川中东北部中—下侏罗统湖相碳酸盐岩碳氧同位素特征及其古环境意义

Carbon and oxygen isotopic composition of lacustrine carbonate rocks of the Lower-Middle Jurassic in NE part of central Sichuan Province and their palaeoenvironmental significance

在钻井岩心观察和岩石学分析的基础上,对川中东北部龙岗地区中—下侏罗统自流井组和凉高山组典型钻井取心段碳酸盐岩样品进行了碳氧同位素分析。40余件样品的测试结果表明,δ13CPDB值在-1.24‰～4.59‰之间,除了1个负值外,都为正值。δ18OPDB值在-15.7‰～-9.83‰之间,都为负值。与海相和陆相碳酸盐岩同位素值对比显示,自流井期和凉高山期古湖泊均表现出内陆开放性淡水湖泊的特点,没有经过明显的海水入侵。在中—下侏罗统广泛存在的碳同位素正偏移是由湖泊初级生产力提高所导致的。岩相和同位素变化显示了古湖平面经历了2次主旋回和6次次级旋回变化,湖平面变化总体表现为东岳庙段沉积期>大一三亚段沉积期>大一亚段沉积期>凉高山期>马鞍山段沉积期。古湖泊初级生产力的变化表现为大一三亚段沉积期>东岳庙段沉积期>大一亚段沉积期>凉高山期>马鞍山段沉积期。

Based on the drilling core observation and petrographic characteristics,this paper analyzed the carbon and oxygen isotopic composition of the typical drilling carbonate samples from the Ziliujing and Lianggaoshan Formations of Lower-Middle Jurassic in Longgang area,NE part of central Sichuan Province.The test result of more than 40 samples shows that the δ13C values are between-1.24‰ and 4.59‰(in addition to a negative value), and the δ18O values are between -15.7‰ and -9.83‰.Compared with the isotopic values of marine and continental carbonate rocks,the ancient lake is characterized by inland open lake without obviously seawater intrusion during the Ziliujing and Lianggaoshan Ages.The widespread existence of carbon isotope positive excursion in the Lower-Middle Jurassic might be caused by the improved lake primary productivity.The petrographic and isotopic change indicates that the ancient lake level had gone through two main cycles and six sub-cycles.The overall lake level might be lower in turn during the depositional periods of Dongyuemiao Member,Dayisan sub-Member of Ziliujing Formation,Lianggaoshan Formation and Maanshan Member,while the lake primary productivity may be decreased gradually during the depositional period of Dayisan sub-Member,Dongyuemiao Member,Dayi sub-Member of Ziliujing Formation,Lianggaoshan Formation and Maanshan Member.