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A NEW EXPLORATION ON THE ORIGIN OF LOESS IN THE SHELF AREA OF THE EASTERN CHINA SEAS
中国东部陆架黄土成因的新探索

Keywords: monsoon,proximal deposit,loess
季风
,近源沉积,黄土,陆架,古土壤

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Abstract:

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

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