末次冰期苏北平原和东延海区的风尘黄土沉积

AEOLIAN LOESS DEPOSITION DURING THE LAST GLACIAL PERIOD IN THE NORTHERN JIANGSU PLAIN OF THE YANGTZE DELTA AND WESTERN AREAS OF THE YELLOW SEA AND THE EAST CHINA SEA

长江三角洲北翼苏北平原和东延海区，晚更新世晚期顶部地层普遍为一层暗绿色至黄褐色的硬质粘土层。本文通过对苏北平原硬质粘土层与西部丘陵区下蜀黄土的地层对比以及粒度、地球化学、矿物组合、微体古生物和抱粉、石英颗粒表面电子显微镜扫描、１４Ｃ测年等实验分析的综合研究，讨论了其成因和年代问题，指出该硬质粘土层为末次盛冰期风尘黄土堆积物经次生变化而演变形成，同期的风尘黄土在黄海、东海海底和陆架区也有堆积，为下蜀黄土在东部平原区和海区的延续。

The top strata of Late Pleistocene on the northern Jiangsu plain of the Yangtze delta generally consist of dark-green or yellowish-brown hard clay. From theanalysis results in stratigraphy, grain size, geochemistry, minerals assemblage, microfauna, pollen and spores, SEM, and 14C age, we consider that the hard clay is aeolian loess and deposited under a dry-cold climatic condition during the last maximum glacial period. A similar aeolian loess is also distributed on the sea floor andthe continental shelf of the Yellow Sea and the East Chilla Sea. The hard clay isthe counterpart of the top strata of the Xiashu loess.The hard clay in northern Jiangsu plain is about 2-10 m in thick ness and issubdivided into the upper part dark-green or grey-green upper part and the lowerpart yellowish-brown. In some areas, however, it may not be subdivided and all isyellowish-brown. It is well compacted with massive argillited structure and contains iron-manganese cutans, slice nodules and plant roots, locally with calcareous concretion. The stratigraphic relationship between the hard clay and the top strata ofthe Xiashu loess is confirmative. Drilling reveals that the hard clay dips southeastand that the submarine hard clay is intermittently distributed and thinner than thecoastal hard clay. The hard clay is similar in sedimentary features to the top strata of the Xiashu loess, indicating that both should be of aeolian dust deposition.(1) Both the hard clay and the Xiashu loess are dominated by silt-sized particles (0.004-0.063 mm), making up approximately 60-70%; the rest is clay (0.004mm) making up about 30$and a very little sand (M0.063 mm). The grain sizetends to become finer and finer from northwest to southeast, which is a typical characteristic of aeolian loess.(2) The hard clay and the Xiashu loess both show similar geochemical characteristics. Both are dominated by SiO2, Al2O3 and Fe2O3, whose total amount reachesabout 80%. The major difference lies in that the FeO/Fe2O3 ratio of the Xiashuloess is higher than that of the hard clay.(3) The mineral composition of both the Xiashu loess and the hard clay aredominated by light minerals, with quartz, feldspars and mica reaching approximately 90%. The main heavy minerals of both are epidote, hematite, amphibole andzircon. Some minerals that are unstable in water condition, such as epidote, amphibole and mica are common to the Xiashu loess and the hard clay, demonstratingthat they were deposited in arid and low temperature conditions.(4) In the Xiashu loess and the hard clay, no microfauna fossils have been identified, but there is a few herb pollen such as Gramineae, Chenopoliaceae and Concentoicystes, indicating that they originated from grasslands.(5) Quartz grains in the hard clay are anchi-rounded or anchi-angular and haveconchoidal fractures, coarse surfaces, apparent traces of mechanical collision andabrasion, and parallel V-shaped scratches, all of which are typical of aeolian quartzgrains.14C-age analyses reveal that the deposited age of:the hard clay is about 25000- 15 000a, when is the maximum of the last glacial period. The sea level fell dramatically so that the east edge of the northern Jiangsu plain shifted eastward about800km. The Yellow Sea and the shelf of the East China Sea became subaeriallyexposed. The aeolian dust transported by the Northwest Wind was spread over thecoastal plain and the continental shelf, and the sediments thus formed become thinner and finer from northwest to southeast. After about 15 000a B. P., deglaciationfollowed. Thus the global climate became warmer and wetter, sea level rose, andloess accumulation terminated, while weathering and the pedological process initiated so that yellowish-brown or grey-brown hard clay was eventually formed. Thenon the eastern plain the hard clay were buried by Holocene transgressive sediments.Some organic matters dissolved from the transgressive sediments permeated the hardclay, and under a reducing environment, Fe3+ and Mn4+ in them were reduced intoFe2+ and Mn2+ so that the upper hard clay changed from yellowish