SEA LEVEL AND δ13C TRENDS IN RIPHEAN PETROLIFEROUS DEPOSITS ON THE WESTERN MARGIN OF THE SIBERIAN CRATON (BAIKIT UPLIFT)

Abstract

The Riphean (1500-1450 to 1000-950 Ma, Ar-40/Ar-39, K/Ar) section of petroliferous peritidal carbonates in the Baikit uplift (western Siberian Platform) contains five sequences, each divided into three to five subsequences, distinguished on the basis of sedimentology and isotope geochemistry. The five sequences deposited in the period from 80 to 100 Ma arid reflect major sea level changes. The stability of delta(13)C estimates is confirmed by petrographical, geochemical, and isotopic data (over 350 analyses) showing that the primary delta(13)C signal in early diagenetic dolomites may remain almost invariable. The delta(13)C values in the lowermost carbonates (1450-1350 Ma, isotopic data) are about zero (-0.2 to 0.5parts per thousand). The overlying section contains long intervals with low positive delta(13)C of 0.5 to 2.0, occasionally to 3.0parts per thousand, alternating with shorter intervals of mostly negative delta(13)C shifts (to -2.2parts per thousand). The negative shifts correspond to the deposition of the lower Yurubchen (similar to1350 Ma), upper Yurubchen-Dolgokta (1270-1250 Ma), Kopcher (1100-1080 Ma), and upper Yukta-lower Tokur (about 1000 Ma) subsequences. The main trends in the sea level fluctuations correlate well with the delta(13)C record: The negative isotopic shifts coincide with the boundaries of the five major sequences; the intervals with medium positive delta(13)C record high sea stand and correspond to the periods of carbonate shelf aggradation; most of low delta(13)C values correlate with the boundaries and lower parts of the subsequences. Isotopic and geological data from other regions indicate that the low sea stand periods marked by the subsequence boundaries in the Baikit uplift are most likely associated with global geological events and reflect eustatic changes.