Study on the linear sand ridges on shelf of the East China Sea

The linear sand ridges (LSR) revealed by newest multi-beam echo soundings bathymetric data (MBES) are distributed extensively on shelf of the East China Sea (ECS). It is not only a kind of ubiquitous geological phenomenon on tide-dominated shelf but also a key step in paleo-environment evolvement history of the ECS. Based on the MBES, high-resolution single-channel seismic profiles, analyzed results of boreholes and numerical simulation of paleo-tidal current fields, the distribution, 3D fine structures , space-time spreading regulars and developing tendencies of the LSR on ECS shelf were studied by quantitative synthetic statistical analysis method. The relationship between LSR and paleo-tidal current field, sea-level curve and the evolution stages of LSR such as formation, growth and buried stages were discussed. The strikes of LSR on ECS shelf emerge at a normal distribution. The azimuth of N155°E is the central point and the azimuth of N125°E,N130°E,N140°E and N180°E are convergent points respectively for the normal distribution. The LSR are aggregating in the centre part of ECS shelf, rarefying at the north and south part, dispersing and bifurcating to the east, aggregating and converging to the west. The LSR on ECS shelf are distributed landward to the isobath of 60m, and seaward to the water depth of 120m at northeast and 150m at southwest. Immature LSR are firstly observed at water depth of 130-180m in the southwestern depressions. Lithology analysis and dating of 4 boreholes and 12 cores have indicated that the widely distributed transgressive sand layers with high content of shell debris which formed in the early to middle Holocene are the main compositions of the LSR on the ECS shelf. The top boundaries of buried LSR in unit 14 are distinguished, and a 3D map of these buried LSR in local area is reconstructed. The features such as length, width, height and strikes of these buried LSR are analyzed quantitatively and compared with those of LSR in unit 2. The paleo-tidal current fields of ECS shelf at different sea level periods since LGM are recurred and their relationships with LSR are discussed. Three sketch models of the shelf LSR are established, and the developing processes such as the formation, growth, buried and moribund stages of the LSR on the shelf are explored. The LSR on ECS shelf are formed mainly in four periods, that is, before 14.5kaBP, 12~14kaBP, 11.5~9.5kaBP and after 9kaBP. The development of these LSR are influenced apparently by 3 Melt-Water Events (MWP-1A, MWP-1B and MWP-1C). On the strength of multiple data such as topographic maps, MBES, single-channel seismic profiles, sub-bottom profiles, side-scan images, and lithology analysis et al., some features of the LSR in nine regions of Bohai Sea, Yellow Sea and ECS are analyzed and contrasted, such as the lengths, widths, heights, strikes, structures, lithology and formation date et al. And these LSR are classified according to the dynamic force, structure, appearance and activity. The sequence stratigraphy formed since 100ka are compared in space-time field based on a long profile acquired on ECS shelf.