Features of the sedimentation, stratigraphy and carbon isotope record of the upper Tournaisian of the Verkhnyaya Usa depression (Polar Urals)

Abstract

Research subject. Upper Tournaisian clayey-siliceous-carbonate deposits of the Malaya Usa section (Polar Urals).Material and methods. The composition and structure of the deposits (50 samples, 51 thin sections) of the section were considered to determine the conditions of sedimentation; 13 samples were chemically disintegrated in 7–10% solution of acetic acid, and thin sections were studied for biostratigraphic analysis (conodonts, ostracods, and foraminifers); the carbon and oxygen isotopic composition of carbonate deposits (49 samples) were analyzed. A three-stage screening test of samples for isotopic-studies was carried out.Results. The Malaya Usa section comprises mainly micritic limestones with fossils silicified to various degree (wackstone to packstone). The features and composition of the sediments indicate sedimentation in low dynamic conditions below the wave base. Biostratigraphic analysis is based on the identification of foraminifers, as well as a few conodonts and ostracods. Foraminifers characterize the Eotextularia diversa–Dainella chomatica Zone, which is approximately comparable to the Upper typicus–anchoralis–latus conodont zonal interval and Entomoprimitia malinovkaensis‑Marginia tschigovae ostracode zones of the top of the Tournaisian. The isotope-geochemical data obtained from the section demonstrate a general lightening trend in the isotopic composition of δ13Ccarb up the section. This trend, along with the biostratigraphic data, mades it possible to compare the δ13Ccarb sequence of the Malaya Usa section with sequences from a number of geographically separated sections of the world (Belgium, South China, USA), which are considered as the Tournaisian isotope carbon event (TICE).Conclusions. The deposits of the studied section were probably formed under the conditions of a very gentle slope (ramp), where the material accumulated at the foot as a filling sequence in the southern part of the Korotaikha intrashelf paleodepression. Using the Malaya Usa section as an example, it is shown that not only the peak positive deviations of the TICE in the upper part of the isosticha Zone and in the lower part of the typicus Zone, but also the isotopic structure of the upper Tournaisian has a significant correlation potential. The use of the isotope-geochemical method as an auxiliary application to biostratigraphy made it possible to improve the age determination of the section from the interval of the Upper typicus–anchoralis–latus conodont zonal interval to the Upper typicus Zone of the standard conodont zonation.