下扬子地区早二叠世末期碳酸盐台地死亡事件的沉积学与碳同位素记录

Sedimentology and Carbon Isotope Records of Latest Early Permian Carbonate Platform Drowning Event in Lower Yangtze Region

华南板块在早二叠世时期位于赤道附近,发育了以栖霞组为代表的浅海碳酸盐岩,随后被孤峰组黑色页岩和硅质岩所取代,代表一次碳酸盐台地死亡事件。文章对下扬子区安徽巢湖平顶山剖面和江苏句容正盘山剖面上栖霞组与孤峰组进行了系统的沉积微相、碳同位素地层学等研究,试图探讨早二叠世末期栖霞组碳酸盐台地死亡的机制。分析表明,栖霞组顶部可划分为9种微相,属于碳酸盐内缓坡—中缓坡沉积环境;孤峰组底部可划分为5种岩相,属于深水盆地环境。栖霞组与孤峰组界处沉积环境和古水深发生突变,代表了下扬子地区早二叠世末期的碳酸盐台地死亡。栖霞组与孤峰组之间的碳酸钙含量突然降低、碳酸盐碳同位素快速负偏、有机质碳同位素明显正偏,指示碳酸盐台地死亡的突发性和事件性。观察显示,无机碳δ^(13)C_(carb)与有机质δ^(13)C_(org)存在镜像关系。文章认为,导致有机和无机碳解耦的原因可能与陆源有机质输入有关,而碳酸盐台地死亡可能为相对海平面上升所导致。

The Lower Yangtze region in South China plate was located near the equator during the Early Permian,where the shallow-water carbonates of the Qixia Fornation widely developed.The latter were subsequently replaced by the black shale and siliceous rocks of the Gufeng Formation,representing a carbonate platform drowning event at the end of Early Permian.In this study,the carbonate microfacies and carbon isotopic stratigraphy of the Qixia and Gufeng formations were systematically studied in the Pingdingshan section of Chaohu,Anhui Province and Zhengpanshan section of Jurong,Jiangsu Province in order to explore the mechanism of this depositional event.Microfacies data shows that the uppermost of the Qixia Formation can be divided into 9 microfacies,belonging to the inner ramp to middle ramp environment.The lowermost of the Gufeng Formation can be divided into 5 lithofacies,indicating a deep-water basin environment.The depositional environment and paleo-water depth at the boundary between the Qixia and Gufeng formations changed obviously,representing the drowning event of the carbonate platform during the Latest Early Permian.At the same place,sudden drop in the content of calcium carbonate,negative carbon isotope excursion and obviously positive carbon isotope of organic excursion indicate the suddenness and eventuality of carbonate platform drowning.The results show that there is a mirror image relationship between δ^(13)C_(carb) of carbon isotope and δ^(13)C_(org) of organic matter.Carbon isotopes of carbonate and organic matter show decoupling changes of negative and positive excursions,respectively,which may be the result of terrigenous organic matter input.We suggest that the drowning of carbonate platform may be caused by relative sea level rise.