Abstract: Abstract:The carbon and oxygen isotopes of the Xixiangchi Group in the Cambrian carbonate rocks of centralsouthern Sichuan basin are less modified by postdepositional diagenesis, and thus provide helpful insights into the original ocean. Based on the carbon and oxygen isotope data collected in this area, the carbon and oxygen isotopic compositions, evolution and geological significance of the Middle and Upper Cambrian sedimentary cycles are studied and discussed. Research suggests that the C and O isotopic profile has obvious cyclicity evolution in stratigraphic sequence. δ13C shows a shift from -56‰ to 3247‰ with a mean value of -0979‰ and most of the samples are in the range of -2‰ to 2‰. δ18O shows arrange from -111‰ to -52‰ averaging at -6934‰ and most of the samples are in the range of -9‰ to -6‰. Through the analysis of carbon and oxygen isotope data of sedimentary cycle in different scale, variation of sea level is the main controlling factor of cyclic sequence. The meterscale cyclic sequence is formed under mixing effect of highlyfrequent sealevel change, paleostructures and paleoclimate. Carbon and oxygen isotope analysis shows that the boundary between the Third Series and the Furongian Series is defined, which provides strong evidence for the division and correlation of the strata. The δ13C shows Xixiangchi Group underwent a slow regression after a rapid transgression in the early and middle stages and in the late stage entered a rapid regression after a slow transgression. Sea level changes suggested by carbon isotope distribution are consistent with the evolution of sedimentary facies.

Key words: centralsouthern Sichuan basin, Xixiangchi Group, carbon and oxygen isotopes, sedimentary sequence, paleoenvironment, geological implication