四川盆地中南部地区洗象池群沉积旋回的碳氧同位素特征及地质意义

摘要：四川盆地中南部地区寒武系洗象池群碳酸盐岩的碳、氧同位素组成受后期成岩作用影响较小，基本保留了原始海洋的同位素组成，根据该地区采集的88个碳、氧同位素数据，研究、讨论了中上寒武统碳酸盐岩不同尺度沉积旋回中的碳氧同位素组成、演化及地质意义。研究表明，四川盆地寒武系洗象池群由下至上的地层序列中，碳、氧同位素具有明显的旋回性演化特征：其δ13C值变化于－56‰～3247‰之间，均值为-0979‰，绝大多数的样品都在-2‰～2‰的区间震荡，δ18O值分布于-111‰～-52‰之间，均值为-6934‰，主要集中在－9‰～-6‰的范围内波动。通过对不同尺度沉积旋回碳、氧同位素数据分析研究，认为在体系域尺度沉积旋回中控制碳氧同位素组成特征的主因是海平面变化，在米级旋回中主要受高频海平面变化、古气候、古构造等多方面叠加的影响；通过对碳、氧同位素数据分析研究，首次明确了该地区芙蓉统的底界，为地层的划分对比提供了有力证据；洗象池群早期和中期经历了短暂而快速的海侵后进入缓慢的海退，在晚期缓慢海侵后的快速海退，碳同位素组成反映的海平面变化趋势与沉积相演化一致。

Carbon and Oxygen Isotopic Compositions and Their Evolution Records of the Xixiangchi Group in Sedimentary Sequences of Central-southern Sichuan Basin and Their Geological Implications

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.