SEDIMENTARY STRUCTURES AND GRAIN SIZE PATTERNS OF THE GEOMORPHIC UNITS IN THE YANGTZE RIVER MOUTH
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摘要: 根据165个柱状样顶部1 m的沉积构造和粒度分析结果,对长江口各地貌单元的沉积物特征进行了规律性总结,以应用于钻孔沉积相的判断。结果显示,河口心滩、上河口段落潮槽和拦门沙为主要砂质沉积区,属于牵引流沉积,分粒级频率曲线的峰值主要出现在63~125 μm及125~250 μm粒级,且峰值在125~250 μm粒级的样品向海逐渐增多。而残留砂沉积物粒径大、分选好,特征明显。其余地貌单元均以泥质沉积为主,典型特征有:(1)潮坪层偶发育,砂质纹层密集;(2)在分粒级频率曲线中,口内涨潮槽主峰以μm为主,口外涨潮槽变粗为31~63 μm占主导,且砂质薄层增多;(3)下河口段落潮槽纹层很多,极薄纹层占主导;(4)南汇边滩分粒级曲线常见双峰现象,平均中值粒径在泥质沉积区最大,部分沉积物位于C-M图的递变悬浮段;(5)三角洲前缘复式河槽沉积物明显细于其他前缘沉积,反映了滞流点附近的弱水动力特征;(6)前三角洲为均质泥,纹层少见,为长江口区最细的沉积。Abstract: 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 atμm 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.
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Key words:
- sandy laminations /
- frequency curve of grain size /
- C-M plot /
- stagnation point /
- Yangtze River mouth
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