三万年前后太湖平原环境变化中的若干问题

根据近400个浅钻（30m）的岩性、岩相及其空间分布规律和¹⁴C年龄、矿物、粒度、微体、孢粉、软体动物化石分析,揭示了组成太湖基底及太湖平原的浅层第四系主要是23000—18000 aB.P.的褐黄色粘土、28000—23000 aB.P.前后的灰色粉砂和亚粘土以及大于33000 aB.P.的暗绿色粘土。上层和下层均属陆相,系海退过程中的泛滥沉积,示干冷环境;中层为海相,反映温湿的季风海洋性气候。区内全新统仅沉积于上层黄土侵蚀沟谷和洼地中,全新世海侵是局部的。

ON SOME PROBLEMS OF ENVIRONMENTAL CHANGES AROUND 30 000 aB. P. ON TAIHU PLAIN

The environmental changes of Taihu Plain for the last 33 000 years are discussed basing on 400 drilling cores and analysises of grain size, mineral composition, foraminifera assemblage, fossil pollen and radiocarbon datings. On the environmental changes of Taihu Plain around 30 000 a B. P. and the Quaternary deposits at the depth of less than 30 m in this area, the conclusion includes:1. Holocene sediments of Taihu Plain are generally 3—5 m thick and most of them are peat and muck. Middle Holocene marine transgression exerted influence only on some lowland, and did not on all of the Taihu Plain.2. The yellow clay which composed of the basement of Taihu Plain was the flood plain deposits in process of regression during 23 000—18 000 a B. P., not the Holocene sediments.3. The marine deposits at the depth of 7—24 m, which were considered as the standard layer of "Atlantic Period" in the middle Holocene, are now dated back to 28 000—23 000 a B. P. according to ¹⁴C dating.4. The terrestrial dark-green clay under the marine deposits, which was thought as the indicator of the boundary between Holocene and Pleistocene, are dated 33 000 a B. P. or even older.5. The paleogeographic environmental changes of the area passed three stages:（1） Around 33 000 a B. P. the climate was dry and cold, the ground was low and flat, and dark-green clay was deposited by the river flooding.（2） In the period of 28 000—23 000 a B. P. the climate became more temperate and humid dominated by moonson and ocean, large scale marine transgression happened, and a set of coastal and shallow seawater deposits which contained some carbonised plant fossiles in the low layer was formed. The sea regressed after 23 000 a B. P., during this process yellow clay which was deposited by the river flooding composed of the basement of the plain. To about 18 000 a B. P., the sea level dropped strongly and climate became dry and cold again.（3） The coastline during the Holocene high sea level was along Fushan-Taicang-JiadingQingpu-Caojin-Fengxian-Zhiling, and only some lower parts of the area were affected by the transgression. Lots of Neolithic relicts on the top of loess indicated that people started their activities here more than 6 000 a B. P. However, the formation processes of the modern city town on the Taihu Plain only have the 2 000—2 500 years'long history.