关键词: 潮坪沉积, 粉砂质泥, 水平纹理, 波痕, 粉砂层, 揭片, 交错层理, 中潮, 交替层, 图版

Abstract: This paper deals with the sediments of intertidal flat of Southern Part of Yangtze Delta. Field study was carried out in two sub-coastal segments: Eastern Intertidal Flat, from Nanhui Cape northward, and Southern Intertidal Flat, from Nanhui Cape westward.The characteristics of Shanghai Intertidal flat are as follows:1. The sediments of the studied intertidal flat are predominantly supplied by Yangtze River discharge, and generally fine in grain size. Even the sediments of low tidal flat, which rank the coarsest in whole intertidal flat, are merely coarse silt, their average size is 0.037 to 0.038 mm in diameter, and the overall content of grains larger than 0.063 mm is below 10%.2. The average tidal range varies from 2.7 to 3.5 m, while wave action is fairly weak in normal sea climate owing to the consumption of wave energy after long jour-ney across wide and gentle subaqueous slope, though no prominent topographic handi-cap such as barrier island exists. Therefore, tidal action plays the dominant role. It can be expected that the grain size of sediments becomes progressively finer, the mud content gradually increases, and sorting becomes worse from low tidal flat landward to high tidal flat. In sedimentary structure, the characteristic features of high tidal flat are horizontal laminae of silt and silty mud. Those of mid tidal flat are predo-minantly alternate beds of silt and silty mud, of which, muddy layers often show wavy structure, while silty layers are in continuous or discontinuous lenticular form; reac-tivation surfaces occur. Those of low tidal flat are predominantly small scale cross stratification; horizontal silt laminae and wavy muddy silt layers are also common; neither herringbone cross stratification nor "B-C" sequences appear.3. Both intertidal and subtidal vertical profiles distinctly show rhythmical stra-tification, which consists of alternate bands of coarse grains and fine grains, Typhoon, occasionally prevailing in late summer and autumn, raises sea level temporarily and is accompanied by violent wind wave action, causes powerful effect of coastal erosion and deposition, and is responsible for the formation of coarse-grained bands. In con-trast, the fine-grained bands are accumulated in the state of low-energy under ordinary tidal and wave condition throughout the year.4. The progradation rate of intertidal flats is very high. In the last 6 years, it was about 100 m/year in the Southern Tidal flat region; while in the Eastern Tidal flat region, it was about 60 m/year. Because of the high rate of accumulation and scarceness of burrowing fauna, the whole intertidal flats are slightly burrowed ordisturbed.5. The environment of intertidal flat is greatly affected by human activities. In accord with progradation rate, embankments were built in time near the edge of mean high sea level. Inside the embankment, instead of natural tidal flat, it becomes cul-tivated field. Outside the embankment, reeds grow extensively, which obviously accelerate progradation rate.