Characteristics and significance of carbon and oxygen isotopes of the Cambrian Xixiangchi Formation in southeastern Sichuan Basin:A case study from the Houtan section in Guizhou
The carbonates in the Cambrian Xixiangchi Formation in the southeastern Sichuan Basin have generally been dolomitized,and the lack of fossils from the area has long hindered a comprehensive understanding of its stratigraphic correlation and division.In this study,we used field observations as well as analyses of the profiles and petrological characteristics of the formation to examine carbon and oxygen isotopes in 41 carbonate samples from the Middle-Upper Cambrian Xixiangchi Formation in the southeastern Sichuan Basin.The results showed that the Xixiangchi Formation was mainly composed of micrite-fine silty crystalline dolomite,with a small amount of residual granular dolomite and coarse silty-finely crystalline dolomite.The δ18O values of the samples were distributed in the range of-11.05‰ to-8.29‰,with an average of-9.67‰,while their δ13C values were in the range of-2.29‰ and 1.36‰,with an average of-0.73‰.There was no significant positive correlation between the values of δ18O and δ13C,and the original composition of the carbon and oxygen isotopes of seawater from the same period had been retained.The analysis showed that the southeastern Sichuan Basin had a marine environment in the Middle-Late Cambrian Xixiangchi period,and salinity in this period was slightly higher than that in the early period.The temperature of seawater in the Xixiangchi period ranged from 15.2 ℃ to 27.5 ℃,indicating that the study area had had a warm or hot subtropical climate.The steptoean positive carbon isotope excursion event in the Houtan section was comparable to such events in other areas of the world,reflecting a global regression event.In addition,a comparison of the characteristics of carbon isotopes of the Houtan section in the southeastern Sichuan Basin with those in the Taihe 1 and Li 1 wells in northeastern Chongqing showed that the latter had a higher salinity than the former.The salt basin evolved from southeastern Sichuan to northeastern Chongqing during the Xixiangchi period,which is important in the search for deposits of salt as well as oil and gas.
southeastern Sichuan BasinXixiangchi Formationcarbon and oxygen isotopespaleoenvironmentstratigraphic correlation
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