CARBONATE CARBON ISOTOPIC COMPOSITION IN THE UPPER RIPHEAN STRATOTYPE, THE KARATAU GROUP, SOUTHERN URALS

Abstract

The Karatau Group of the Southern Urals furnishes an opportunity to reveal the C-isotopic composition for the major part of the Upper Riphean in a single type succession comprising, according to radiometric and paleontological evidence, both the lower and upper horizons of this unit (1000 ± 50-650 ± 20 Ma). In the studied sections, the group is dominated by carbonate deposits, which in terms of cathodoluminescent, geochemical and isotopic-geochemical data can be considered insignificantly altered and retaining the primary C-isotopic signature. Limestones of the lower Katav Subformation, lowermost in the carbonate succession of the group, show the descending trend of δ 13C values from 4.4 down to -0.8‰ PDB, whereas limestones of the upper Katav Subformation and the lower part of the overlying lower Inzer Subformation demonstrate the ascending trend up to 3.9%o with three positive excursions. The upper part of the lower Inzer limestones is characterized by low positive values of δ 13C and the upper Inzer limestones, by the low negative values. Limestones and dolomites of the overlying Min'yar Formation display the largest for the Karatau Group range of δ 13C variations from -2.2 to 5.9‰ with four peaks. The Uk Formation limestones, which terminate the Karatau carbonate succession, show again the low positive values of δ 13C. Variations of δ 13C in the Karatau Group are basically consistent with the general Upper Riphean trend. However, contrary to other Upper Riphean sections, the Karatau Group is devoid of long intervals characterized by high (>6‰) values and demonstrates much smaller δ 13C range. These distinctions may be due to significant stratigraphic hiatuses in the Karatau carbonate succession.