长江河口及其水下三角洲晚第四纪地层和环境变迁

笔者系统地研究了长江河口地区的晚第四纪地层。长江三角洲前缘区晚第四纪地层可划分为上更新统上段中部、全新统下部、全新统中部和全新统上部。上更新统顶部地层缺失。前三角洲区晚第四纪地层齐全，无缺失。武木冰期晚期，现今的长江河口和大陆架均出露成陆，古长江在大陆架外缘入海。１５０００ａＢ．Ｐ，海面上升，海水逐渐向大陆架上部侵入；１２０００—１００００ａＢ．Ｐ，海水沿谷地侵入；６０００ａＢ．Ｐ，长江人海口在扬州、镇江附近。随着人类活动的增强，长江流域水土流失加剧，人海河口不断外移，并以南岸边滩推展，北岸沙岛并岸的形式使河口不断束狭，逐渐形成现今长江口三级分汉四口入海的态势。

STRATA AND ENVIRONMENTAL CHANGES OF THE CHANGJIANG ESTUARY AND ITS SUBAQUEOUS DELTA DURING THE LATE QUATERNARY

The paper is focused to discuss the sedimentary environmental changes of the Changjiang Estuary and its subaqueous delta area since the late Pleistocene based on the analysis of the estuarine sandbars and subaqueous delta from stratigraphic sequences, sedimentary features, sporopollen assemblage, micropalaeobiology and sedimentary facies.I. Late Quaternary strata1. Delta front areaUpper Pleistocene: fine sand. The sporopollen assemblage is Pinus-Quercus-Gramineae-Artemisia, and foraminifer assemblage is Ammonia beccarii var.-Elphidium magellanicum- Quinqueloculina sp. This layer belongs to the littoral faciies,but its top lacks a continental factes layer.Lower Holocene: clay, interbedded with silt and fine sand. There are two sporopollen and foraminifer assemblages. In the lower part, its sporopollen assemblage is Pinus-Quercus-Betula-Gramineae-Artemisia, and foraminifer assemblage is Ammonia beccarii var. -Elphicdium magellanicum-Cribrononion subincertum. This belongs to the submerged valley factes. In the upper part, the sporopollen assemblage is Quercus-Pinus-Castanea-Artemisia-Chenopodiaceae and foraminifer assemblage is Ammonia beccarii var. -Elphidium magellanicum-Nonionella jacksonensis. This layer belongs to the shallow sea factes.Middle Holocene: silty clay, fine sand with clayey silt. Its sporopollen assemblage is Quercus-Castanea-Q. glauca-Artemisia-Chenopodiaceae, and the foraminifer assemblage is Ammonia beccarii var. -Florilus decorus-Quinqueloculina akneriana rotunda. This layer belongs to predelta facies.Upper Holocene: silty clay, interbedded with fine sand and silt. It has Pinus-Quercus-Gramineae-Artemisia-Pteridium sporopollen assemblage and Ammoniabeccarii var. -Epistominella naraensis foraminifer assemblage. This layer has plenty of authigenic pyrite. The stratum belongs to the delta front and its top is the delta plain factes.2. Predelta areaUpper Pleistocene: The lower part is silty-fine sand and silty clay. The sporopollen assemblage is Quercus-Betula-Rhus-Pinus-Artemi and the foraminifer assemblage is Globigerina falconensis-Neogloboquadrina pachyderma-Cribrononion incertum. It is sea factes. The upper part is clayey silt, carbonaceous clay and silty clay. The sporopollen assemblage is Pinus-Quercus-Betula-Ephedra-Abies No foraminifer is found. It belongs to river-lake sedimentary factes.Lower Holocene: silty clay interbedded with fine sand and silt. The sporopollen assemblage is Quercus-Pinus-Castanea-Betula-Artemisia and foraminifer assemblage is Cribrononion incertum-Elphidium magellanicum. It belongs to the littoralshallow sea facies.Middle Holocene: silty clay interbedded with silt. The sporopollen assemblage is Quercus-Castdhea-Q. glauca-Polypodiaceae and the foraminifer assemblage is Bulimina marginata-Neogloboquadrina pachyderma-Globigerina falconensis.This layer belongs to shallow sea facies.Upper Holocene: clayey silt and silty clay. The sporopollen assemblage is Pinus-Quercus-Polypodiaceae and the foraminifer assemblage is Neogloboquadrina pachyderma-Globigerina falconensis-Epistominella naraensis. This layer belongs to the predelta facies.Ⅱ. Environmental evolutionAt about 15 000 years ago, the sea level began to rise, so the sea water intruded along the valley at 12000-10000 years ago. Then the predelta area was turned into the littoral-shallow sea environment, and the delta front area was transfered into the submerged valley-littoral sea environment.Due to a warm climate, the sea level at the middle Holocene was close to the present level, the delta front and predelta became predelta and shallow sea respectively. At about 6000 years ago, the Changjiang Estuary was sitllated in the zone of Zhengjiang or Yangzhou, and it was a funnel-shapped estuary. During the last 3000-2000 years, with the exploitation of the Changjiang basin,abundant sediments from the downstream made the delta expand seaward. Because the south bank expanded, and the sandbars were attached into the northbank, the width at the mouth was narrowed. Meanwhile, in the different parts of the Estuary,the islands of Chong