CARBON ISOTOPE CHARACTERISTICS OF PERMIAN QIXIA FORMATION IN SI-CHUAN BASIN AND ITS STRATIGRAPHIC IMPLICATION
The Permian system in the Sichuan Basin is the most extensively developed stratum in South China.This paper utilizes data from two drilling wells in the Sichuan Basin to conduct a correlation study of Permian lithostratigraphy and carbon isotope stratigraphy.The objective is to clarify the stratigraphic division of the Middle Permian Qixia Formation and to understand the complex and diverse changes in sea level during its deposition.The results indicate that the Qixia Formation in the Sichuan Basin is primarily composed of micrite,bioclastic micrite,dolomitic limestone,and dolomite.In well MX150 and well LT1,the carbon isotope values of the rocks range from-1.72‰ to 4.75‰ and 2.75‰ to 4.91‰,respectively,showing an overall increasing trend.However,the isotopic values in the lower part of the Qixia Formation are relatively scattered.The upper part of the Qixia For-mation exhibits sections with increasing isotope values,displaying alternating trends of positive bias-negative bias-positive bias-negative bias.These data suggest that the Qixia Formation corresponds to the lower part of the Gonggu stage through the Luodian stage,spanning approximately from 283 to 270 Ma.The observed positive and negative carbon isotope cycles in the Qixia Formation are closely related to global sea level changes and lithology cycles.The negative carbon isotope anomalies commonly found in the lower part of the Gonggu stage may be linked to volcanic activity-associated wildfires and methane leakage events.These events likely led to a substantial input of CO2,causing climate warming,glacier melting,and sea level rise,which resulted in the deposition of muddy limestone in the lower part of the Qixia Formation.The overall increasing trend of carbon isotope values in the upper part of the Qixia Formation reflects an increase in primary productivity and significant organic carbon burial,which ultimately reduced atmospheric CO2 concentrations and global sea levels.The presence of bioclastic granular limestone in the middle and lower parts of the Qixia Formation suggests a decrease in sedimentary water depth.The positive isotope biases correspond approximately to low sea level periods,while the negative biases cor-respond to high sea level periods,reflecting the correlation between glacial-type sea level changes and carbon cycle fluctuations.The development of dolomites near the points of maximum positive skewness may indicate low sea level periods,which are conducive to contemporaneous and quasi-contemporaneous dolomitization.
万方数据
维普
