountain zone, deglaciation started somewhat earlier than 13,800 cal yr BP. The grain size, petromagnetic, and physical properties of the lacustrine sediments point to a general weak hydrodynamics throughout the 13,800 yr sedimentation history in Lake Khikushka. However, a gradual reduction in fine fractions from base to top of the lake section suggests a gradual drawdown of the water level following the change of the water source from glacial melt waters to atmospheric precipitation. Biogenic silt accumulation in Lake Khikushka started parallel with the onset of the Holocene ~11,700 yr BP, suggesting a quick response of the natural environment of the lake basin to major global climatic changes. The short-term episodes of enhanced hydrodynamics in Lake Khikushka ~9000, 8200, 6500, and 2000 yr BP confirm the high sensitivity of the lake geosystem to climatic changes, not only on a millennial but also on a shorter-time scale. The characteristics of the sediments of Lake Engarginskoe are evident of lacustrine-alluvial sedimentation in the reservoir in the Early and Middle Holocene. New data on the grain size and petromagnetic parameters of the lacustrine sediments from two lakes located within different climatic belts of East Sayan show their high potential for a wide range of paleoecological reconstructions.

The paper presents the U-Pb age of zircons and the geochemical and isotope characteristics of granitoids from the Kitoi block (Sharyzhalgai uplift, southwestern Siberian craton). The studied granitoids compose different-scale vein bodies. According to the structural relations among the rocks and their U-Pb zircon ages, there were three stages of early Precambrian granite formation: ~2.99, 2.54, and 1.88 Ga. The Neoarchean and Paleoproterozoic granites contain inherited zircons dated at ~2.97-3.00 Ga, which are close in age to magmatic zircons from the Mezoarchean granite-gneiss. The Kitoi granitoids of different ages show similar enrichment in SiO₂, K₂O, and incompatible trace elements. In trace element composition and crystallization temperatures the Mesoarchean granite-gneisses are similar to A-type granites, while the Neoarchean and Paleoproterozoic granitoids, to I-type ones. The inherited ~2.97-3.00 Ga zircons and narrow range of model Nd ages ( T _Nd(DM) = 3.0-3.3 Ga) indicate that the Neoarchean and Paleoproterozoic granitoids formed through the recycling of the Mesoarchean crust. The Neoarchean (~2.54 Ga) and Paleoproterozoic (~1.88 Ga) stages of granite emplacement in the northwest of the Kitoi block correlate with the formation of granitoids and high-temperature metamorphism in the Irkut block and the border zone between the two blocks. The subsynchronous granite formation and metamorphism (~2.54 Ga) give strong evidence that the amalgamation of the Kitoi and Irkut blocks resulted from collisional orogeny in the terminal Neoarchean.

Coal fires are a phenomenon that can be observed worldwide in areas where rocks containing coal seams are exposed and can pose major environmental threats. A coal fire can begin through spontaneous combustion when coals are exposed to dry and oxygen-rich near-surface conditions. Burning, depending on the temperature of heating, causes baking or even melting of the surrounding rocks and the formation of different types of combustion metamorphic rocks. In Northwestern China, coal fire occurrences are concentrated at the edges of the sedimentary basins or at the margins of orogenic belts, where coalrich units were exposed owing to the Indo-Eurasian collision. On the northern margin of the Tianshan range, evidence of coal fires is widespread in the Jurassic sedimentary units containing coal seams which outcrop along the Central Asian Orogenic Belt. In some cases, coal fires are active and can be linked to ongoing mining activity, but outcrops of combustion metamorphic rocks not associated with fires are also found and are indicative of past burning events. We examine combustion metamorphic rocks outcropping in the Toutunhe River valley (Liuhuangou area, Xinjiang, Northwestern China). Combustion metamorphic rocks in the study area were mapped and classified according to their morphological and mineralogical characteristics. Outcrops are exposed at various heights on the valley flanks, which are characterized by the presence of multiple levels of fluvial terraces. These terraces are indicative of the phases of erosion and deposition of the Toutunhe River and testify to tectonic uplift. The investigation of the stratigraphic and crosscutting relationship of combustion metamorphic rocks with terrace deposits and apatite fissiontrack dating made it possible to determine that at least four phases of coal fire activity occurred from late Miocene to Quaternary. The first and oldest burning phase dates back to 10 ± 1.3 Ma and terminated prior to 2-3 Ma; the second was active before ~550 ka; the third had terminated by ~140 ka; the fourth began later than ~5.7 ka. The relationships between combustion metamorphic rocks and fluvial terraces further suggest that coal fire ignition/extinction in the area since the Miocene have been linked to the interplay between the uplift of the Central Asian Orogenic Belt and the phases of fluvial erosion and deposition in interglacial periods.