SEDIMENTATION AND SEDIMENTARY FACIES OF THE ZHUJIANG (PEARL RIVER)MOUTH

The Zhujiang river mouth is a drainage basin of Zhujiang river system. The amount of yearly average runoff is totally 341.2xl0⁸m³, and the amount of annual suspended silt is 83.36 million tons, 80% of which enters into the Zhujiang river mouth area. A large area of silt-clay or sandy deposits has been formed under the coaction of the river and the sea. Dynamic and environmental features at the Zhujiang mouth are as follows: 1 The discharges of runoff and suspended silt mainly occur during the flood season, and respectively account for 80% and 81--95% of the annul amounts; 2 Water mass of the South China Sea is a constant influence on the river mouthj 3 River flow and tidal current at each river mouth or estuary are different from one another in intensity) 4 Water currents flowing at river mouths or estuaries are to-and-fro currents, but outside they are rotative currents; 5 The nearshore current mainly flows southwe-stward; 6 A large amount of organic matters and chemical elements is carried into the estuary. According to the dynamic and environmental condintions,three types of sedim- entation can be classified at the Zhujiang mouth: 1 Distributary mouths(including Jiaomen,Honggili,Hengmen,Medaomen,Jitimen,Hutiaomen),at which runoff affection is dominent and tidal affection is weaker; 2 Esturies(river mouths are mainly affected by tide), such as, Lingdinyang(Humen) and Huangmaohai(Yamen). Their yearly runoff is far smaller than the tidal current and varies a lot. At estuaries, salt water is mixed with fresh water and a sharp variation occurs because of the flood season and low-water season; 3 Shelf shallow sea, which is not only affected by tidal, sea and inshore currents, but also affected by river fresh water. However, its sedimentary environment is dominated by sea factors. Its main sedimentary facies are river-mouth sand bar, forelandaccretion bed, alluvial-river bed, subaerial or subaquatic levees, interdistributary bay, flood-tide trough, predelta, tidal channel, tidal sand bar, tidal flat and shelf shallow sea.