The С–О–Sr–S Isotope Geochemistry and Chemostratigraphic Correlation of Ordovician Deposits in the Vilyui Structural-Facies Zone, Siberian Platform

The C–O–Sr isotope composition of carbonates and the S–Sr isotope composition of gypsum were studied in the Ordovician section and the Upper Cambrian–Lower Silurian boundary horizons of the Vilyui structural-facies zone located in the central Siberian Platform. The upsection decrease of the 87 Sr/ 86 Sr ratio within 0.70920‒0.70795 and the value of δ 34 S ~ 25‒28‰, typical for the reference sections of this stratigraphic interval, indicates a good connection between the basin and the World Ocean in the Ordovician and Early Silurian, but its isolation in the Late Cambrian (Kholomolokh Formation) when the 87 Sr/ 86 Sr ratio (0.7085) was lower than the oceanic one. The sharp 87 Sr/ 86 Sr ratio decrease from ~0.7087 to 0.7080 in the Khar’yalakh Formation (~45 m thick) can be correlated with the interval from the upper Darriwilian-to-upper Sandbian stages of the ICC (Middle Ordovician Volgian horizon–Upper Ordovician Baksanian or Dolborian horizon of the Russian GSS). Based on this benchmark and lithological features of the section, we discuss the causes of wide variations in δ 13 С (‒7.2 to 1.6‰) and δ 18 О (16.9‒29.8‰) in the carbonates, as well as a possible correlation of carbon isotope excursions in the Vilyui section with the global carbon isotope events.