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扬子浅滩成因探讨
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摘要
东海陆架以宽平的地形、充分的陆源沉积物供应、快速沉降和强动力场为特征,是世界上最宽阔的大陆架之一。扬子浅滩位于东海陆架北部30.7°N~32.6°N、122.5°E~125°E,具有水深浅、坡度缓、沉积物供应丰富和复杂的海洋动力环境等特点,在海平面上升过程中,潮流作用强盛,是物质运移的主要动力,其表面广泛发育砂质沉积,我国先后有许多学者从不同角度对其成因进行了研究,综合各家学说,有残留沉积和现代潮流沉积两种主要观点。
     本文以分析探讨扬子浅滩东南边缘海底底形特征及成因为基础,结合扬子浅滩地形地貌特征,特别是典型微地貌特征,对扬子浅滩成因进行探讨。
     依托国家908-1-DX19项目,利用SM2000多波束探测系统对扬子浅滩东南边缘进行高精度探测,对该区域海底底形特征进行分析,并探讨其成因,结果表明:
     “三脊夹两槽”是该区域大型地形地貌特征,三个大型的沙脊之间有两个凹槽;沙脊顶部发育有冲蚀陡坎、冲蚀沟等微地貌,三个沙脊顶部微地貌几乎处于同一水深,在-39m左右;凹槽顶部发育有冲刷沟和冲蚀坑地貌。
     区域内沙脊方向与现代潮流方向不一致,是海平面上升过程中,潮流冲蚀原有的沉积物质,残留的滨岸砂。凹槽顶部的冲刷沟和冲蚀坑地貌是潮流在侵蚀沙脊的过程中形成的。三个沙脊顶部浅点位于古海岸线上,发育的冲蚀沟槽、冲蚀陡坎为海浪对海岸线侵蚀造成,残留的古岸线地貌。
     扬子浅滩与东海陆架潮流沙脊以及苏北浅滩是一系列在海平面上升过程中形成的,具有继承性的潮流沙脊体系。
     扬子浅滩区域的两个大型沙脊北侧相对平缓而南侧较陡,在沙脊北侧发育有大量平缓的沙波。扬子浅滩南部发育沙脊,沙脊北部发育有沙波,这种形成演变规律基本符合Belderson等(1982)提出的潮流地貌与潮流流速关系,是在海平面上升停顿期,在较强往复潮流作用下,形成与古潮流NNW—SEE方向一致的侵蚀—堆积潮流沙脊地貌。
     综合表明:扬子浅滩是在末次冰期以来海平面上升过程中形成陆架残留砂,表面正遭受现代潮流动力侵蚀改造,并与现代水动力环境相适应。
The East China Sea shelf, which is one of the most wide continental shelf in the world, has three features as follows:a wide and flat terrain; a variety of land-based sources for sediments; a rapid subsidence and strong power causes. Yangtze shoal is located in the north of the East China Sea shelf 30.7°N-32.6°N, 122.5°E-125°E, with water depth, slope slowly, rich sediment supply and complex Marine dynamic environment, etc. In the process of sea levels rising, trends were very strong and were the main force to move the sediments. Sand deposition is widely developed on its surface. Many Chinese scholars had researched the reason with different ways, comprehensive different views to have two major points:residual depositions and modern tide depositions.
     I analyze the seabed topography in the southeast of Yangtze shoal and discusses the reason about it in this artical. Then I discuss the reason about the Yangtze shoal on its basis combining with the modern trend, particularly with characteristics of a typical micro-topography.
     This article is based on the national 908-1-DX19 Project detected the seabed topography in the southeast of Yangtze shoal with SM2000 multi-beam detection system. Then I analyze the features on the seabed and discuss its reasons. The results show that:
     There are three sand ridges and two grooves in this areas, two grooves between three large sand ridges; some erosion gully、erosion scarps on the top of the sand ridges whose micro-topography of the top of the three Sand ridges is almost at the same depth, around the -39m; some erosion gullies and scouring pit on the top of the grooves as well.
     The direction of the sand ridges in this area is inconsistent with that of the modern trend. That turns to be the coastal sand, the residues which is from the original sedimentary materials eroded by the trend, during the process of sea-level rising. The erosion gullies and the scouring pit on the top of the grooves were the result of tidal erosion. The shallow spots at the top of the three sand ridges are in the ancient coastline. The developing erosion grooves and scarps were formed as the waves eroded the coastline. That proves to be the residual ancient shoreline topography.
     Yangtze shoal sand ridges, the East China Sea continental shelf and the northern Jiangsu Shoal are a series of formations in the process of the sea level rising, having the succession in the tidal sand ridge system.
     Two large sand ridges of the Yangtze Shoal are relative flat in the north, steep in the south. Meanwhile there are a large number of flat sand waves at the north side of the ridges. The sand ridges were developed in the south of Yangtze Shoal, while sand waves were developed in the north of the bridges. Its formation and evolution law basically conforms to the relationship between the geomorphology and current velocity of the trends. It is the erosion-accumulation tidal sand ridge topography with consistent direction of the ancient trend of NNW-SEE. It is formed by the strong effect of reciprocating trends.
     Synthesis that:Yangtze Shoal is the residual sand of continental shelf formed during the sea-level rising since the last glacial. Its surface is subjected to erosion and transformation of the dynamic of the modern trend, and is adapting to the modern hydrodynamic environment gradually.
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