北京西山地区下苇甸剖面寒武系碳酸盐岩碳氧同位素研究

Study on carbon and oxygen isotopes of Cambrian carbonate rocks in Xiaweidian section,West Hill,Beijing

北京西山下苇甸剖面寒武系的碳酸盐岩发育良好且连续,在岩石学和地层学研究基础上,通过实测剖面和碳酸盐岩碳氧同位素,探讨了下苇甸地区寒武纪海洋环境及沉积相演变。研究表明,下苇甸寒武系发育碳酸盐岩镶边台地相沉积,自底部昌平组至顶部凤山组依次发育潮坪、潟湖、障壁岛礁滩、滩外斜坡沉积相。碳酸盐岩氧同位素表明该区域寒武纪为温暖至炎热的亚热带气候,同时对古沉积环境也有明显响应,沉积环境的盐度自潮上带至滩外斜坡逐渐减低至正常海水的水平,碳同位素与下苇甸沉积相及沉积旋回的变化契合度良好,表明寒武系整体为一个大的海侵过程,晚期凤山组沉积期海平面略下降。

Cambrian carbonate rocks are well developed and continuous in the Xiaweidian section of West Hill,Beijing.Based on the study of petrology and stratigraphy,the Cambrian marine environment and sedimentary facies evolution in the Xiaweidian section are discussed through the carbon and oxygen isotopes of carbonate rocks.The results show that carbonate rimmed platform facies was developed in the Cambrian,and tidal flat,lagoon,barrier island reef beach and outer slope sedimentary facies were developed successively from the underlying Changping Formation to the overlying Fengshan Formation.The oxygen isotopes of carbonate rocks reveal that the study area was in a warm to hot subtropical climate during the Cambrian,and it also had an obvious response to the Cambrian sedimentary environment.The salinity of the paleo-environment decreased from the supratidal zone to the slope outside the beach being the level of normal seawater gradually.The carbon isotopes agree well with the changes of the sedimentary facies and sedimentary cycle,indicating that the study area experienced a large transgression process during Cambrian,and the sea level droped during the deposition of the Fengshan Formation.