山东平邑盆地古近系卞桥组一段湖相碳酸盐岩碳氧同位素特征及其地质意义

Carbon and Oxygen Isotopic Characteristics of Lacustrine Carbonate Rocks from the First Member of Paleogene Bianqiao Formation in Pingyi Basin of Shandong, China and Their Geological Significance

对山东平邑盆地柏林剖面古近系卞桥组一段19件湖相碳酸盐岩样品的岩性特征、碳氧同位素特征、主量和微量元素进行了分析。结果表明:平邑盆地古近系卞桥组一段的岩性从地层底部到顶部整体变化依次为粗碎屑岩、核形石灰岩、生物碎屑灰岩、颗粒灰岩、泥晶灰岩/泥灰岩。碳酸盐岩δ13C值主要为-3.5‰^-2.5‰,平均值为-2.9‰;δ18O值主要为-8.5‰^-6.5‰,平均值为-7.6‰;碳氧同位素具有良好的相关性,说明平邑盆地在古近系沉积时期是水体停留时间较长的封闭性湖泊系统。碳酸盐岩Sr/Ba值主要为0.25~31.55,可以认为平邑盆地古近系沉积时期水介质主要为咸水环境;Mg/Ca值主要为0.0042~0.0119,结合氧同位素组成,反映平邑盆地古近系沉积时期的古气候变化经历了潮湿环境→半干热环境→干热环境3个阶段。根据岩性序列垂向演化及薄片特征,可以推断出平邑盆地在古近系卞桥组一段沉积时期,湖盆水动力随着地层变新而减弱,湖盆水体逐渐由高能转入低能环境。

The lithological characteristics, carbon and oxygen isotopic composition, major and trace elements of 19 samples for lacustrine carbonate rocks from the first member of Paleogene Bianqiao Formation in Pingyi Basin of Shandong were analyzed. The results show that the lithological changes from the bottom to the top of the first member of Paleogene Bianqiao Formation in Pingyi Basin are coarse clastic rock, oncolite limestone, bioclastic limestone, grain limestone, micrite limestone/marlstone. The δ13C values of carbonate rocks are-3.5‰--2.5‰ with an average of-2.9‰, δ18O values are-8.5‰--6.5‰ with an average of-7.6‰, and the carbon and oxygen isotopes show good positive correlation, suggesting a relative closed lake system with a relatively long resident time at Paleogene in Pingyi Basin. The Sr/Ba ratios of carbonate rocks are 0.25-31.55, and it can be considered that the aqueous medium at Paleogene in Pingyi Basin is mainly salt water;the Mg/Ca ratios are 0.004 2-0.011 9, revealing that the paleoclimate evolution at Paleogene in Pingyi Basin experiences 3 stages, including humid climate→semiarid climate→arid climate. According to the vertical evolution of lithologic variation and thin section characteristics, it can be inferred that the hydrodynamic force of lake basin gradually weakens as strata becoming upper, and the water body of lake basin gradually converts from high-energy to low-energy environment at Paleogene in Pingyi Basin.