海洋地质与第四纪地质

<正>苏北岸外浅滩位于苏北新洋港——吕泗港之间的岸段外围,它是以弶港湾为中心向海呈幅射状展布的巨大砂体。南北长约200公里,东西宽近100公里,面积宽广,形态特殊,地形复杂,是南黄海西南部独特的地貌类型。关于浅滩的成因至今说法不一,有的认为是海面上升过程中形成的水下阶地,有的认为是早期的残留砂体,还有的认为是两股沿岸流在琼港的相遇,水位抬高产生梯度流向海扩散而形成的辐射砂体。因此,进一步探讨它的物质来源、形成机理、发育过程及演变趋势,有很重要的地质意义。近几年来,我们对浅滩进行了海洋地质综合调查,为研究它的成因提供了重要的依据。

The shoal situated to the north of the Yangtze River mouth and off the coast of Jiangsu province, represents an unique geomorphological type in the southwest part of the South Yellow Sea. Basically it takes the shape of several striped sand bodies spreading radially towards the sea with Qiong Gang Bay head beach as its center, and is characterized by shallow water, highly fluctuating and quickly changing relief. The sediments are dominated by fine sand, well-sorted, rich in heavy minerals and with high Eh value, indicating the oxidative sedimentary environment of high energy. In early Holocene, the ancient course of the Yangtze River once entered the sea near Qiong Gang and, consequently, Sand bars and sand beaches were formed there. Later on, the Yangtze River deflected towards the south, leaving the old river mouth abadoned so that Qiong Gang Bay first made its appearence. Besides, ever since early or middle Holocene, the Yellow River has changed its course many times to enter the sea from North Jiangsu, which has resulted in a giant subaqueous delta of the abadoned Yellow Riverand made the configuration of Qiong Gang Bay still more clear-cut. The Hol-ocene composite delta formed by the Yangtze River and the Yellow River is the base upon which the shoal off North Jiangsu has been developing. The shoal area is an area with semi-diurnal tidal current. The tidal current converges to the bay during the flood and diffuses out of it at the ebb. The biggest tidal range can reach 7.63 meters. The piledup water of the Yangtze from the south and the longshore current of North Jiangsu from the north meet near the bay and, as a result, the tidal current is highly intensified. The process from flood to high slack water to ebb, which repeats twice a day, leads to the change of water body from dynamic energy to potential energy to dynamic energy, the later in turn resulting in the re-sedimentation of the submarine mud and sand from transportation to deposition to differentiation. It is due to this repeated back-and-forth movement that the submarine geo-morphology corresponding to the hydraulic conditions has been gradually formed. In its future evolution, the central part of the shoal off North Jiangsu will go on to be lifted and the area of it will continue to be enlarged. The tidal channels and sand bars will still extend along the northeast direction, but the extending speed tends to slow down. The modern beaches in the inner side of the shoal area are favourable to the accretion of sediments, its water depth will get shallower little by little and, at the end, the beach will emerge from under the water and merge into the shoal, making the land, prograde towards the sea. (Translated by Wang Shaoming)