Discussion On The Genesis Of The Yangtze Shoal

The East China Sea shelf, which is one of the most wide continental shelf in the world, has three features as follows:a wide and flat terrain; a variety of land-based sources for sediments; a rapid subsidence and strong power causes. Yangtze shoal is located in the north of the East China Sea shelf 30.7°N-32.6°N, 122.5°E-125°E, with water depth, slope slowly, rich sediment supply and complex Marine dynamic environment, etc. In the process of sea levels rising, trends were very strong and were the main force to move the sediments. Sand deposition is widely developed on its surface. Many Chinese scholars had researched the reason with different ways, comprehensive different views to have two major points:residual depositions and modern tide depositions.I analyze the seabed topography in the southeast of Yangtze shoal and discusses the reason about it in this artical. Then I discuss the reason about the Yangtze shoal on its basis combining with the modern trend, particularly with characteristics of a typical micro-topography.This article is based on the national 908-1-DX19 Project detected the seabed topography in the southeast of Yangtze shoal with SM2000 multi-beam detection system. Then I analyze the features on the seabed and discuss its reasons. The results show that:There are three sand ridges and two grooves in this areas, two grooves between three large sand ridges; some erosion gully,erosion scarps on the top of the sand ridges whose micro-topography of the top of the three Sand ridges is almost at the same depth, around the -39m; some erosion gullies and scouring pit on the top of the grooves as well.The direction of the sand ridges in this area is inconsistent with that of the modern trend. That turns to be the coastal sand, the residues which is from the original sedimentary materials eroded by the trend, during the process of sea-level rising. The erosion gullies and the scouring pit on the top of the grooves were the result of tidal erosion. The shallow spots at the top of the three sand ridges are in the ancient coastline. The developing erosion grooves and scarps were formed as the waves eroded the coastline. That proves to be the residual ancient shoreline topography.Yangtze shoal sand ridges, the East China Sea continental shelf and the northern Jiangsu Shoal are a series of formations in the process of the sea level rising, having the succession in the tidal sand ridge system.Two large sand ridges of the Yangtze Shoal are relative flat in the north, steep in the south. Meanwhile there are a large number of flat sand waves at the north side of the ridges. The sand ridges were developed in the south of Yangtze Shoal, while sand waves were developed in the north of the bridges. Its formation and evolution law basically conforms to the relationship between the geomorphology and current velocity of the trends. It is the erosion-accumulation tidal sand ridge topography with consistent direction of the ancient trend of NNW-SEE. It is formed by the strong effect of reciprocating trends.Synthesis that:Yangtze Shoal is the residual sand of continental shelf formed during the sea-level rising since the last glacial. Its surface is subjected to erosion and transformation of the dynamic of the modern trend, and is adapting to the modern hydrodynamic environment gradually.