我国西沙群岛第四纪生物沉积物及成岛时期的探讨

QUARTERNARY BIOLOGICAL SEDIMENTS IN THE XISHA ARCHIPELAGO, CHINA AND A DISCUSSION ON THE AGE OF ISLAND-FORMATION

Bioclastic sediments in the Xisha archipelago were formed during the period from late Pleisloeene to Πolocene. Except for loose coral shell sand, they can be dividcd,according to petrological features, into coral reef limestone, phosphate-cemented foraminifera (or red aglae and Eehinodermate, etc.), calcarenite or calcirudite. All of them have been subjected to raring degrees of deuterogene alterations such as phosphatization, calcitization as well as dissolution and filling. The age of formation of the Xisha archipelago, i.e., the time it finally emerged from under the sea may be dealt with as: (1) Sand bars. (1.5-3.0 meters above sea level) took shape about 2000--2500 years ago; (2) Coral reefs (3--5 meters above sea level, with the exception of rock islands) formed 400-5000 years ago, which is in agreement with the high sea level stage in the post-glacial period; (3) Rock islands(about 15 meters above sea level) formed at late Pleistocene or earlier, which can probably be correlated with the high sea level period some 80000--125000 years ago.

Bioclastic sediments in the Xisha archipelago were formed during the period from late Pleistocene to Holocene.Except for loose coral shell sand,they can be divided,according to petrological features,into coral reef limestone,phosphate-cemented fora-minifera (or red aglae and Bchinodermate,etc.),calcarenite or calcirudite.All of them have been subjected to varing degrees of deuterogene alterations such as phosphatiza-tion,calcitization as well as dissolution and filling.The age of formation of the Xisha archipelago,i.e.,the time it finally emerged from under the sea may be dealt with as.(1) Sand bars (1.5-3.0 meters above sea level) took shape about 2000-2500 years ago; (2) Coral reefs (3-5 meters above sea level,with the exception of rock islands) formed 4000-5000 years ago,which is in agreement with the high sea level stage in the post-glacial period; (3) Rock islands (about 15 meters above sea level) formed at late Pleistocene or earlier,which can probably be correlated with the high sea level period some 80000-125000 years ago.