A PRELIMINARY STUDY OF THE -50M PALEOCOASTLINE OF THE NORTHERN SOUTH CHINA SEA
Chen Jun-ren Fang Wen-ke (Comprehensive Research Brigade of South China Sea, Ministry of Geology and Mine, Guangzhou) Zhao Xi-tao (Institute of Geology, Academia Sinica)
In this article, the authors, after analyzing the data which were collected during an integrated marine geological investigation of the Northern shelf of the South China Sea, carried out in 1970-1978, especially on the sediment samples and echo Sounder, shallow stratigraphy profiles and Sidescan Sonar records, acknowledged that their knowledge on the features, divisions and geological significance of the- 50 m paleocoastline have been greatly improved. Evidences of the-50m paleocoastline of the northern shelf of the South China Sea are fairly ample. Geomorphologically, there are cracks on the sea bottom at the -50m depth on the northern shelf of the South China Sea, and the sea bed at the depth of 50-65m is a broad platform. On the bottom at the-50m, a number of ancient littoral relieves such as tiebars, beaches, barriers and lagoons etc., have been found. Sedimentological, su-bmerged beachrocks, nearshore pebbles have been collected on the sea bottom of-50m, and deposites become coarser abruptly; kinds and contents of heavy minerals are similar to those of the present coast. Various parameters of grain sizes distribution, such as sorting coefficient Qd /, medium diameter Md , possibility cumulative curve, C-M pattern curve and texture parameter scattered diagram all have the features of that of sea coast, river mouth or delta. Chemically, component and contents of sediments are similiar to those of the present coast too. Biologically, most of molluscs are intertidal and subtidal species and their shells have always been abraded. In the foraminiferas, the Pararotalla genus is the dominant species, in which both F. ozawai and P. taiwanicalia are of the littoral facies. In the ostrocada the recent shallow water complex is a common component. The pollen complex is a mixture of bog land and mangrove swamp environment. This paleocoastline has the following basic features: 1. Its configuration is similiar to that of the present; 2. The degree of the development of the submerged river courses, inner and outer of the paleocoastline are obviously different; 3. There are ancient littoral relieves developed at the sea bottom; 4. The preserving water depths are different from place to place; 5. There are obvious cracks on the sea bottom and 6. Sediments are coarser. According to the geological and geomorphological features of the paleocoastline, it can be divided into: 1. The Baibu Gulf, 2. Southeast Hainan Island, 3. Guangzhou Bay, 4. Zhu-jiang (Pearl) River Mouth and 5. East Guangdong. Compare the coastline of the northern South China Sea with the related coastline ofthe neighboring seas and other oceans, the forming age of the-50m paleocoastline of the northern South China Sea is about 12,000-11,000 y. B. P.. Discovery of the-50m paleocoastline in the northern South China Sea have the fo-llowing theoretical and practical significance: 1. It is an important indicator of the ancient sea level. 2. It is an important evidence of the vertical differential crastal movement. 3. It is one of the proof that it is the division line of the inner and outer shelves of the northern South China Sea. 4. It is an important ancient littoral placers on sea floor.