淀山湖太湖形成的古生物证据

M ICROPALEONTOLOGICAL EVIDENCE FOR THE FORMATION OF DIANSHAN LAKE OF SHANGHAI AND TAI LAKE OF JIANGSU PROVINCE OF CHINA

通过对湖泊周围钻孔样品的微体古生物分析,根据化石群的生态特征,结合岩性、孢粉、^(14)C等资料探讨晚更新世中期以来淀山湖、太湖沉积环境的演变过程,并比较其异同。现代湖泊的形成于全新世晚期。

Some core samples have been collected from nearby the Dianshan Lake's bridge and round the Tai Lake. By qualitative and quantitative micropaleon-tological analyses of them, abundant foramini ferids, ostracods, oospore membranes of Chara. molluscs, echinoid spines, pollen of pinus and shieldform fossils were found. Based on the ecological characteristics combining with sed imentological, paly nological as well as 14C data, this paper suggests that two episodes of transgression have occurred in the Dianshan Lake and Tai Lake since mid-Late Pleistocene (Q23 and Q4) . However, both present lakes were formed in Late Holocene and five stages of geological development of them are recognized as follows;1. Mid-Late Pleistocene A large-scale transgression occurred in thisarea, Dianshan Lake was located in littoral zone and Tai Lake, in lagoon-bayenvironment with warm water forams. such as Asterorolalia and Globorotalia.2. Latest Pleistocene A large-scale sea-level subsidence caused the occurrence of regression. The two lakes changed into fresh-water lakes. No fossil has been found.3. Early Holocene With the beginning of Holocene-Transgression, Dian-shan Lake and the eastern Tai Lake turned into lagoons while the other part into shallow-water environment on shore and only with euryhalline forams.4. Mid-Holocene When the Holocene high sea-level occurred (0500 -7500 years ago) the two lakes became estuary, after then the estuaries disappeared. The Dianshan Lake had been separated from and changed gradually into a fresh-water lake again, and lastly, emerged to a land. But the transgression remained its influence in southeastern and northwestern sides of Tai Lake to form the transitional facies sediments. The blackish forams and open sea small-sized forams were found there.5. Late Holocene Because the lake outlets were narrowed or silted up, the surface runoff converged into depressions to form a group of lakes. As a result, the extent of the two lakes enlarged constantly. Forams were seldom found in sediments