Grain-size characteristics of the extant tidal flat sediments along the Jiangsu coast, China

Five types of sediments have been identified for the tidal flat deposits along the Jiangsu coast: (1) fine sand, (2) sandy silt, (3) silt, (4) clayey silt, and (5) clay. Among these, the silt deposits (including sandy silt, silt and clayey silt), with median grain diameter of 4-7 phi, are the dominant sediments within this particular intertidal environment; they normally have good sorting, and are symmetrical or positively skewed and single and/or double modal in grain-size distribution. This type of sediment, in terms of grain-size characteristics, has a very close relationship to its source sediments provided by the two large rivers in the region, i.e. Changjiang River and Huanghe River. The dominant mode for the sediments transported into the environment is suspended load due to the active tidal regime over the region. Grain-size diameters decrease normally from offshore towards onshore in two forms along the Jiangsu coast: (a) a step-like curve, with sharp change occurring at the mud flat; and (2) a smooth concave-up curve. Comparing these results with their coastal topographic profiles and sediment dynamic features, it is found that form (1) is related to steady accretional/progradational tidal flat profiles in the middle part of the coastline, and form (2) to erosional/stable profiles near the old river deltas. The differences in the grain sizes and distribution patterns of the sediments between the northern and the southern parts of the region are mainly due to the difference in sources, i.e. Huanghe River sediments dominate the northern part and Changjiang River sediments prevail in the southern part. Except this fluvial control, the modern tidal hydrodynamics has also played an important role in the distribution of the coastal and nearshore sediments: this is particularly so over the tidal flats, rather than in the subtidal/offshore area.