黄淮海南部第四纪沉积地层与环境演变研究进展

Progress of research on the Quaternary sedimentary strata and environment in the southern Huang-Huai-Hai

黄淮海南部位于长江、黄河、淮河等重要水系长期相互作用的关键区域,其沉积地层记录了中国东部第四纪构造活动、海平面变化和人类活动等诸多环境演变的信息。研究显示黄淮海南部第四纪广泛发育海侵—海退旋回层序,然而海侵次数和时间仍存在争议,相关层序地层研究开展较少。基于黄淮海南部地区公开发表的34口典型钻孔,结合3个露头剖面,对地层进行了分区。将黄淮海南部第四系岩石地层划分为连云港平原区、里下河平原区、长江三角洲北翼和南黄海陆架区四个地层区。基于钻孔的岩相和生物相特征,识别出暴露相、风暴潮相和浅海相冷水团沉积三个标志性沉积特征,并将地层划分为五个典型的沉积相组合,即河道—河漫滩相组合、潮间—潮下—浅海相组合、滨岸沼泽—障壁海岸—滨海相组合、潮坪—潮上相组合和三角洲前缘—前三角洲相组合。在此基础上,重建了黄淮海南部第四纪的环境演变历史,结果表明首次海侵时间为早更新世(不早于1.9 Ma),中更新世后才出现明显的海、陆交替特征。最后,还对年代地层的新方法(如宇生核素)和层序地层的建立进行了展望。

Background,aim,and scope The Yellow River,Yangtze River and Huaihe River are the three largest rivers flowing into the Yellow Sea.Since at least the Late Quaternary,they have shaped a wide lowland area,known as the southern Huang-Huai-Hai Plain(HHHP).The study of the sedimentary record in the southern HHHP may provide important insights on these rivers’geological histories as well as the interaction between tectonic movements and sea level changes in the coastal area of eastern China.Although hundreds of boreholes have been drilled since the 1980s in southern HHHP,there is no consensus on stratigraphic correlation,and frequency and timing of the marine transgressions within this area remain poorly defined.Therefore,a review of the lithostratigraphy,chronostratigraphy and sedimentary facies from the available boreholes in the southern HHHP is of critical importance for understanding the evolution of the sedimentary environments and strata in the area.Materials and methods In this review paper,we synthesize data from 37 well-documented sections(34 cores and 3 outcrops)from published literature to establish a new stratigraphic correlation in the southern HHHP.Such a stratigraphic correlation is based on hiatus boundaries,sedimentary facies and age constrains from 14C,OSL,and paleomagnetism.Results The Quaternary lithostratigraphy of the southern HHHP is generally divided into four units(Unit 1-4,from bottom to top).The age of Unit 1 may be constrained to Lower Pleistocene by paleomagnetism,since it displays a clear Matuyama/Gauss boundary at the base and Matuyama/Brunhes boundary at the top.The ages of Unit 2 and Unit 3 may be constrained to the Middle Pleistocene and Upper Pleistocene,respectively,based on OSL dating and sporopollen analysis.At the top of Unit 3,there is a clear depositional hiatus characterized by gray-yellowed paleosol known as the“first hard-clay”which has been dated to cal.12.5 ka in age.Unit 4 lies above this paleosol and is thus regarded as Holocene,as confirmed by 14C ages from boreholes and outcrops.In the studied area,these four units can be well correlated in the cores located onshore(in the Lianyungang Plain,Lixiahe Plain,northern Yangtze River Delta)and offshore areas(South Yellow Sea Shelf)based on the lithofacies and biostratigraphic characteristics.Typically,three types oflithofacies have been used for stratigraphic correlation:subaerial exposures(e.g.,the“first hard-clay”),cold-water mass deposits,and storm deposits(e.g.,cheniers).The sedimentary facies within these units were reinterpreted and grouped them into five facies associations(FA):channel-floodplain deposits(FA1);intertidal-subtidal-shallow marine deposits(FA2);coastal marsh-barrier coast-coastal deposits(FA3);tidal flat-supratidal deposits(FA4);and delta front-prodelta deposits(FA5).Discussion Based on our new stratigraphic correlation and sedimentary interpretation,which lead to the reconstruction of the paleoenvironmental evolution of the southern HHHP during the Quaternary,the first transgression occurred not earlier than 1.9 Ma,and obvious transgression-regression sequences appeared after the Middle Pleistocene.In the Early and Middle Pleistocene,the southern HHHP was dominated by fluvial lacustrine sediments,and clear evidence of transgressions have been observed only in the offshore cores.During the Late Pleistocene,there is evidence of 2-3 large-scale transgressions,which are observable in both offshore and onshore cores.These transgressions may be related to MIS(marine isotope stage)5(128-74 ka)and MIS3(60-24 ka).In the Holocene,the southern HHHP was dominated by coastal sand bar and tidal sand ridge deposits in the offshore area,and was dominated by salt marsh and tidal flat sediments in the onshore area.Conclusions In summary,the Quaternary lithostratigraphy in the southern HHHP is subdivided into four stratigraphic units,whose deposits have been reinterpreted into five depositional paleoenvironments.The reconstructed paleoenvironmental evolution of the southern HHHP indicates that recurring transgression-regression cycles occurred after the Middle Pleistocene.However,there are still limited boreholes and geochronological data for detailed analysis of the transgression-regression cycles in high spatial and temporal resolution.Moreover,the evolution of the three rivers(Yellow River,Yangtze River and Huaihe River)and their roles on the development of the southern HHHP require further comprehensive studies.Recommendations and perspectives Future research should focus on new dating methods(e.g.,cosmogenic nuclide,feldspar OSL),to improve the dating results,and on the establishment of a more reliable sequence stratigraphic model,with the correlation of more boreholes.