南京地区新近纪河流沉积物的年代与物源示踪

The Neogene strata in Nanjing area spread widely along the Yangtze River and its tributaries Qinhuai and Chu, which were regarded as the standard Neogene sequence in the lower Yangtze drainage. However, the age and provenance of these strata were still under disputed. Different scholars got different age of these strata, ranging from Miocene to early Pleistocene. As for the provenance, some people think it's local origin, while some workers think it's the paleo-Yangtze sediments. During this study, we notice that some of these strata distributed in Juhuatai and Luhe, Nanjing may be the product of proto-Yangtze River as proposed by some earlier scholars. So, these strata may be the crucial material to record the evolution history of the Yangtze River during the Neogene. In this proposal, we intend to determine the age of the strata by means of measuring the 40Ar / 39Ar ages of overlying basalts as well as fossil identification. Meanwhile, the provenance of these strata will also be investigated by comparing the element, Neodymium isotope as well as the detrital zircon U-Pb age and Hafnium isotope composition of the sediments with the rocks and the present Yangtze River sediments from the Yangtze drainage. These results will find out how long is the proto-Yangtze and the Three Gorges were cut through or not during the strata were deposited. Our study will also provide some clues for us to understand the timing of the uplift of the Tibet Plateau as well as the inception of the Asian moonsoon.

在南京地区长江及其支流两岸广泛出露一套新近纪河流沉积物，又被称为著名的雨花台组或雨花台砾石层。当前人们对这套地层的年代和来源都存在争议，认为其年代可能是中新世或上新世-早更新世，物源可能是局部物源或长江物源。申请人根据新生代长江流域区域构造与沉积演化过程的分析认为这套地层可能处于古长江流向苏北盆地的古河道上，可能是研究长江早期地质演化的关键沉积记录。这套地层不仅富含动植物化石，还被玄武岩覆盖，可通过玄武岩40Ar / 39Ar定年和化石鉴定标定其年代。除砾石层外，这套地层还含有砂、粉砂和泥级沉积物，可通过元素、Nd 同位素和碎屑锆石U-Pb 定年及Hf 同位素的综合分析，准确示踪其来源。通过这套地层年代和物源的准确确定，可以重建当时长江流域的古地理格局，揭示长江水系的贯通时间，也有望为与长江演化密切相关的青藏高原隆升和亚洲季风形成的时间提供一定的年代学约束。

河流是构造与气候共同作用的产物，长江是发源于青藏高原最大的一条河流，长江水系的贯通通常被认为是新生代青藏高原隆升和亚洲季风形成的直接结果。近年来，在长江三角洲及江汉盆地新生代沉积物物源示踪研究结果表明，长江贯通三峡的时间可能要比早期多数人认为的第四纪要早。在南京地区长江及其支流两岸广泛出露一套新近纪河流沉积物，以富含雨花石而著称，称为雨花台砾石层或雨花台组，可能与古长江有关，可能记录了长江早期演化的关键信息，然而前人对其地层序列、年代及物质来源均存在争议。. 本项研究以南京地区新近纪河流沉积物为研究对象，通过沉积物上覆玄武岩年龄测试及所含生物化石的系统总结，探讨了这套地层的年代，并通过系统的元素、Sr-Nd同位素和碎屑锆石U-Pb年龄组成，对这套地层的物质来源进行了系统的分析。研究结果发现这套地层上覆玄武岩年龄普遍在10-22.9 Ma，地层中普遍发现中新世特征生物化石，由此提出南京地区不同剖面新近纪河流沉积地层形成的年代基本一致，应为中新世初期。江宁方山剖面、江浦砂矿剖面、六合灵岩山剖面的元素、Sr-Nd同位素和碎屑锆石U-Pb年龄组成与现代长江河流沉积物差别较大，而六合唐公山、小盘山、灵岩山剖面的沉积物则与现代长江河流沉积物基本一致。. 根据南京地区新近纪河流沉积物年代和物源示踪的结果，并结合区域构造演化，本项研究认为在六合-仪征一带大面积分布的含雨花石砾石层以及南京江南、江北呈线状展布、与长江河道平行的砾石层，为古长江沉积物，而南京浦口江浦砂矿剖面、六合灵岩山剖面为古滁河沉积物，南京江宁方山剖面为古秦淮河沉积物，长江在中新世初期贯通三峡东流。本项研究结果对于约束青藏高原隆升和亚洲季风形成的时间具有重要意义。