中国东部陆架黄土成因的新探索

晚更新世末期出露了的陆架平原成为北半球沙漠带的最东端,并在那里出现了面积不等的沙漠一黄土堆积系列。许多研究者已经发现陆架沙漠－黄土堆积群中普遍含有有孔虫、放射虫、海绿石和粒度组成较粗等特点,大多数黄土沉积都属于近源沉积,真正能随高空气流进行洲际输送的黄土物质,是一些非常细粒的尘埃。中更新世期间,苏北宿迁一带的第三纪紫红一红色粘土就已经出露地表,并经常受到风暴的侵蚀。当紫红－红色粘土被搬运到丘陵一带时,就形成了下蜀土沉积；当它落在湖相环境时,就形成了岩芯中所见到的杂色粘土。因此,下蜀土也属于近源沉积。

A NEW EXPLORATION ON THE ORIGIN OF LOESS IN THE SHELF AREA OF THE EASTERN CHINA SEAS

Since the 1990s, a series of progresses have been made in the research of both exposed and buried loess layers in the shelf area of the Eastern China Seas. Records of sub-bottom profiling of thousands of kilometers have further verified the existence of buried loess layers and aeolian dunes. As a matter of fact there are many paleo-dunes and loess layers in the coastal zone of the Bobal Sea, and there are a series of buried loess layers and dunes in the shelf area of the Eastern China Seas. Basedon these facts, it can be deduced that the loess in the shelf area was not transpededfrom the west part of the mainland of China. Instead, it was mainly formed in situ, i.e., just from the shelf area itself.This is the "proximal theory of loess origin".During the maximum of the last glacial period, the sea level of the EasternChina Seas was lowered with great magnitude. The coastline of Eastern China Seasthen was located approximately along the present 130m isobath in the outer shelfzone of the East China Sea. As a result the Bobal Sea and Yellow Sea as a wholeand most part of the East China Sea were exposed to form a part of the Asianmainland. The area of the exposed shelf plains was over 1 000 000 km2. The wintermonsoon then was obviously strengthened, hence the cold and dry air currents fromthe west could spread to the exposed shelf plains. The deflation of storm winds notonly accelerated the development of inland desert-loess systems, but also led to thedevelopment of desert-loess deposition systems in the exposed shelf plains. Manyresearchers have found that the desert-loess deposition sequences in the shelf areascontain the fossils of foraminifera and radiolaria and glauconite, and consist of coarseparticles. These characteristics indicated that the loess material in the shelf plains didnot come from the west inland desert area. Most part of the loess deposit, consistingmainly of silt and silty clay, was proximal deposit and only the very fine-graineddust can be transported intcrcontinentally in the upper air currents.In the coastal zone of the Bobal Sea, the desert-derived loess deposit is mainlydistributed in the west and northwest parts of Liaodong Peninsula, the islands in theBobal Strait, and the north part of Shandong Peninsula (from Penglai to Lingezhuang) in the pattern parallel to the coasts. The loess platform in Xiazhupan-Lingezhuang-Pozi in the north part of Shandong Peninsula forms a straightly steepcliff along the coast, which is the unique loess coast in China.A series of geological drillings in the Bobal Sea have revealed that there is athick sand layer beneath the Holocene transgression sequence in the desertifled areas.The development of the deserts in the exposed shelf plains can be divided into tWophases f the expansion phase and the contraction phase. During the desert expansionphase, a thick sand layer formed in the 15-29m warer-depth zone from the west tothe east of the Bobal Sea as revealed by geological drillings. The desert contractionphase began just before the Holocene transgression, when the deserts in the shelfplain contracted, while the area of loess deposition spread. The thin loess layerformed in the desert-contraction phase directly overlay the sand layer of the desertexpansion phase.Dune groups to the south of the Bohai Sea are mainly distributed near theWeifang City. Here many villages are named as "FU", watch means sand dune inthe localism. Sand dunes containing fossils of fortaminifera were developed in manyplaces, indicting that their origin was related to the exposure of the Bobai Sea shelf.Records of sub-bottom profiling have revealed the distribution patterns and scalesof buried aeolian dunes in the Yellow Sea. During the maximum of the last glacialperiod corresponding tO the desert expansion phase, the whole sea shelf plain was adesertization area, where the original marine deposition sequence was disintegrated,various aeolian dunes were formed, and the "angle of repose" stature was formedand preserved in the sub-bottom stratigraphic sequences. At about 15 000 aB.P., theworld cli