长江河口地区晚更新世晚期以来沉积环境的变迁

本文所述长江河口地区,指河口附近的陆上部分,包括北部的三角洲平原,东部的滨海平原及西南部的冲积一湖积平原。全区除局部见有一些基岩山体以外,绝大部分为第四系覆盖区。整个第四系岩相层序比较复杂,反映沉积环境有过多次更替。

EVOLUTION OF SEDIMENTARY ENVIRONMENT OF THE CHANGJIANG ESTUARY AREA SINCE THE LATE STAGE OF LATE PLEISTOCENE

Based upon the lithofacies criteria, the vertical sequence of the sediments of the Chang-Jiang River(Yangtze)estuary area since the late stage of Late Pleistocene can be divided into three portions in. ascending order:(1)the late stage of Late Pleistocene terrestrial,(2)the Early Holocene fluviallittoral-offshore and(3)the Middle to Late Holocene offsho-relittoral and estuary-fluviolacustrine sediments.The late stage of Late Pleistocene was a period of falling sea level and the sediments were mainly fluvial. By the end of the period as the rivers intensified their erosional activity, an erosion surface was formed with a dark palaeosol zone in the eastern part.Early Holocene was a period of transgression, which reached its maximum at early Atlantic stage, except a few higher portions. The pre-existing erosion plain was submerged totally below the sea level.The Middle and Late Holecene were marked by the extensive development of deltas and coastal plain. They may be further divided into three sub-stages, i.e., the early(7000 -4000 years B. P.), the middle(4000-1500 years B. P.)and the late substage(1500 years B. P. to present). The progradation rate gradually increased from early to late substage.The development of palaeogeographic environment since the late stage of Late Pleistocene was related closely to the changes of sea level. For example, the Holocene sea level was characterized by:(1)succession of a rapid rising and relatively stable periods before and after the most intensive Holocene transgression;(2)reaching about O M at the early Atlantic stage, 7000 years B. P. with the maximum transgression at the high level of about 2.5-3 M above present sea level during 7000-4000 years B.P.. Since that time i.e., 4000 years ago, the sea level did not show any langer fluctuations with changes in the range of 2-3M.