珠江磨刀门河口交杯四沙现代沉积特征及其形成机理

Present Deposits and Its Formation of No.4 Jiaobei Sand at Modaomen Estury,Zhujiang River

通过对交杯四沙表层现代沉积考察,分析了滩面沉积微地貌类型,并结合粒度分析和成分分析,探讨不同微地貌沉积特征,进而讨论了交杯四沙的形成过程。研究结果表明:交杯四沙表层发育有低潮线以下、低潮位波浪冲洗带、高潮位冲洗带、风暴潮冲洗带、滩顶冲越带、滨后冲越带、冲越扇中部、冲越扇前缘斜坡以及分流间湾区等9个微地貌单元,代表了9个微相分区。交杯四沙滩面主要以粉砂和细砂为主,整体上分选性中到差,矿物成分主要为石英和黏土矿物。交杯四沙东侧是磨刀门主河槽,以径流为主;西侧沿白藤海、灯笼沙至三灶岛水道以潮流为主。交杯三沙以南,四砂以北构成了分流间凹地环境。交杯四沙以南为东南向的波浪作用带。交杯四沙和其他交杯沙系列沙体具有相同的形成过程,首先由河槽底流将拦门沙沉积物向西搬运沉积形成浅滩,浅滩受潮流和波浪的改造其平面形态呈酒杯状,浅滩的东南面受东南向波浪的作用,前坡遭受侵蚀,在后坡产生堆积,由此沙滩逐渐向陆迁移,先后形成交杯一沙、二沙、三沙和四沙,交杯一沙、二沙和三沙已经合并成陆。随时间的推移,交杯四沙也将与交杯三沙相接成陆,新的交杯五沙有望形成,磨刀门西侧浅滩区将演变成由一系列反曲沙脊和潮滩相间而成的三角洲"滩—脊"平原。

Present sediments investigation have identified 9 microgeomorphic units on the surface of No. 4 Jiaobei Sand,Modaomen estury,Zhujiang river,which represent 9 micofacies of this sand. These microgeomorphic units and microfacies include：（ 1） subwater zone,（ 2） lower tidal swash-backwash zone,（ 3） higher tidal swash-backwash zone,（ 4） storm swash-backwash zone,（ 5） bar top washover zone,（ 6） backshore washover zone,（ 7） middle washover fan,（ 8） marginal washover fan and（ 9） inter-distributary wetland. Sediments are mainly fine to silty sand with middle to poorly sorting,minerals are mainly quartz and clay. The fluvial dominated Modaomen main channel developed in the east of the Jiaobei 4 sand while a tidal channel developed along from Baitenghai to Denglongsha and Sanzao island in the west. The southeastward wave played a dominate role in the south of Jiaobei 4 sand and an inter distributary wetland was situated between No.3 and No.4 Jiaobei sand area. Similar processes have been for forming No.4 and other Jiaobei sands. Bottom current within the main channel transported the mouth bar sediments laterally to form a shoal in the right side of the channel,then the shoal was reconstructed into cup-like No.1 Jiaobei sand by tidal and wave. The wave eroded the shore face of the No.1 Jiaobei sand and deposited the sediment at the wave-shaded area,which make the sand accrete landward and then form the No.2,No.3 and No.4 Jiaobei sand in front of it one after another. No.1,No.2 and No.3 Jiaobei sand are now connected to form Jiaobei island above the sea. The No.4 Jiaobei sand may connect with No.3 Jiaobei sand,and then a new No.5 Jaiobei sand will appear in the future. As a result,the western bank of the Modaomen main channel will be changed into a sand ridge plain with tidal flat and recurved ridges.