Submarine Bedforms And Formation Cause In The Outer Shelf Of The North Of The East China Sea

The East China Sea shelf has three characteristics as follows: a wide and flat terrain; a variety of land-based sources for sediment; a rapid subsidence and strong power causes. On the outer shelf of the East China Sea undersea ridges and troughs are widely distributed consecutively, the study referred to as tidal sand ridges shaped by the postglacial transgression process. A lot of researches on the submarine bedforms of the East China Sea continental shelf ridges have been done by both domestic and foreign scholars, and a large number of meaningful results have been obtained. But the study area mainly concentrated in the southern part of the East China Sea. The selected area in this paper is located in the outer shelf of the north of the East China Sea near Cheju Island. It's an extension part of the Yellow Sea Trough turns towards the Okinawa Trough. It's in the pathway of the Yellow Warm Current which is one of the branches of the Kuroshio Warm Current. The analysis about the seabed morphology, sediment distribution and formation mechanism will contribute to an overall understanding of the formation, evolution and the characteristics of the pathway.Around the study region high-precision multibeam sonar data, geological sampling data, shallow stratigraphic data and hydrological data are all collected, applied with modern data processing technology,quantitative analysis methods and tidal power geomorphologic theory, a detailed report including Rippled Scour Depressions (RSDs), three-dimensional morphology of the seabed sand ridges and distribution of seafloor sediment is characterized. Under such a strong hydrodynamic condition, the formation cause and evolution process of the ridge-trough submarine bedforms in this area are analyzed. On this theme, following aspects are mainly discussed:Based on the high-precision multibeam sonar data on the study area, a high-precision seabed Digital Terrain Model (DTM) and sonar mosaic image are built to generate high-resolution three-dimensional seabed morphologic maps, as well as slope and aspect maps, special terrain profiles and other surface texture maps. Among those maps accurate seabed RSDs and sand ridges'height, width, length, direction and other characteristic parameters can be measured. A deep submarine gully with the depth of 153.2 meters is found in the northeast corner of the study area. The relative height difference is up to 32 meters indicates strong erosion with dramatic changes in seabed topography. There are more than ten large-scale straight ridge RSDs and sand ridges in the study area. They are all NW-SE direction, in length from a few kilometers to 40 kilometers, between ridge and trough the general elevation difference is about 5 meters, the maximum reach up to10 meters.According to previous research and seabed geological sampling data from the seabed, a detailed analysis of granularity characteristic belonged to thirty seven surface sediment is conducted, the sediment can be divided into six categories: Fine Sand (FS), Silty Sand (TS), Clayey Sand (YS), Sand - Silt - Clay (STY), Clayey Silt (YT) and Silty Clay (TY), combined with the distribution of sediment thickness on the study area the distribution discipline is revealed. Based on multibeam backscatter intensity data and 19 geological seafloor sediment sample data, a statistical model which presents the relationship between seabed backscatter signal and sediment type is set up. Using Self-Organizing Feature Map (SOFM) and improved Learning Vector Quantization (LVQ) neural network methods, a fast and accurate automatic identification for three seabed sediment types (TS, YS, STY) implementation is feasible.Applied with the Princeton Ocean Model, the M2 tidal current velocity, direction and other relevant data in the study region are calculated. And the surveying ocean current information is collected from Japan Oceanographic Data Center (JODC). Advanced data analysis methods are applied to explore the relationship between marine dynamic environment and the submarine bedforms formation. The relationship between modern hydrodynamic and RSDs is the further study. The study found that the basic tidal current direction NW-SE is consist to the seabed shape. But the bottom tidal current rate is generally about 30cm/s, too weak to transport sandy or smaller particle of seabed sediments. The impact of the modern hydrodynamic to the submarine beforms in this region has been quite debilitated. In the other hand, the largest ocean current velocity in the seabed could reach to about 69cm/ s, so it's possible to stir-up and move marine sediments such as fine sand or silty sand. That might lead to a certain erosional and depositional morphology. So, the modern hydrodynamic still have the power to transform and maintain the seabed RSDs and sand ridges.Based on the previous research results, the evolution processes of the M2 tidal current fields can be obtained since the postglacial transgression. Combined with the previous tidal sand ridges research in the East China Sea, a thorough research is conducted to find out how the ancient hydrodynamic environment shapes the formation, distribution and evolution of the RSDs and sand ridges bedforms. It comes to a conclusion that shows submarine beforms on the outer shelf of the north of the East China Sea has been strongly eroded since the postglacial transgression, the seabed material like sandy or silty sediments are started and transported. The seabed erode RSDs and erode-deposit tidal sand ridges morphologies which have the same NW-SE direction coincided with the ancient tidal current are shaped by a strong reciprocating tidal current during the standstill period of the sea level ascending.