The Study of Carbon and Oxygen Isotopes from Transitional Beds at the Bottom of Phosphorites in Meishueun Section, Yunnan

Based on the study of the characteristics of stable isotopes of carbon and oxygen from dolomite in phosphatic dolostone and dolomitic phosphorite samples successively collected from the strata above or below the boundary between the Xiaowaitoushan and Zhongyicun members in Meishucun section, the δ¹³C values have been determined within the range from -1.65 to -2.42‰, indicating that the rocks are ascribed to marine sedimentary carbonate rocks. The increasing tendency of δ¹²C values for the samples from bottom to top shows that there is a conformable gradational contact between the two lithologic members, in general agreement with the results on the study of petrology, petrography and palaeontology of Meishucun section made by the previous workers. The δ¹⁸ O values range between -7.08 and -7.67‰, similar to the average δ¹⁸O values of seven Cambrian dolomite samples (-7.9‰) determined by Veizer and Hoets (1976), thus providing another evidence for the determination of the stratigraphic ages for the boundary of Meishucun section.