| As one of the terrestrial sediment sinks of the Yangtze River,the northern Yangtze River Delta(YRD)has unique advantages in studying environmental evolution and identifying origins of the sediments of the Yangtze River basin.Owing to frequent migration of the Yangtze River and the Yellow River since the Quaternary,combined with frequent sea level fluctuations,the depositional environment and sediment sources of the North Jiangsu Plain is extremely complex.In this paper,two drill cores(Core M and Core Y)in the northern YRD were obtained.Based on analyses of the paleomagnetism,AMS 14C,ESR,OSL dating results and the marked stratums of Core M and Core Y,combined with many other studies in this area for reference,a relatively reliable chronostratigraphic framework of each core was established.On the basis of the reliable chronostratigraphic framework of these two cores,comprehensive analyses of the testing results of Core M and Core Y,e.g.sediment colors and components,sedimentary textures and structures,grain size,magnetic susceptibility,macro(i.e.,bivalves and gastropods),micro(i.e.,foraminifer)fossils,element geochemistry and mineralogy were carried out,sedimentary environment evolution process and changes of sediment supply to the northern YRD were discussed in detail.The results could reveal the extent of the Yangtze River and the Yellow River have supplied sediments to the northern YRD area,and provide important information for better understanding of the stratigraphic formation and evolution and changes of regional paleo-fluvial geomorphologic process system of the study area.The results show that:(1)The land-ocean interaction records since the middle Pleistocene of Core M were not remarkable.From the middle Pleistocene to the marine isotope stage(MIS)7,river floodplain-shallow lake-lacustrine deposits were developed during this period.Estuarine-river floodplain-shallow lake-coastal marsh-river floodplain-lacustrine swamp-shallow bay-coastal marsh deposits were developed respectively since MIS 7.(2)The complex and variable sedimentary faces of Core Y might be controlled primarily by sea-level fluctuations and frequent migration of river course in the study area since MIS 7.Fluvial channel-lacustrine-estuarine-river floodplain-fluvial channel-natural levee-lacustrine swamp-river floodplain-tidal flat-river floodplain-lacustrine-shallow lake-tidal flat-coastal marsh-river floodplain-coastal marsh deposits were developed respectively since MIS 7.The multiple occurrences of river-binary structure at bottom of Core Y were supposed lateral migration results of the Yangtze riverbed in the valley.(3)From the middle Pleistocene to MIS 8,shallow lake-river floodplain deposits were developed in the northern region of the study area,and the Yangtze River sedimentary system was developed in the southern region.The region of Core Y might be one of the northernmost ancient channels in the estuarine section of the Yangtze River during the high sea level period of MIS 7,and developed fluvial channel sand-lacustrine-estuarine deposits,respectively;the northern region of the study area might be river floodplain,with a certain distance to the south and?50 m higher than that of the south ancient channel,estuarine deposits were developed.During MIS 6,as the global sea level decreased with fluctuations,the ancient Yangtze River channel migrated southward,with the high sea level drawing to a close in the study area and then regression occurred,the study area was completely exposed and developed river floodplain deposits.During MIS 5,north of the study area was?35 m higher than that of the south,the Yangtze River channel migrated northward again,the southern region of the study area developed fluvial channel sand-natural levee-lacustrine swamp-river floodplain-tidal flat deposits,respectively,and estuarine deposits were developed in north of the study area.During MIS 4,the study area was completely exposed again and developed river floodplain deposits,to the end of MIS 4,the topography on both sides of the study area was quite similar.During MIS 3,nearly 20 m thick of lacustrine-shallow lake-tidal flat deposits were developed in the southern region of the study area,and shallow lake-coastal marsh deposits were developed in the northern region,topography in the south side of the study area was?8 m higher than the north.During MIS 2,the study area was completely exposed again and developed river floodplain deposits.During MIS 1,the step-like and rapid rise of postglacial sea level occurred as a result of global melt water pulses,freshwater lacustrine swamp,shallow bay and coastal marsh deposits were developed in the north side,respectively;the south region of the study area was lack of the early postglacial sediments,and coastal marsh deposit was developed during middle-late period of the postglacial owing to the higher topography.(4)Provenance analysis of mineralogy and geochemistry in sediments of Core Y and Core M showed that sediment supplies to the northern YRD since the middle Pleistocene were variable.From the middle Pleistocene to MIS 8,sediment from the Yellow River may occasionally influence north of the study area.During transgression period,e.g.,MIS 7,MIS 5 and MIS 1,sediments from the Yellow River dominated the study area,while during recession period sediments from the Yangtze River dominated the study area.Sediments from the Yellow River in the study area were considered to might be transported from previously deposited sediments by strong tidal current and intense wave to this area during the process of transgression,or a reflect of southward shift of the Yellow River during this period and then were transported to this area.(5)It can be concluded the northern YRD area has experienced complicated environmental evolution and changes of sediment supplies since the middle Pleistocene,which was controlled primarily by rivers,wave,tide,climate change,sea-level fluctuations,tectonic subsidence,etc.The global sea-level fluctuation under the control of climate change is the main factor controlling the sedimentation process in this area. |
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