Variations in carbon isotope composition in a reference section of the Upper Viséan-Lower Serpukhovian interval (Lower Carboniferous) of the Moscow Syneclise (Polotnyany Zavod quarry)

Research subject. Carbon isotope composition of carbonates and conodont elements in the Upper Visèan–Serpukhovian interval (Lower Carboniferous) in the Polotnyany Zavod and Luzha reference sections in the southwest of the Moscow Syneclise. Aim. To evaluate the possibility of tracing the horizons identified in the basin of the Moscow Syneclise on the basis of data on the carbon isotope composition of carbonates and conodont elements. Materials and methods. Carbon isotope composition was studied in the Upper Aleksin–Protva stratigraphic interval. Carbon isotopic composition of carbonates (δ¹³C_carb) was determined in 14 samples, and that of conodonts (δ¹³C_con) – in 20 samples. Comparative material was obtained from the reference section of the Serpukhovian Stage on the Izyayu River (Tchernyshev Ridge, east of the Pechora Plate). Results. The Upper Aleksin–Tarusa interval is characterised by δ¹³C_carb values of about 1.5‰ VPDB with decreases to –2.6 and –0.2‰ VPDB in the Upper Aleksin and Upper Mikhaylov intervals respectively. A broad “plateau” (values of 1.2–1.6‰ VPDB) covers the upper part of the Venyev and Tarusa horizons. The carbon isotope composition of conodont elements shows a decrease to –30‰ VPDB in the Venev Horizon and an increase to –24‰ VPDB in the middle part of the Tarusa Horizon. In the Steshev Horizon, the δ¹³C_con values decrease to the average values in the section (about –27‰ VPDB). Conclusion. The study of variations in the carbon isotopic composition of conodonts and carbonates in the Upper Viséan–Lower Serpukhovian interval at the Polotnyany Zavod quarry enabled the identification of two correlation intervals characterized by a negative shift in δ¹³C_con in the Venyev Horizon and a positive shift in δ¹³C_con in the Tarusa Horizon. These intervals are also traceable in sections of the Pechora Plate and may be used for interregional correlation.

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