C-isotope chemostratigraphic correlation of the Upper Riphean in the Uralsand Arctic region

Two secular variation curves of C-isotope composition in the Late Riphean c arbonates are discussed. The curve approximating data on the Upper Riphean deposits of Spitsbergen, Greenland, northwestern Canada, and southwestern A frica (Kaufman and Knell, 1995) is now accepted as the model one for the se cond half of this stratigraphic interval add as useful far global correlati ons. The careful analysis of the used chronometric calibration of that curv e shows that it is vulnerable in the lower, relatively older part of the cu rve. Comparison of the model curve with another one recently obtained for t he Karatau Group, the Upper Riphean stratotype in southern Urals (Podkovyro v et al., 1998) demonstrate that they are of the in-phase character. They b oth depict the identical number of large positive and negative peaks despit e the lower amplitudes of delta C-13 variations in the Uralian section that is conditioned, in general. by the lower values of extreme positive points . The in-phase character of two variation curves suggests, along with biost ratigraphic, isotopic-geochronological and regional geological data, that t he model one well characterizes not only the second half, but also the olde r part of the Late Riphean time. The available difference in configuration of uppermost parts of the curves under comparison appears to be a consequen ce of the Upper Riphean stratotype incompleteness, as there is a large pre- Vendian stratigraphic hiatus and another one below the Uk Formation, which was underestimated in previous works.