EVOLUTION OF HOLOCENE SEDIMENTARY ENVIRONMENT IN THE HANGZHOU-JIAXING-HUZHOU PLAIN
Yan Qinshang Huang Shan (Department of Geography, East China Normal University)
This paper tackles this problem based upon a variety of sedimentological evidence, such as paleomicrofauna, pollen and spores, sediment grain size, sedimentary structure, heavy mineral content and 14C radio-carbon dates. These data were mainly, in conjunction with nearly 200 subsurface drilling cores available in this area, collected from seven newly drilled cores at Gou-ting, Gaoqiao and Shuanlin etc. The results obtained from this study could be summarized as follows: 1. A tenacious-clay layer with a greyish or yellowish brown colour identified as river-lakemarsh deposits of late Pleistocene exists, with few exceptions, beneath surface Holocene deposits. This layer can be used as a marker defining the boundary between Holocene and Late Pleistocene depositions. 2. The topography of the palaeo-surface of the Late Pleistocene, Which presenting an inter-fingered pattern, is in general a major relatively elevated platform with elevation within -5m and is crossed by a south-to-north orienting valley about 15──25 m in elevation running between the platform and the western hilly land from Taihu lake to the Qiantang Jiang River. Another depressed valley exists southeast of the platform trending from Haiyan to Jinshanwei, whose elevation is no greater than -20 m. 3. The evolution of Holocene sedimentary environments in this area can be divided into three periods: 1) Early period (about 10000-7500 Y. B. P.) Environmentally, the platform was terrestrial transited to low-salinity marshes; while west of the platform estuarian and south of it a coastal embayment. 2) Middle period (7500-2500 Y. B. P.) In the early stage of this period, the western part was an estuary, the area around Ganpu and Zhapu a coastal bay and the platform was altered into tidal flats and low-salinity marshes. The northeast portion and parts of the southwest portion then were low-salinity marsh environment. During the second stage, the platform was turned into low-salinity marshes except for its northeast and southwest portion which were fresh water marshes; the southern shallow bay was silted up into land and the western estuary was shallowed and silted into a narrow boy connecting Tai Lake with the Qiangtang River. In the last stage, the platform developed further into an area fresh-water lakes and marshes, and a number of fresh-water lakes developed in the northeast; the western channel became silted up. 3) Late period (the last 2500 years) In most of the study area fresh-water lakes and marshes dominated. At the time of about 2000 Y. B. P. the western channel became isolated from the Hangzhou Bay and was transformed into some small fresh-water lakes.