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

The Karatau Group of the Southern Urals furnishes an opportunity to re veal the C-isotopic composition for the major part of the Upper Riphea n in a single type succession comprising, according to radiometric and paleontological evidence, both the lower and upper horizons of this u nit (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 insignif icantly altered and retaining the primary C-isotopic signature. Limest ones of the lower Katav Subformation, lowermost in the carbonate succe ssion of the group, show the descending trend of delta(13)C values fro m 4.4 down to -0.8 parts per thousand PDB, whereas limestones of the u pper Katav Subformation and the lower part of the overlying lower Inte r Subformation demonstrate the ascending trend up to 3.9 parts per tho usand with three positive excursions. The upper part of the lower Inte r limestones is characterized by low positive values of delta(13)C and the upper Inter limestones, by the low negative values. Limestones an d dolomites of the overlying Min'yar Formation display the largest for the Karatau Group range of delta(13)C variations from -2.2 to 5.9 par ts per thousand with four peaks. The Uk Formation limestones, which te rminate the Karatau carbonate succession, show again the low positive values of delta(13)C. Variations of delta(13)C in the Karatau Group ar e 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 parts per thousand) values and demonstrates much smaller delta(13)C range. These distinctions ma y be due to significant stratigraphic hiatuses in the Karatau carbonat e succession.