Hypostratotype of the Bashkirian Stage of the Carboniferous System (Askyn River, Bashkortostan): Lithology, Isotopes (δ13C, δ18O), and Carbonate Depositional Settings

— The isotopic composition and lithology of the hypostratotype of the Bashkirian Stage (Pennsylvanian) in the Askyn River section (Bashkortostan, western slope of the South Urals) are studied. The isotopic study (δ 13 C, δ 18 O) was conducted taking the carbonate lithology into account. The succession studied is composed of Upper Serpukhovian, Bashkirian, and Lower Moscovian carbonates of various lithological types, changeable throughout the section. The Serpukhovian carbonates are characterized by a relatively light carbon isotopic composition (negative values of δ 13 C), whereas the Bashkirian and Moscovian sediments have a heavy composition (mainly positive values of δ 13 C). The carbon isotope anomaly of the first order, fixed at the Mississippian–Pennsylvanian boundary and having a global distribution, is recorded in the studied hypostratotype section slightly higher than the uniformly accepted boundary level. This may indicate that the section contains “transitional beds” at the base of the Bashkirian. In the studied carbonates, a dependence (of the second order) of the isotopic composition of carbon and oxygen on the lithological type of rocks is sometimes recorded. The variation in the values of δ 13 C and δ 18 O in carbonates reflects changes in the conditions in which they were formed (temporary and local changes in paleotemperature, bioproductivity, desalination) and epigenetic changes.