晚更新世末期中国陆架沙漠化及其衍生沉积的研究

CHINA SHELF SEA DESERTIZATION AND ITS DERIVED DEPOSITS DURING THE LAST STAGE OF LATE PLEISTOCENE

根据中美联合调查的浅地层剖面仪记录,经室内分析结果表明,晚更新世末期,影响渤、黄、东海环境变化的动力因素是古季风活动;古季风活动的结果,使陆架部分地区发生沙漠化,并产生一系列衍生沉积;内陆架区的辽东浅滩、苏北浅滩、台湾浅滩有可能都为埋藏沙漠体;位于渤海海峡中的庙岛列岛,以及蓬莱西部的黄土沉积,为渤海中部沙漠化的衍生沉积;位于南京附近的黄土沉积,为苏北浅滩的衍生沉积;山东半岛和华北平原为继承性的干旱化环境;陆架区各类沉积物间具有一定的亲缘关系。

The author made a detailed analysis of data on the shallow stratigraphic profile of the Yellow Sea shelf with total length of more than 3 000 km, found that in the regression during the last stage of late Pleistocene, several areas on the shelf were desertized and holds that desert deposits and their derivative sediments constituted the base of the so-called 'residual sediments' in the shelf areas of the East China Sea (including Bohai Sea, Yellow Sea and East China Sea).1. Environmental conditions of the shelves of the East China Sea in the la-st stage of Late PleistoceneThe climax stage of the last glacial period began from about 25 000a B.P. The Mongolian high pressure became much stronger, and caused the characteristic monsoon climate in Eastern Asia. As drainage systems degenerated under the cold climatic condition at that time, rivers became shorter and narrower, and even dried up or disappeared. Therefore, fluvial process gave way to wind process, and the exposed shelf areas of loess sediments with sparse plants, were subject to strong deflation, and became desertized.2. Desertization of the shelf areas of the East China Sea and its deriveddepositsEolation was not only a major exogenetic force on land but also on the shelf region in the ice age. At the climax of the last ice-age, a thick layer of loess deposit was developed in northern China. The shelf region at that time had similar climatic condition and the same sediments. Because the shelf region was not protected by vegetation in the ice-age, primary deposit strata, under the intensive effect of wind storm, were naturally remoulded to produce new sedimentary differentiation and new series of land sediment.(1) The desertized areas Much evidence of shelf desertization was obtained from the shallow stratigraphic profiling data on the shelves of the Yellow Sea and Bohai Sea.1) The boundary of late/mid-Pleistocene sequences is well preserved and can be traced and correlated over wide areas.2) As an indication of the high intensify of desertization, the transgressive sequence of the last interglacial period, well preserved for the most part, had the top surface cut by later deflation, and was characterized by relatively high relief in some places.3)The transgressive sequences formed in the two interstadial stages of the last glacial period and the regressive sequence intercalated between them are mostly destroyed and replaced by new desert deposits.4) A series of about 30m thick sand deposits with 'steep repose-angle' were formed on the deflated base.5) Dune chains and vast sand flats are widely found on the shelves.6) A thin Holocene depositional sequence with horizontal beddings is completely preserved.(2) The derivative deposition areas The derivative deposits from the desertized shelf areas of the East China Sea can be classified into three types: completely submerged, partly submerged and completely exposed ones.1) Completely submerged typeThe muddy deposit areas in the centre of the North Yellow Sea, in the northern part of South Yellow Sea and on the shelf area o the southwest of Jizhou (Cheju) Island are typical.2) Partly submerged typeThis type of derived deposits from the shelf deserts can be represented by the loess deposits distributed along the southeastern coastal areas of the Bohai Sea, such as those on Miao-dao Islands and along the northwestern coastal zone of Penglai County.3) Completely exposed typeThe North Jiangsu Shoal was the largest desert in the western part of South Yellow Sea during late Pleistocene. As the ancient monsoon wind coming from Mongolian High Pressure Zone turned northeastward as it reached the eastern side of Shandong Peninsula, the dominant wind here was also northeastly. Therefore, the derivative deposits of the North Jiangsu Shoal Desert must be distributed to the lee of it. They are the loess deposits in Nanjing and its adjacent areas.