苏北海岸带潮成辐射砂脊群的形成及其古地理意义

Formation of radial tidal sand ridge system and its palaeogeographical significance in coastal zone of northern Jiangsu Province

南黄海潮成辐射砂脊群的面积约为 2 0 0 0 0km2 ,以 16 0°的角度从弓京港向海展开。它与以弓京港为顶点的辐聚辐散潮流场相伴而生。 6 0余个钻孔揭示 ,毗邻海区辐射砂脊体系的江苏沿岸平原上存在一个面积约30 0 0km2 潮成砂区 ,其顶点位于东台 ,同样呈扇形以 130°的角度向东展开。在潮成砂区内潮成砂质沉积单元位于冰后期海侵型砂坝 -湖沉积层之上 ,二者之间具明显的冲刷面。砂坝 -湖沉积层位于晚更新世基底硬粘土层之上 ,二者之间有较长的沉积间断。潮成砂沉积层上覆潮坪沉积层 ,二者呈渐变关系。以潮成砂层底部的侵蚀面为界 ,其下为海侵序列 ,其上为海退序列。古潮流的研究揭示 ,潮成砂区内同样存在辐聚辐散的古潮流场 ,其顶点位于东台附近。由此推断 ,沿海平原的潮成砂区内也是辐射状潮成砂脊体系 ,它形成于全新世海退时期。由于长江和黄河三角洲的前展 ,以东台为顶点的潮成砂脊体系逐渐暴露成陆。陆上和海域潮成辐射砂脊群形成于相同的潮汐动力环境 ,但处在不同的发育阶段 ,前者形成于全新世中期 ,后者发育于全新世晚期。矿物分析揭示 ,陆上和海区的潮成辐射砂脊体系主要由长江和黄河沉积物组成 ,其中长江沉积物由南向北运移 ,且时间较早 ;黄河沉积物由北向南运移 ,时间较迟 ,这种泥沙的运移?

A radial tidal sand ridge system with an area of more than 20 000 km 2 in the southern Yellow Sea is fan-shaped and stretches from Qianggang toward the sea with a central angle of 160°. The marine radial tidal sand ridge system is associated with a convergent-divergent tidal current field.More than 60 drilling holes reveal that an area of 3 000 km 2 of tidal sand strata in exists Jiangsu coastal plain on land,which is adjacent to the marine radial tidal sand ridge system and stretches eastward from Dongtai and is also fan-shaped with a central angle of 130°. The tidal sand unit lies on a transgressive coastal barrier-lagoon unit with an erosional surface between them, and the latter unit is in turn lying on the Late Pleistocene stiff mud, with a long-term hiatus between them. The tidal-sand unit is overlain by the littoral tidal-flat unit and together they constitute the Holocene regressive succession. The erosional surface at the bottom of the tidal sand unit may serve as a surface, below and over which a transgressive succession and a regressive succession exist respectively. Paleo-current study reveals that a paleo-current pattern with a convergent-divergent tidal current field similar to the modern marine was present in the Holocene subaerial tidal sand strata. Therefore, the subaerial tidal sand bodies may be inferred to be an ancient radial tidal sand ridge system that was formed during the Holocene regression, and gradually exposed and changed into land as the Changjiang and Huanghe deltas prograded. Both subaerial and marine radial tidal sand ridge systems were formed in the same tidal depositional environment but at different development stages. Thus, the radial tidal sand ridges in southwestern Yellow Sea should have been formed in the process of the Holocene regression. Mineralogical analyses demonstrate that the sediment sources of the subaerial and marine radial tidal sand ridge systems mainly came from the Changjiang River and the Huanghe River.Sediments of the Changjiang River were migrated from south to north in the earlier period,while sediments of the Yellow River were migrated from north to south in the later period.The tendency of mud and sand migrating has continued until today. As the sea-level rise slowed down, the Jiangsu coastline has prograded seaward as the Changjiang delta expanded and the radial tidal sand ridge system in northern Jiangsu gradually exposed to land.This progradation was accelerated when the Huanghe River debouched in the north of the study area in 1128.The convergent-divergent tidal current field has shifted its apex three times since the Holocene maximum transgression, from the apical area of the Changjiang palaeo-estuary to Dongtai of the northern Jiangsu tidal sand area, and then to Qianggang. It means that the radial tidal sand ridge system has also experienced three stages of development, and has changed its location from Chengdong to Dongtai, and then to Qianggang. However, the radial tidal sand ridge system developed in the Changjiang palaeo-estuary has not been found so far,which maybe due to the strong reworking of river currents.