Study On The Sedimentary Environment Evolution Of Xiaomiaohong Tidal Channel Of Radial Tidal Sand Ridge System Since The Late Pleistocence In Jiangsu Offshore, South Yellow Sea

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.