COMPOSITION AND STRUCTURE OF THE NEPSKY AND TIRSKY FORMATION OF THE PRILENSK-NEPSKY STRUCTURAL-FACIES ZONE OF THE NEPA-BOTUOBA ANTECLISE BY THE RESULTS OF CORE MATERIAL STUDIES

Background. Clarification of the structure of sedimentary formations located on the Siberian Platform by means of lithological and petrographic core studies has a theoretical and practical significance for the creation of geological models for oil and gas prospecting and exploration works.

Aim. To characterize and compare the composition of the upper Vendian of the central and southern parts of the Prilensk-Nepsky structural-facies zone.

Materials and methods. Core samples for analysis were obtained from boreholes in the Preobrazhenskaya, Verkhnechenskaya, Danilovskaya, Yaraktinskaya and other areas. The article is based on the results of a layered lithological description of the cores from 15 wells with a total core output of 560 m in the interval of the Nepsky and Tirsky formations; the petrographic analysis of 540 thin sections; the determination of grain size distribution of rocks by laser light scattering in 220 samples; and X-ray diffraction analysis of 540 samples.

Results. The Nepsky formation is represented by the following lithotypes: conglomerates, gravelites,  sandstones, aleurolites and argillites. The section of the southern part of the structural-facies zone, compared to its central part, is characterised by increased claying. This is associated with the location of this zone in transitional coastal-marine environments, where large river systems were discharged carrying sandy-clay material into the basin. The source of the terrigenous rocks was felsic igneous rocks. The section of the Tirsky formation of the central part is represented (from bottom to top) by a transition from argillites and aleurolites to dolomites. In the section of the southern part, basal sandstones lie at the base, alternating upstream with clay-carbonate rocks.

Conclusion. The material composition of the studied sediments reflects the specifics of sedimentation during the Nepsky and Tirsky time periods. For the first time, the Tirsky formation of the southern part  of the structural-facies zone was divided into four lithological members. The formation of basal sandy rocks at the bottom of the sub-formations is associated with the stages of tectonic activity and positive tectonic movements that initiated relatively rapid weathering and transportation of terrigenous material from adjacent heights to the sedimentation basin.

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