【作者】 李延军；

【导师】 殷勇；

【作者基本信息】 南京大学 ， 海洋地质学， 2012， 硕士

【摘要】 南黄海辐射沙脊群是晚第四纪长江、黄河沉积物受潮流作用改造形成的巨型陆架地貌单元,是海陆交互作用的典型产物。小庙洪潮流通道位于沙脊群区最南部,也是深槽距岸最近的一条大型滨岸潮流通道,作为该地区继承古河谷性质的潮流通道的典型代表,研究小庙洪水道晚更新世以来沉积环境的长周期演变以及近10年来的悬沙浓度变化,对于了解该水道乃至辐射沙脊群南部地区的成因机制、动态发展趋势和古长江下切河谷的发育具有重要意义。2008年2月在江苏吕泗港岸外小庙洪潮流通道的牡蛎礁上(32°8.988’N,121°32.821’E)实施07SR11孔的野外施工,钻孔总进尺70.9m,实际取得岩芯51.2m,取芯率达到了72%。之后在室内进行钻孔描述和编录工作,并采集样品进行粒度、磁化率、有孔虫、粘土矿物及地球化学元素的测试和分析,结合同位素测年及其它环境替代性指标,对小庙洪潮流通道的长周期演变和成因机制进行综合分析。同时运用遥感手段对1999～2010年间27景ETM+图像进行悬沙浓度的反演,分析在长江输沙量剧减的情况下该海域的悬浮物质含量变化,预测在物源减少的情况下小庙洪潮流通道及辐射沙脊群南翼地区未来可能的演变。小庙洪07SR11孔沉积物以粉砂和砂为主,平均含量为48.9%和47.2%,黏土含量较少,平均含量仅为3.9%,平均粒径在1.6～6.5Φ中之间波动,平均值为4.3Φ中,分选性较差。从整个钻孔来看,沉积物下粗上细,下部以细砂、中砂为主,向上粉砂、黏土含量增多。黏土矿物以伊利石为主,其次为高岭石、蒙皂石,绿泥石含量最少,根据黏土矿物可以判断该地主要以长江物质为主,但在钻孔底部和顶部,可能有少量黄河物质掺混影响。根据沉积构造、粒度特征、古生物特征等,将该孔自上而下划分为5个沉积单元：(1)河床相(33.71-70.9m),沉积物以细砂、中砂为主,黏土含量很少,沉积物中几乎没有有孔虫,在底部见咸淡水双壳类化石混杂堆积,推测该河流环境受到一定的潮水影响。(2)河漫滩相(23.05-33.71m),沉积物以砂质粉砂和粉砂质砂为主,发育粉砂—黏土互层的水平层理、微波状层理和爬升交错层理,有较多的粉砂质透镜体,未见有孔虫,个别层位有铁锈斑和暴露标志。(3)硬黏土层(22.25-23.05m),中黄棕色粉砂与黏土,已硬结,见很多红褐色的铁锈斑、黑色的碳质斑点以及淋溶淀积形成的钙质结核。属于全球低海平面时期(MIS2)暴露环境下形成的陆相沉积,广泛分布于长江三角洲地区,与上覆地层有明显侵蚀界面。(4)河口湾浅海沉积相(4.02-22.25m),沉积物以粉砂和砂质粉砂为主,黏土与粉砂交互成层,多发育水平或波状层理,多沙斑透镜体,生物扰动明显,有孔虫丰度整体上较高,以广盐型属种为主。属于全新世海侵层序。(5)潮控滨岸相(0-4.02m),发育为以泥质沉积为主的沙泥互层,具向上变细的沉积序列。发育波状交错层理,夹有厚层砂质透镜体,生物扰动作用明显,含有大量的牡蛎壳。有孔虫组合面貌与下覆地层类似。粒度分析显示,跳跃组分多呈双跳跃特征,显示出双向水流特征,与潮流通道中往复流特点一致。该段沉积显示潮流通道充填沉积的特点。小庙洪潮流通道长周期演化显示它是沿袭长江古河道,并在此基础上发育而来。晚更新世大约60-70ka前(相当于氧同位素4期),古长江由此入海,形成下切河谷,在氧同位素3期的海侵过程中(当时最高海平面要比现在低50-60m),古切谷相继充填河床相→河漫滩相沉积,总厚度达47.86m,占整个钻孔的67.5%。在氧同位素2阶段(14-29ka cal B.P.),随着海平面下降,该地广泛暴露,发育标志性的硬黏土层。暗示此时古长江已离开研究区,南迁到现在的长江三角洲区域。进入全新世,随海平面上升,该地被淹没成为古长江河口湾(鼎盛时期的顶点在镇江—扬州)的一部分,发育河口湾—浅海相沉积。直至距今2000-3000年,随着河口湾淤积速度的加快,至唐宋年间古长江北支淤塞,形成了古三余湾,伴随着海堤的逐步完善及黄河南泛带来的大量泥沙,古三余湾逐渐淤积成如今的形态,该地最终由河口湾变为潮控滨岸环境,原残留古河道在潮流作用改造下逐渐发育为现今的潮流通道。通过对该地区1999-2010年27景遥感图像进行悬沙浓度的反演表明,该地区悬沙浓度平均值基本上是冬半年明显大于夏半年,岸滩、沙脊附近大于深槽,近岸大于外海。长江的输沙量与海区夏季悬砂浓度的平均值之间呈正相关关系,在三峡大坝修建后长江输沙减少,势必会减少该地区的物质来源,有可能会加剧沙脊群南翼沙洲与岸滩的侵蚀以及槽道的刷深。更多还原

【Abstract】 The South Yellow Sea radial sand ridge system is a giant geomorphic unit on the Yellow Sea shelf, coused by the tidal influent on the sediment accumulation body of the Yangtze River and the Yellow River since the Late Quaternary, and is a typical product of the sea and land interactions. The Xiaomiaohong tidal channel located at the most south of the sand ridges area, north of the Yangtze River delta. Also, the distance from the shore to the deep channel is nearest. As a typical which inherited the ancient river valley, study on the sedimentary environment changes in a long time and the SSC in1999-2010of the Xiaomiaohong tidal channel is of great significance to understanding the genesis and development of channel and even the southof the radial tidal sand ridge systems and the acent Yangtze River valley.We got the corel at the February2008, located on an oyster reefs in the Xiaomiaohong tidal channel, Lvsi Port offshore. The bottom of the hole is70.9m, but only got51.2m sediments; the rate is about72%. Then the core was logged in lab, and then series of analys such as clay minerals, grain size, geochemical elements, paleontological and so on were carried out. Combined with datings and other environmental indicators, the formation and long term evolution of the tidal channel was analyzed. Meanwhile we used27ETM+images between1999and2010to get the changes of the suspended sediment concentration, under the background of the Yangtze River’s sediment discharge sharply declined. Forecast the evolutions of the Xiaomiaohong tidal channel and even the south of the radial sand ridge areas under the background of the sand source reduction.The results have shown that the size distribution is dominated by sand and silty, with the average of48.9%and47.2%, clay only with the average of4.0%. The mean size changed between1.6-6.5φ, average of4.3φ, with poor sorting. From the all of the corel colums, partical size is fine and have more clay and silty at the upper, but the lower part is corse, mainly of fine sand and medium sand. X-ray diffraction has shown the lllite has the highest content, followed by kaolinite, smectite and chlorite, shown that the sediments mainly from the Yangtze River, but in the bottom and top of the corel the Yellow River materials may impact the areas, and mixed in the sediments.Five sedimentary faces have been distinguished:(1) River bed deposits (33.71-70.9m):it consists of fine sand and medium sand,with little clay. In the sediments there are almost no foraminifera, but at the bottom salt and fresh water shells mixed together, the tidal may impact the river at that time.(2) Floodplain (23.05-33.71m):composed of sandy silt and silty sand, Horizontal laminations, small ripple beddings and climb staggered bedding are common in this interval by silty and clay. There are many silty lenses, but no foraminifera, only at2315cm, we found a shell fossil, and called Turbonillanonlinearis Wang some, at some layers iron rust and expose signs can be found.(3)Paleosol (22.25-23.05m):composed by hard yellow-brown silt and clay. There are many reddish-brown iron rusts, black carbon spots and calcareous nodules in this layer. The palesol formed under oxidizing conditions in MIS2when the global sea level was very low, paleosol are widely distributed in the Yangtze River Delta region. There is an erosion interface at the top.(4) Shallow marine estuaries (4.02-22.25m):mainly of silty and sand silty, which formed in MIS1and is s transgressive sequence. There are mainly horizontal laminations and wavy beddings with clay and silty interval, have many sandy lensse and bioturbation. In the sediments foraminiferal is abundant, mainly to the wide salt type.(5) Littoral facies controlled by tidal (0-4.02m):the particle generally top-down coarsening, mainly of sand and mud interbedded, formed wavy cross-beddings. There are many sandy lenses and bioturbation. The particle size analysis shows that the jump was a double jump characteristics, reprenst two-way flow characteristics which is similar in the channel. At the top of the corel there are many oyster shells, the foraminiferas ware similar to the overlying strata. The sedimentary shows that the tidal channel has a filling deposited features.The long term sedimentary environment evolution shows that Xiaomiaohong tidal channel fllowed the ancient Yangtze River valley. In Late Pleistocene the region, about60-70ka B.P.(MIS4) the Ancient Yangtze River into the sea at Xiaomiaohong, and formed the incised valley, but durning MIS3transgression when the highest sea level was50-60m lower than today, the valley had been experienced the Yangtze River bed and floodplain sediments,the thickness up to48m(the length of the corel is70.9m). Then in MIS2(14-29ka cal B.P.), the sea level is very low, the land exposed and developed paleosol which means that the Acent Yangtze River had go away to the south where is the Yangtze delta now. After entering the Holocene, the sea level rised rapidly, the channel became part of the ancient Yangtze River estuary which the mouth was at Zhenjiang and Yangzhou City at its heyday, and developed shallow marine estuaries sediments. But from2000-3000aBP the speed of estuarine siltation accelerated, until Tang ang Song dynasties the north branch of the ancient Yangtze River blockaged, became the ancent Sanyu Bay. Along with the building of seawall and the Yellow River brought a lot of sediment after1128AD especially after1592AD, Sanyu Bay gradually filled, and became littoral environment controlled by tidal from marine estuaries, the residued river bed developed into the modern tidal channel under the impact of tidal fluents.We used27ETM+images between1999and2010to get the suspended sediment concentration, shown that SSC mainly lower in summer half year, higher in winter half year, near shore and sand ridges mainly higer than deep sea. The aveage of SSC in summer have a positive correlation with the Yangtze River’s sediment discharge. After the Sanxia Dam was built, the Yangtze River’s sediment discharge sharply declined, the Xiaomiaohong tidal channel and even the south of the radial sand ridge areas may be sesiously erased under the background of the sand source reduction.更多还原