Lithology, bio and chemostratigraphy of the upper Serpukhovian lower Bashkirian deposits in the Pacha Shor section (Ilych river, Northern Urals)

The trend of increase in the carbon isotopic composition at the Bashkirian base is noted in many sections of the world. However, detailed works in the Urals show that this trend does not always coincide with the Serpukhovian/Bashkirian bound ary. Biostratigraphic, lithological and isotopic analysis of the Upper Serpukhovian and Lower Bashkirian deposits in the Pacha Shor section of the Northern Urals revealed the main variations ofthe carbon isotopic composition (δ13Ccarb). In the Protvian Suranskian interval, represented by thebrachiopod microbial organogenic buildup, a gradual decrease of δ13Ccarb from 2.10‰ to 0.35‰ is noted. Between Suranskian and Akavaskian hori zons, the carbon isotopic composition increased. On the Akavaskian Askynbashskian interval, represented by bedded bioclastic and algal limestones, a gradual decrease of δ13Ccarb from 2.71‰ to 1.36‰ is noted. This indicates that the carbon isotopic composition increase in the Pacha Shor section at the Suranskian Akavaskian boundary isronm nts. At theronm nts. At theame time,ame time,he carbon isotopiche carbon isotopicthe result of changing sedimentation conditions from semiisolated lagoons to open marine envie s tcomposition analysis of the individual components of the limestone sample (Suranskian horizon)showed higher values in the carbonate of brachiopod shells (δ13Cbrach=1.9 4.79‰), while in individual components δ13Ccomp vary within 0.07 0.5‰. These data clearly indicate that the carbon isotopicvalues of the entire rock and the brachiopod shell carbonate can have significant differences. The explanation of this fact should be the subject of future research.