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

Key words: 
• mouth bar /
• microgeomorphic units /
• microfacies /
• Zhujiang river /
• present deposits

Abstract: 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 betweenNo.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.