Upper Bashkirian deposits in the Sharyu river basin (the Chernyshev ridge): lithology, carbon and oxygen isotopes, conditions of sedimentation

The Upper Bashkirian deposits in the Srednie vorota section (the Sharyu River, the Chernyshev Ridge) are composed of unchanged limestones. In their composition δ13CPDB = 0.3...3.9‰, δ18ОSMOW = 19.0...24.3‰. Isotopic ratios depend on changes of sedimentation conditions during Late Bashkirian. Formation of studied deposits took place in two stages. At the first stage, during the period of maximum regression and activation of tectonic mode, rock associations of microbial-pelitomorphic and bioclastic limestones were formed. δ13C gradually increases from 1,4 to 3,1‰, and then decreases to 1,2‰, and δ18О is first equal to 22,3...23,2‰, and then decreases to 20,9‰. At the second stage rock associations of algal, microbial-bioclastic and bioclastic limestones with crinoids and corals were formed on a shallow carbonate shelf during the gradual deepening of the sea. In the isotopic system, there is a gradual increase of average values of δ13C from 2,75 to 3,69‰, and average values of δ18O are equal to 22,18...22,88‰ with deviations of 2-3‰