钱塘江河口周边钻孔磁性地层学研究及意义

长江三角洲地区的沉积历史和特征,是揭示我国东部沿海区域新生代以来构造演化与地理格局形成过程的重要窗口.本文基于磁性地层学方法,结合AMS14 C和光释光测年数据以及该地区硬质黏土标志层,对分布于钱塘江河口两侧的五个钻孔建立年代地层框架.结果表明,除河口北部一个钻孔底部年龄至高斯正极性期外,其余钻孔显示南部地区约200ka BP以来才开始沉积,最南端晚至约60ka BP.与该地区已有钻孔磁性地层学资料进行对比,认为长江三角洲部分区域沉积年龄超过百万年和"北厚南薄、东厚西薄"的特征,主要是受早期区域构造背景的影响,而长江三角洲大范围的沉积发生自约200ka BP以来,受构造缓慢沉降和海平面变化控制的细颗粒(黏土-粉砂粒级)从晚更新世才开始沉积,约60ka BP以后扩大至整个三角洲区域.

Magnetostratigraphy characteristics of several cores around the Qiantang River mouth and its significance

The sedimentary history and characteristics of the Yangtze delta can help understanding the tectonic and geomorphological evolution in the coast areas of East China since Cenozoic. In this study, palaeomagnetic dating, AMS¹⁴C dating, optically stimulated luminescence (OSL) dating as well as the hard clay layer analysis were jointly used to establish a chronostratigraphy framework of five drilling cores from the coast area of the Qiantang River estuary. Results show that the age of the bottom of core CSB-6 suggests Gauss normal polarity chron. While for other cores, the deposition time only started from 200-60 ka BP. Combined with referenced magnetostratigraphy of cores in this area, we suggest that it is the regional tectonic movement at the earlier age that resulted in the much longer depositional age in some parts of the Yangtze River Delta as well as the sedimentary characteristics of thick in north and thin in south, and thick in east and thin in west. In conclusion, a wide range of deposition in the Yangtze River Delta occurred since~200 ka BP. The deposition of fine particles (clay-silt), which was controlled by slow tectonic subsidence and sea-level changes, started form the Late Pleistocene and began to expand to the whole delta region after~60 ka BP. These data provide a time scale for further study on the changes of geology, paleoclimate and paleoenvironment of the Yangtze delta.