SEDIMENTARY STRUCTURES AND GRAIN SIZE PATTERNS OF THE GEOMORPHIC UNITS IN THE YANGTZE RIVER MOUTH

The sedimentary structures and the grain size distribution patterns of the top 1 m of 165 short cores are described and summarized in this paper in accordance with their geomorphic units in the Yangtze River mouth, and applied for identification of depositional facies. Results show that the traction sandy deposits dominated in channel bars, upper-estuary ebb channels and mouth bars characterized by the frequency curves of grain size peaked at 63~125 μm and/or 125~250 μm with the 125~250 μm-peaked curve increasing seaward. Remaining geomorphic units are mainly dominated by muddy sediments with typical features as follows:1) sand-mud couplets generally occur in tidal flat deposits rich in sandy lamina. 2) The frequency curves of grain size have a main peak at<4 μm in the inner-mouth flood channel, while in the flood channel offshore grain size composition peaks at 31-63 μm with increasing sandy laminas. 3) Laminations, predominantly very thin laminations are popular in the lower-estuary ebb channel. 4) Sediments from the Nanhui Shoal are characterized by double peaks in frequency curves with the highest average value of the median in the muddy area. Some samples appear in QR region in the C-M plot. 5) The sediments deposited in the compound channel offshore are significantly finer than that in other units of the delta front, which reflect the weak hydrodynamic near the stagnation point. 6) The prodelta mud is rather homogeneous, which consists of the finest sediment at the Yangtze River mouth.