四川盆地东部下寒武统龙王庙组碳、氧同位素组成及古环境意义

摘要

Based on the study of field sedimentary characteristics and the observation of indoor thin sections, carbon and oxygen isotopes were tested for 22 samples of Lower Cambrian Longwangmiao carbonate rock from eastern Sichuan Basin. After the validity of these samples was fully demonstrated, the characteristics of carbon and oxygen isotopes were analyzed and the paleoclimate and paleo-ocean environments during the Longwangmiao sedimentary period were discussed. The δ13C values of carbonate distributed between-1.533‰ and 2.619‰ with an average of 0.046‰, while the δ18O values varied from -9.916‰ to -3.58‰ and-7.746‰ on average. The overall variation trends of carbon and oxygen isotope in this area are basically the same as the other areas in Yangtze platform. It is shown that eastern Sichuan Basin was marine environment during the Longwangmiao sedimentary period, and the salinity of seawater had a decrease at first, followed by an increase and second decrease at the end. Seawater temperature mainly distributed between 20℃ and 30℃, being a warm or hot subtropical climate. Under the influence of depositional environments, such as slowly fluctuated decline of sea level, the exposure of shelf and biologic depopulation, the δ13C values had the evolutionary trend of negative drift. Those indices reflect that the lowest seawater depth and the highest salinity were present in the middle period of Longwangmiao Formation deposition, which was the best time favorable for development of dolostone.%在野外沉积特征研究、室内薄片观察的基础上,对四川盆地东部龙王庙组22个碳酸盐岩样品的碳、氧同位素进行测试,在充分论证样品的有效性后,分析研究区龙王庙组碳、氧同位素特征,进而探讨龙王庙组沉积期的古气候及古海洋环境。样品实验数据显示δ13C值分布在-1.533‰~2.619‰,平均为0.046‰；δ18O值分布在-9.916‰~-3.580‰,平均为-7.746‰；碳、氧同位素整体变化趋势与扬子地台其他区域基本相同。古海洋环境恢复表明,龙王庙组沉积期四川盆地东部整体处于海相环境,海水盐度呈先降低、后增高、复又降低的多段式变化；海水温度主要分布在20~30℃,属于温暖或炎热的亚热带气候；在海平面缓慢波动性下降、陆架暴露、生物灭绝的沉积环境影响下,龙王庙组δ13C值具有负漂移的演化趋势。龙王庙组沉积中期的水体深度最浅,古盐度最高,是白云岩最为有利的发育阶段。