长江三角洲之沉降

大量第四纪钻孔揭示了长江三角洲南北平原在基底构造和地层分布上存在着明显的差异。前第四纪的三角洲北部是一个巨大的沉积盆地。新第三系沉积地层可达一千多米 ,而此时南部却是由古生代和中生代火成岩、碳酸岩和沉积岩组成的台地 ,第三系沉积地层几乎缺失。然而 ,第四系地层厚度在长江三角洲南北两地的分布已经持平 ,且地层最厚处已转移到长江口崇明岛一带 ,全新统地层更有继承此演化特征。这表明了本区沉降中心从北部逐渐向南转移的趋势 ,反映了南部基底构造在第四纪以来有明显下沉的迹象 ,是长江主河道南迁的主要地质引力。由于大量全新世沉积堆积在崇明岛前缘的沉降中心 ,重力压实作用导致该中心前部出现“泥火山” ,周围地层变形明显。本研究提出在长江河口区进行大型桥遂工程建设时应充分考虑到上述由基底改造沉降和重力沉降产生的效应

Yangtze delta subsidence

The Yangtze delta's fluvial channel system and overall depocenter configuration have been controlled largely by movement of active subsurface structures during most of the Quaternary.Data from numerous borings reveal marked variations in lithic distribution and thickness of Pleistocene and Holocene sequences across the delta plain. These stratigraphic difference records shift in the Yangtze's depocenter through time and area in large part a direct response to the interaction of land motion with sea level variations and fluvial and onceanographic processes. Delta plain subsidence has progressed from north to south since early Quaternary time, and this has induced a southward shift of the Yangtze river channel from the early Pleistocene to the present. 　　During much of Quaternary time, the Yangtze river flowed across the northern part of the delta's plain. This is primarily the results of two factors:differential subsidence of the northern sector from the upper Tertiary to the late Pleistocene;and a “topographic barrier effect”produced by highlands which blocked the Yangtze from flowing to and across the southern sector of the plain. Thus,fluvial Quaternary deposits in the southern delta plain. Therefore, the strata configuration,texture and composition can serve to distinguish Yangtze river deposits concentrated in the north from the more proximal fluvial channel sediments in the south. Our investigation suggests that tectonically induced channel migration and southward shift of the Yangtze deltaic depocenter are continuing at present. These ongoing depositional changes related to differential subsidence have potentially serious ramifications for this low-lying, densely populated delta plain area. Hence, accurate measurements of land subsidence are needed throughout the plain, and particularly in the lower reaches of the Yangtze. 　　The Holocene Yangtze depocenter is located on the outer Chongming island, where more than 60m of sediment sequences are accumulated over the past 7000 years. This thickened sediment unit has caused a gravitational subsidence in the unconsolidated to have formed a series of mud diapirs in the front of the depocenter and sediment strata adjacent to the depocenter are largely deformed.