A PRELIMINARY STUDY ON THE HOLOCENE TRANSGRESSION ON THE COASTAL PLAIN ALONG THE NORTH-WESTERN BOHAI BAY
Wang Yiman (Institute of Geography, Academia Sinica)
The plain at the lower-reaches of Jiyun and Chaobai River to the north of Hai-ho River along the north-west coast of Bohai Bay is a lowland on which rivers meander and shallow lakes scatter. The Holocene transgression here was rarely systematically studied before because of the discontinuance of the cheniers that usually appear south of Haiho River.Three cores were drilled after the field observation. The association of spore-pollen and mollusc and foraminifer in the strata was analysed. It shows that the climate fluctuated during the period of the Holocene from cold to damp and hot and then became cool. The sedimentary environment changed correspondingly as limnetic-fluvial-fluvial-marine-limnetic-fluvial. So the Holocene stratum can bedivided into three layers with the lower depth of 17-20m., 12-15m.and 2-3m.respectively. The Holocene transgression took place at the stage of Atlantic when the climate was optimum and the sea level rose in the Middle Holocene.According to the marine sediment and the foraminifer association combined with the relief and the distribution of underground saline water, etc., the limits of the Holocene transgression on this plain are drawn from east to west as follows. Anniou-dian in Luannan County, Hongqiao in Yutian County, the south of Zhangguzhuang in Baodi County, eastern Yungqing County and Xinglunggung in Werian County. The outline of the transgressive extent on the plain along northern Bohai Bay is then linked as a whole.There are crowded and well-developed oyster reefs and shells buried just below the land surface in the north-west coastal plain.It presents a contrast between oyster reefs and cheniers on the west coastal plain. The difference is due to the different dydro-dynamic of river and sea. On the north-west coastal plain, rivers were small with clear waters. The tides and waves were weakened in this small bay inside the Bohai Bay.All these provided a very good ground for the growth of the oyster reef although the lack of high energy of the coast made it impossible for chenier to be formed.