pdf № 380, August

DOI: 10.19110/geov.2026.8.1

The Rechitskian (Mendymian) deposits of the Frasnian Stage, Upper Devonian, in eastern Tatarstan forms part of the Upper Devonian petroleum source-rock complex and are of interest as an interval combining features of unconventional rocks and locally developed conventional reservoirs. This study aims to reconstruct the depositional setting of the Rechitskian deposits and to assess the role of diagenetic transformations in the development of their pore space. The study is based on core material from wells in the eastern Tatarstan and integrates core description, petrographic thin-section analysis, X-ray diffraction data, organic-matter geochemical studies, and porosity-permeability measurements. The Rechitskian horizon is shown to consist of variable-scale alternation of carbonate and carbonate-siliceous beds and packages of different thickness, including limestones, organic-rich argillaceous carbonate-siliceous and siliceous rocks, and thin intervals of secondary dolomite. Deposition took place in a facies-differentiated shelf basin that included intrashelf depressions, slope zones, and elevated shallow-water areas. Improved reservoir properties are mainly associated with leached wackestones and packstones, as well as with secondary dolomites.

Keywords: Rechitskian (Mendymian) horizon, Upper Devonian, Volga-Ural petroleum province, depositional settings and diagenetic history, dolomitization, reservoir properties

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DOI: 10.19110/geov.2026.8.2

Miocene sections of Severoshirvanskaya and Tsurevsky formations in the Pshekha river valley, North-Western Ciscaucasia is the object of study. The subject of the research is foraminifera, ostracods, bivalves, gastropods, algae, angiosperms, gymnosperms, insects, and the sedimentary rocks containing them. The collected samples have been processed in complex by lithological, palaeontological, facies, and geochemical methods. The first data on the isotopic composition of carbon and oxygen show a negative excursions d¹³С (–7.71 ‰) for the lower Chokrakian and Konkian (–0.42 ‰), and an increase in the isotopic composition of clay rocks d¹⁸О from 21.1 to 31.1 ‰ from Tarkhanian to middle Sarmatian. Several criteria have been identified for recognizing Pshekha section as unique. There are a) distribution of more than 15 groups of aquatic and terrestrial animals and plants; b) abundant eufossils with preserved microstructures; c) coastal marine, relatively deep-water and lagoon facies; d) replacement of organic matter by pyrite and siderite. Sections of the Pshekha river are compared with Miocene lagerstättes of the world to prove their value.

Keywords: Late Cenozoic, West-Kuban trough, Tarkhanian, Chokrakian, Konkian, Karaganian, Sarmatian, flora, fauna

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DOI: 10.19110/geov.2026.8.3

Comprehensive research of Quaternary deposits heve been conducted using lithological, microteriological, microfaunistic, and geochronological methods, verified by the interpretation of aerial photographs and aerial visual observations. The aim of the investigations is to identify lithostratigraphic features of the upper moraine for substantiated subdivision and correlation of sections, as well as palynological and microtheriological criteria for determining the age of interglacial horizons. The Upper Neopleistocene sequence, which lasted approximately  from 130,000 to 11,700 years ago, is divided into four horizons. The base is composed of Sula (Mikulino) horizon interglacial sediments, overlain by the Laya (Podporozhye) and the Polar (Ostashkovo) glacial horizons and the Byzovaya (Leningrad) interstadial separating them. Paleogeographic features of interglacial horizons have been identified, indicating a relatively warm Sula interglacial, during which spruce-birch forests with fir and a large admixture of dwarf birch grew, and Byzovaya interstadial with colder climatic conditions and sparse spruce-birch forest-tundra communities. Polar moraine is occupying only the far north of the Pechora Lowland, where it shapes the surface topography.

A palynozonal correlation of the Late Glacial-Holocene sections in the Kuya and Chernaya rivers valleys has been carried out, and the sedimentation environments during the transition from the Last Glaciation to the Holocene and during the Holocene have been reconstructed. Based on the results of spore-pollen and diatom analyses, accompanied by radiocarbon dating of the sediments, the landscape and climatic conditions of sedimentation over the past 13.7 thousand years have been described.

Keywords: Polar moraine, lithological analysis, palynological composition, heavy mineral assemblage, glacial feeding province

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DOI: 10.19110/geov.2026.8.4

Conodonts have been discovered for the first time in limestones of the upper subformation of the Kurtash Formation of the Suvanyak complex of the Uraltau region. The conodonts are represented by the species Panderodus cf. unicostatus (Branson et Mehl), Dapsilodus sp., ?Distomodus sp. and Walliserodus sp. and determine the Silurian-Lower Devonian age interval of the host strata. The terrigenous deposits of the Uraltau region are compared with the Paleozoic terrigenous complexes of the eastern part of the Zilair synclinorium and the Paleozoic framing of the Kraka ultramafic massifs, where limestone bodies in the sections of the Ferma Roshcha and Kzyl-Balyk terrigenous deposits (East Zilair zone) were formed in conditions of dissected underwater relief. The discovery of conodonts in limestones allows us to refine the age of the Kurtash Formation and understand the geological formation conditions of the Suvanyak and Maksyutovo complexes of the Uraltau.

Keywords: conodonts, limestone, Uraltau, Suvanyak complex, Maksyutovo complex, Silurian, Devonian

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