Isotope-geochemical characteristics of Wenlock carbonate and terrigenous-carbonate deposits of the Subpolar Urals and the southern part of the Chernyshev ridge

Research subject. Wenlock deposits in the Subpolar Urals and southern part of the Chernyshev Ridge. Materials and methods. Carbonate and terrigenous-carbonate rock samples from sections of the Subpolar Urals (outcrops 212, 217) and the southern part of the Chernyshev Ridge (outcrop 479) were studied by isotope analysis of carbonate carbon and oxygen.Results. The isotopic composition of the studied sections varies across a wide range of δ13С (–6.4…–0.05‰) and δ18O (20.0–26.9‰). Therefore, three time intervals characterized by distinctive isotopic characteristics can be distinguished, roughly corresponding to early Sheinwoodian (I), late Sheinwoodian (II) and Homerian (III). Interval I is characterized exclusively by the rocks of outcrop 479 with δ13С (–3.6…–3.0‰) and δ18O (22.4–23.6‰). In Interval II, the average values of isotopic characteristics of outcrop 479 indicate a tendency to weighting carbon (–5.5…–3.5‰) and somewhat heavier oxygen (23.2–25.2‰) isotopes. In outcrop 212, the average isotopic values for carbon and oxygen vary from –2.9 to –1.3‰ and from 21.9 to 24.3‰, respectively. In outcrop 217, the average values are δ13C (–1.8…–0.8‰) and δ18O (22.4–25.4‰). In Interval III, the average values of carbon isotopic composition in outcrop 479 are becoming heavier from –2.5 to –0.7‰. In outcrop 212, the isotopic values of oxygen (21.9–23.1‰) and carbon (–4.9…–2.5‰) tend to become weighting; however, in outcrop 217, the average isotopic values of δ13C (–1.9…–0.5‰) and δ18O (22.3–24.5‰) remain constant. The conducted litho-facial studies showed that the weighting carbon isotopic composition ranging from –6.4…to –3.0‰ is associated with an increase in microbial activity in sediments, the manifestation of vadose-phreatic conditions, and the intake of light carbon dioxide with a flow of terrigenous material from the earth. In the latter case, oxygen isotopic values are also the most lightweight (20.0–23.0‰).Conclusions. The obtained isotopic characteristics of the Wenlock rocks under study indicate the expedience of identifying three time intervals and their correlation with paleogeographic reconstructions of Wenlockian sedimentation in the Timan-northern Ural region.

Pseudo-morphogenesis in the shells of the Silurian brachiopods

The analysis of paleontological data, the study of mineral matter transformation involving live organisms in various rocks is important for the understanding of the events of the geological past. The aim of this work is to identify the bio-genic factor in the process of pseudo-morphogenesis in the shell sashes of the Early Silurian brachiopods from the car-bonate rocks of the Chernyshev Ridge. Structural and microscopic methods (Shimadzu XRD 6000; Fourier spectrometer InfraLum FT-02; JSM 6400 JEOL; VEGA3 TESCAN; MIN-8) were used to study quartz pseudomorphoses in the sashes of four shells of the Early Silurian brachiopods Borealis sр. of Pentamerida order from the carbonate strata of the Chernyshev Ridge. The salicification of the brachiopod shell walls resulted in the complete replacement of the original carbonate skeleton with a change in its structural organization. It is established that quartz is the main mineral component of the intra-shell material and secondary dolomite crystals and newly formed calcite aggregates. The paper discusses the participation of Early Paleozoic and modern cyanobacterial symbionts in the replacement of primary calcite with quartz and the formation of quartz crystalline aggregates on the shell walls. It is shown that quartz on the surface of shell sashes and in the intra-shell substance of brachiopods has undergone repeated transformations due to the combination of ancient and modern weathering processes.

Silurian carbonate reservoir rocks of the joint zone of the Khoreyver depression and the Chernyshev ridge

The paper presents a comprehensive study of low-capacity Silurian carbonate reservoir rocks in the northeastern regions of the Timan-Pechora province. Silurian deposits in the sections of the Khosedayu swell, the Talbey block, and the Makarikha-Salyukin anticlinal zone are mainly secondary dolomites and, to a lesser extent, limestones, as well as with low-power interlayers of agrillites, clay dolomites, dolomite-anhydrite rocks. The deposits were formed in coastal-marine and shallow-marine conditions. As a result of the conducted studies, it was noted that dolomites with a shadow biohermal structure have the best filtration properties in the Lower Silurian, and bioclastic limestones – in the Upper Silurian. The reservoir rocks of the Talbey block of the Chernyshev ridge have the best filtration-capacitive properties. The main volume of the void space in the first case is associated with cavities and cracks, in the second one – with matrix porosity. However, the signs of modern karst in the salts of Malo-Tavrotinsky strata observed from the core indicate a possible reformation of the deposits, which requires more careful attention when predicting deposits. The results obtained can contribute to the design of a rational system for the development of hydrocarbon deposits.