冰后期海岸层序的初步研究

PRELIMINARY STUDY ON POSTGLACIAL COASTAL SEQUENCES OF CHINA

本文着重讨论了冰后期海岸层序下伏层和不整合面的性质和特征,层序本身的沉积相组合、厚度变化规律,风暴沉积的类型和分布,层序内部的连续性和改造,以及构造沉降带和隆起带海岸层序的差异。

The postglacial coastal sequences formed during the processes of trans-gression and regression with unconformity at the bottom.Two basic types ofpostglacial sequences in coastal zones of China have been distinguished.Theone in uplift belts consists of superimposed littoral facies,with underlyingbed being bedrocks or old Quaternary beds.The other one was formed indepressed belts,the lower and upper parts of whick are littoral facies withneritic one in between.Terrestrial stiff sand clay of dark green colour thataccumulated during low sea level is the underlying bed.The depressed beltspinched out progressively from coastline to continent,leading to superimposi-tion of lower and upper littoral facies,which then were replaced by terres-trial facies.The storm deposits are well developed in postglacial sequences and showvarious characteristics in different areas and sedimentary units.A few fora-minifera contained in dark green stiff sandy clay are considered as a resultof storm effect.Sand beds with shell fragments and glauconite are storm de-posits and are widely distributed in littoral facies at the northern flank of theChangjiang Delta.In neritic facies of the coastal sequences,the storm depo-sits are represented by sandy gravels with shell fragments in the coastal zones of Beibu(Tonkin)Gulf,and by sand layers that have lower inverse-graded bedding and upper graded bedding in the north flank of the Chang-jiang Delta.In tidal flat deposits of upper littoral facies,there are boulderclay and sand layer of 10-15 cm thick,which are also considered to bestorm deposits.The postglacial sequences in delta areas had been regarded to be conti-nuous without any serious erosion.The study of coastal sequences in deltaareas shows that the coastal sequences developed in ancient valleys and atinterfluve with strong tidal currents,such as Jianggang,often subjected toevident reworking.Only coastal zones where currents are usually weak,se-quences are continuous.