Abstract: Several dark-green or yellow-brown layers with different thickness have been found in the late Quaternary strata in the area of the Changjiang River and Jiantangjiang River. They are very hard,called “hard clay”. The top hard clay layer is called the first hard clay. There are the marine sediments on the first hard clay. The boundary of Holocene has been discussed based on the first hard clay. The first hard clay is found in Ningbo region too. 19 samples from the first hard clay,the marine sediments on the first hard clay and the layer under the first hard clay,were dated using optical luminescence and 14 C dating method to provide age control of its development. According to our dating results,the age of the original material of the first hard clay is 45～55ka BP. The first hard clay is uncontinuously distributed,being eroded by the later stages transgression in some places,and unconformable with the upper marine sediments. The lower limit of the marine bed may not be simply delineated as the Holocene boundary. It is possible that the marine sediments occurred at the end of late Pleistocene.

Key words: the hard clay, age, Ningbo region

摘要： 在长江三角洲、钱塘江及东海陆架广大地区的晚第四纪地层中发育若干层厚度不等的暗绿色、黄褐色的硬质黏土层,简称硬土层。最上面的硬土层称“第一硬土层”,第一硬土层之上为海相地层。第一硬土层一直被认为是全新统的基底而被作为划分全新统与更新统的重要标志层。文中对采自宁波地区第一硬土层及上覆海相层和下伏地层中的19个样品进行了光释光测年和¹⁴C测年,结果表明,第一硬土层的母体形成于45~55ka BP,它并非连续分布,局部被后期海侵剥蚀而未保留,与上覆海侵层是突变关系。在宁波地区,全新世的底界不能简单地划在第一硬土层和海相层之间,海侵可能早在晚更新世末期就开始发生了。

关键词: 硬土层, 年代, 宁波地区