THE CHENIERS OF THE NORTHERN COASTAL PLAIN OF JIANGSU PROVINCE
Gu Jiayu Yan Qinshang Yu Zhiying ( Department of Geography,East China Normal University )
Four cheniers exist in the northern coastal plain of Jiangsu Province,namely the West Ridge,the Middle Ridge,the East Ridge and the New Ridge in order from west to east.They run nearly parallel to the present shoreline.The chief depositional features of the cheniers are as follows: 1.Thirty different species of molluscs have been found,of which the predominant species are Mectra quadangularis Deshayes, Cycline sinensis ( Gmelin ) , Meretrix Linne, Chlamys,Ostrea, Umbonium Thomosi( Crosse ) , Ballacta exarata ( Philippi ),Sinonouacula constricia Lamarck, etc.They belong to the fauna of intertidal and subtidal shallow-water environment. 2.Chenier deposits,on the average, are composed of well-sorted fine sands (2.5φ in grain size ),occupying 60-80% .Medium sands come next,ranging from 7 to 33%,while very fine sands occupy 1-7%.Silt and clay are trace in amount.It can thus be deduced that the winnowing force of wave action in the formation of cheniers is rather moderate.Furthermore,the fact that the areal variation of components of different grain sizes becomes gradually small from north to south,demonstrating that chenier sands are transported by the southward longshore current. The cumulative probability curves of chenier deposits show four populations.Two of them are those of saltation,illustrating that the chenier deposits are affected by swash and back wash. 3.The chenier deposits are mineralogically composed of 70-74% quartz,16-20% feldspar and 7-12% heavy minerals.Twenty species of heavy minerals have been identified, of which the dominant minerals are amphibole, epidote. garnet .zircon, magnetite,hematite and toumaline .The assemblage of heavy minerals especially garnet as an indicator,is similar to that of the Huanghe River ( Yellow R. )and that of the Huai River. 4. The internal sedimentary structures are very simple, mostly plane beddings,and low-angle cross beddings with long stretches of 4 - 6 ° seaward dippings and short stretches of 10° landward dippings. After examining the different characteristics of four cheniers and their respective underlying beds, it is realized that they have different historical development .The West Ridge is the remnant of an ancient barrier island, formed at the end of postglacial maximum transgression.The other three cheniers are beach ridges in origin,formed in late Holocene.As a whole, the prerequisite for their formation is a variable longshore sediment supply.In the coastal region studied, when sediment supply from the Huanghe River is abundant, the shore build forward rapidly as muddy silt flat; when the supply is less,wave attack cuts back these flats and concentrates the coarse fraction, mostly fine sands and shells into cheniers. 5.According to radiocarbon ( C~(14))dating.together with historical records and archaeological studies, the development of the coast can be divided into two stages.In the early stage ( from 6500 year B.P.to 850 year B.P. ) the progradati-onal rate of coast is only 1-3 m year, but in the late stage ( from 850 year B. P. up to the present ), the rate rapidly increases to 36-65 m/year. After the abandonment of the old Huanghe River channel in 1855, the rapidly reworking process of irregularities of coastline has been going on. At present, the general coastal outline is being gradually straightened, deposition prevails in the seaward concave segment, and erosion dominates in the convex segment.