Study On The Long Term Sedimentary Environment Evolution Of Kushuiyang Region Of Tidal Radial Sandy Ridge Syatem, Northern Jiangsu Offshore, South Yellow Sea, East China

On northern Jiangsu offshore area, a characteristic tidal radial sand ridge system (TRSRS), which contains more than 70 ridges and tidal channels between them, has developed under a complex tidal current field. The TRSRS, which has developed under a land-sea interaction, is a giant sedimentary system on the western coast of Yellow Sea by sediment input from old Yangtze River and Yellow River since the Late Quaternary. The TRSRS, which contains a lot of information about sea level changes, land-sea interaction and climate changes, is the best carrier for studying inner shelf system and coastal evolution since the Late Quaternary. So far, most research is focusing on the southern and western onshore areas. Little attention has been paied on Kushuiyang tidal channel although it has been trated as an important long offshore tide channel (107.5 km in length) in the middle of the TRSRS region. The channel has an average water depth of 15m.Five cores in Kushuiyang region were chosen for the studing, among which five lie on ridges and three on the Kushuiyang channel. The boreholes were drilled to depth between 36.9m and 58.7m. The thesis is aimed to reconstruct the long term evolution and analyze the mechanism and time of ridge-channel system, on the basis of sedimentary facies, grain size, trace elements and chronological datings.Based on lithology, color, grain size, sedimentary texture and structure, fossils and contacted relationship between overlying and underlying strata, four types of sedimentary facies have been distinguished from boreholestidal flats, flooding plan (terrestrial stiff mud), tidal sand ridge and tidal channel.Tidal flats:Tidal flat facies is composed of olive gray clay, black gray clay, silt and a small amount of fine sand. It includes flaser bedding, lenticular bedding, wavy bedding and massive bedding, with amount of shell fragments. The tidal flats can be further divided into high-intra tidal flat, mid-intra tidal flat and low-intra tidal flat.High-intra tidal flat is composed of olive gray, dark gray clayey silt and clay, intercalated with thin layer of silt and lenticular sand body. Horizontal lamina (<1mm in thickness) and lenticular beddings occurred in the facies. Mid-intra tidal flat is mainly composed of light olive gray silt and sandy silt, intercalated with olive gray clayey silt bands or sometime displaying as interlayers of light olive gray silt and clayey silt often occurred in the facies. Flash beddings and wavy beddings often occurred in the facies. Low intra-tidal flat consiss of light olive fine sand, intercalated with dark olive gray clayey silt bands (1-2mm thick), with flash or massive beddings.Flooding plain (Terrestrial stiff mud):The flooding plain is composed of mottled yellow brown clay and silty clay, with very thin horizontal to wavy laminars (<1 mm in thickness). The flooding plain deposits changed into so call terrestrial stiff mud after encountering a series of pedogenesis processes. The stiff mud, which extensively occurs in Yangtze River delta areas, developed on an expourse environment during sea level low stand. It has become a symbolized strata of TRSRS region.Sand ridge:The sand ridge is mainly composed of light olive gray sand, yellowish silty sand and grayish brown-olive gray silt with clayey silt thin layer (1 to 6 mm thick). Flash and massive beddings are common in the facies. It displays a coursing up succession.Tidal channel:The tidal channel is composed of olive gray silt, brown and grey silt and clayey silt and interlayer of silty clay. Horizontal lanimars and wavy beddings are common in the facies, occurasionly with light olive gray fine sand. Carbonated plant debris and shell fragments occurred in the facies.On the log-probability curves, the sand ridge and tidal flats display 3 or 4 subpopulations, among, with which the salutation subpopulation account for a high proportion. The salutation subpopulation has the characteristic of double salutation population. The proportion of salutation subpopulation of sand ridges, which displays a convex up type is higher than that of tidal flat.Long term evolution has shown the study area was dominated by tidal flat during 60 kaB.P. and 25 kaB.P. and followed by a subaerial exposure symbolized as "stiff mud", which has indicated a sea level decline during 25kaB.P. and 16kaB.P. At the beginning of the Holocene transgression (16-7kaB.P.), the area received tidal flat deposits again. The tidal sand ridge-channel system developed after the area was completely drowned due to sea level rise. The increasing Holocene tidal currents strongly eroded and reworked underlying strata after 7 ka B.P., piled up deposits and resulted in the formation of sand ridges.