河海交互作用与苏北平原成因

RIVER-SEA INTERACTION AND THE NORTH JIANGSU PLAIN FORMATION

苏北平原向海侧分布着大冈、上冈两列古贝壳堤以及依此而建的范公堤;西侧有低山丘陵环绕,江淮水系河流汇入平原内侧的湖泊群,发育了伸出型三角洲;兴化、里下河地区是平原的最低洼处.这一系列地貌组合与分布特点,启迪着探究平原的成因.146m深的宝应望直港钻孔,穿透了2.58Ma以来的第四纪地层.沉积相、微体古生物、古地磁测定与沉积层常量元素分析等综合研究,阐明了4次主要的海洋作用沉积环境,尤其是晚更新世3.9～2.6万年前的浅海环境沉积更为显著.河湖相泛滥平原沉积与潮滩浅海沉积交互成层,证实了苏北平原是第四纪时期由河-海交互作用,从古海湾发育堆积成宽大平原.进一步工作需探索相关时期的江-海分布变化及黄河和黄渤海通道与苏北平原发育的关系.

The North Jiangsu Plain is located on the northern side of the Changjiang River and the south of Lan-Shan-Tou Cape in the area of 32°10′ -35°05′N and 118°40′ -120°30′E. The landform is mainly lowland declined from the Grand Canal in the west towards the east along the Yellow Sea coast, with the lowest place around the Xinhua-Sheyang lake area in the middle of the plain. A sufficient water network of rivers, canals and lakes is the major feature of the plain with the abandoned Huanghe River-Huaihe River system in the north, and the Changjiang River system and numerous lakes in the south. Previous studies have discovered that the eastern part of the plain was formed from the sea during Holocene high sea level period which is evidenced by a series of shell beach ridges located in Longgang, Dagang, Dongtai, and Hai＇an for about 200km long from north to south, about 60km west of the present coastline. ^14C datings have shown that it was formed 6500 ～ 5600aB. P. A manmade dike, named Fan-Gong Dike, was built combined with the shell beach ridges to form a complete dike during 960 ～ 1127A. D. This study is to trace the genetic formation of the plain inside of this old dike. Geomorphologic features of the plain west to the Fan-Gong dike indicate seemingly its following natures ： （ 1 ） The North Jiangsu Plain is circumscribed by a series of rocky hills as an arc-shaped ＂Bay＂. （2） A series of lakes such as the Hongze Lake, the Gaoyou Lake, and the Shaobo Lake, located on the west side of the Grand Canal, are almost connected to each other either by lowlands or by swamps. There are several rivers entering into these lakes with deltaic forms on the west shore of the lakes. The Grand Canal was actually built by partly using this series of connected lakes. （3） A series of manmade islands were built based on the natural bars or barriers of the lakes or water swamps. These islands are locally named as Duo, Dun or Wei, each of which in Chinese means ＂manmade bar or crossway＂. These islands and other natural barriers are arranged in the North to South direction, paralleled to the present coastline. Sea shells and fragments have been found from these islands supporting the notion that the area was origated from the sea. The BY1 Core of 145m thick sediment from the Wangzhigang Town of Baoying County in the middle part of the lowland area between the lakes and the old coastal ridges displays only 97m of sediment as hard to get the whole section of the pure sandy layers, although the core penetrated through almost the whole strata of Quaternary. Comprehensive study on sediment grain size, structure, facies, mineral and chemical element analyses, paleomagnetic, and paleo-microfossil analyses has indicated a river-sea interacted formational process of the plain. Marine foraminifera fossils found mainly from the layer between 39.16m to 14.70m indicated that the water depth was between 20～50m, the water salinity of 31‰～32‰ and 15‰, and the water temperature about 15℃. The time forming that layer might be Late Quaternary, earlier than the formation of the hard clay layer of the Changjiang River Delta plain region, but later than 0.78Ma of B/M boundary shown at the depth of 39.16m of core. The ^14C datings show that the layer between 16. 15 ～ 16.27m was formed at 26470 ± 60aB. P. （by AMS） ; 19.15 ～ 19.25m at 32910 ± 170aB. P. ; 23.90 ～ 23.95m at 39385 ± 170aB. P. Marine facies have been also discovered from the layers of 8.7 ～ 1.0m, 65.4～58.0m, and 81.00～79.66m. Although only few forams have been found from these layers, they have clearly indicated that sea water from time to time flooded over the drilling area during Late Pleistocene, although it is now 120km away from the present coast. Terrigenous facies of alluvial plain or shallow lakes have been found in the core at 97～81m, 79.66 ～65.40m, 58.00 ～ 39.16m, 14. 72 ～ 8.70m, and also at the surface. Sedimentary facies of the core show a series of environment changes from coastal with water depth less than 100m to a wide plain during Pleistocene. It became a fluvial plain bay of a shallow sea during Holocene. To sum up, the current study sets up a sample of tracing the evolutionary history of a plain by geomorphologic analyses combined with multidisciplinary analyses on sedimentary facies. It can be also used for a better planning of the region development.