Sedimentation Features In The Subaqueous Yangtze Delta

Due to three-decade changes of natural and human impacts and the ongoing Three Gorge Dam project in the upper reach of the Yangtze River, it is necessary to investigate and study ad hoc the subaqueous Yangtze delta following the two systematic surveys during 1960s to 1980s. This paper is based on the author's 2002 geological samplings and geophysical surveys, in situ.In September to November 2002, 163 surface samplings, 24 vibro cores up to 3m, 440km sub-bottom profiles and 240km sonar scanning with synchronous echo sounding were carried out under the author's leadership in order to have a comparative study after the completeness of the Three Gorge Dam project in the subaqueous Yangtze delta and also to probe the recent change in these 30 years accordingly.Based on analyses of grain size, foraminifera, ostracoda, carbonate, organic carbon and other geochemical data of surface sediments, the study area can be divided into two parts, i.e. the modern sub-aqueous delta and late Pleistocene delta. The modern sub-aqueous delta can further be classified into Zone-A, Zone-B, Zone-C and Zone-D. Zone-A is delta front with clayey sands and fine sands as its dominant components. Zone-B is prodelta with clayey silt. Zone-C is the transition from prodelta to continental shelf with sand-silt-clay. From A to C, grain size tends to be finer with weakening dynamics from Pejrup diagram. There is a good positive correlation between organic carbon and clay percentage with low carbonate. Zone-D is relict late Pleistocene delta sand with high contents of oxidized tests, high abundance and percentage of foraminifera and ostracoda and high percentage of carbonate. Based on comparisons with surface sediment types in 1960-1970s and 1980s, the modern sub-aqueous delta is expanding seaward and mixing with the relict late Pleistocene delta sands, meanwhile nearshore sediment is coarsening and sub-aqueous sand ridge is developing. The decrease of fluvial sediment to the estuary has led to coarsening of bottom sediments in the modern delta and shrinking of modern depocenter. The total concentration of LREE in the study area with an average of about 160ppm is 4 times greater than that of HREE with an average content of about 42ppm. LREE , HREE and total REE concentrations in modern delta front area are greater than those in modern prodelta, transitional area from modern to Late Pleistocene delta, and Late Pleistocene delta area in turn. Chondrite normalized REE distribution patterns of different areas show the similar mode like shale but different from deep-sea clay, which indicates a terrigenous sediment source. With the intensifying of marine factors and the weakening of river agents, Eu depletion increases with an average content of 0.67 at a range of 0.65~0.72. Ce depletion cut of South Branch around 122° 30' E, 31° N have relation with the estuarine front and water mixing while Ce depletion in the Late Pleistocene delta maybe caused by Taiwan warm current and reduction condition. REE desorption and dissolution in the surface sediments dominates their REE distribution. Heavy minerals, grain size and desorption control REE in thesurface sediments of the subaqueous Yangtze delta.There are sub-aqueous and buried sand ridges between modem and late Pleistocene delta. Plastic deformations including diapirs are easily developed in the water-saturated trough parts of ridges due to erosion by strong marine dynamics and pressure loss of sediment layer. The author explains strata plastic deformation using a simplified model. But the phenomenon needs co be proved by further investigation.