pdf № 331, July

DOI: 10.19110/geov.2022.7.1

Poorly studied deposits of the Upper Visean section of the south-east of the Orenburg region are one of the promising objects of geological exploration. A powerful stratum of dolomite breccias is described in one of the wells of the Zalesovo area in the lower part of the undifferentiated the Venev, Mikhaylov and Aleksino strata of the Visean section. The section is composed of secondary dolomites with relics of primary textures and structures. Dolomitization complicates interpreting the genesis of deposits and conduct detailed sedimentological studies. Lithotypes of rocks are identified based on the analysis of a set of data from lithological and mineralogical studies. Lithotypes: 1) secondary dolomites with relics of primary textures, 2) dolomite breccia with clay-carbonate cement, 3) secondary breccia dolomites with relics of primary textures, 4) dolomite breccia. A detailed lithological description of the lithotypes is given. The features of their formation are determined. Reservoir properties of rocks are analyzed. It has been established that the sediment complex encountered in the section of the Uppen Visean section of the Zalesovo area are the result of different stages of karst formation and following transformation of rocks. Reservoir properties of rocks depend on the development of post-sedimentation transformations. Karst processes and their products require further study at the regional level and consideration when conducting geological exploration on productive reservoirs of the Uppen Visean section.

Keywords: Zalesovo area, Upper Visean section, dolomite breccia, karst process.

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DOI: 10.19110/geov.2022.7.2

The paper presents the results of the study of carbonate psephitolites developed in the Lower Serpukhovian deposits in the Central part of the Chernyshev Ridge. They have different morphology and genesis. There include sedimentation breccia and post-sedimentation intra-formation units, however thick strata of brecciated rocks have the most problematic genesis indicating the formation of these clastic carbonate rocks during sedimentation and post-sedimentation processes.

As a result we determined that a shallow sea basin existed on the territory of the central part of the Chernyshev Ridge in the Early Serpukhovian period. In the western part, shallow sediments, including intraplate karst breccias and coastal-marine detrital limestones, were deposited in shoals with island uplifts and lagoons. At the same time, dark-colored limestone silts were accumulated in the deeper sea in the east, and intercalated with removed debritic flows. During the Ural orogeny, due to the layer-by-layer breakdown at the base of the Serpukhovian, thick strata of intensively breccated carbonate deposits were formed. Post-sedimentation processes of changes in the Lower Serpukhovian sediments (recrystallization, dolomitization, partial dissolution) could occur during major Middle and Upper Carboniferous regressions, as well as after the change of the elysial hydrogeological regime to infiltration at the Triassic-Jurassic boundary. They have continued up to the present time. This is especially evident in the eastern part of this territory in areas of increased permeability of rocks. There are intensive processes of leaching, dolomitization, calcitization, and karst developing.

Keywords: carbonate psephitolites, Serpukhovian, Lower Carboniferous, Chernyshev Ridge.

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DOI: 10.19110/geov.2022.7.3

The Upper Silurian proximal lagoon deposits, widely developed in the Ilych River basin, were previously studied in detail. One of the surprising fabrics in them is large (up to 1.0 m in diameter) areas with zonal cements (cementstones), there are different points of view on their origin. The samples of these cementstones investigated by microscopic and isotopic methods, have a complex morphology characterized by horizontal and vertical element alterations. This internal texture is conditioned by centripetal zonal radial-fibrous calcite growth with thin layers of fine-grained dolomites. Most of the proposed interpretations (recrystallized stromatoporoids, stromatactis fabric, or methane spring cannels) were rejected because one or more cementstone features contradicted them. The paleokarst interpretation at least has no contradictions in all three characteristics considered: morphological, lithological, and geochemical. The d¹³C values of cementstones (and the host rocks at the boundary with them) vary within 0.9–1.95 ‰ (PDB), while the d¹⁸O values have narrower variations (25.51–25.88 ‰ SMOW in one case and 25.11–25.48 ‰ SMOW in the other case). In addition, these d¹⁸O values are higher than those in the host rocks.

Keywords: Upper Silurian, cementstones, paleokarst, C and O isotopes.

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DOI: 10.19110/geov.2022.7.4

Lithology and geochemistry of carbonate filling in ancient karst cavities in the Famennian-Tournaisian limestone sequence of the Rezh subzone at the eastern slope of the Middle Urals are considered. Mineral filling of paleocavities differs from typical modern subaerial speleothems. Caves are mainly filled with white massive coarse-grained calcite (cave spar), red-colored argillaceous-carbonate microgranular deposits (carbonates with an admixture of terra rossa, cave laminites) and collapse breccias, composed of host limestones fragments. The rare earth elements (REE) patterns and the isotopic composition of carbon indicate a very weak effect (or its absence) of soil or meteoric waters on the carbonate system: the speleothem calcite inherited these characteristics from the host limestones. At the same time, the isotopic composition of oxygen indicates the primary meteoric genesis of solutions and/or relatively high temperatures of calcite crystallization. Most likely, the studied speleothems were formed from solutions of surface origin circulating in the sedimentary strata for a long time (hypogene sediments).

Keywords: paleokarst, phreatic speleothems, terra rossa, collapse breccias, groundwater, REE, carbon and oxygen isotopes.

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