试论西沙群岛石岛的形成

石岛是西沙群岛宣德环礁上的一个小岛,地处北纬16°21’、东经120°21’,与永兴岛共处于一个珊瑚礁坪上(图1)。

A PRELIMINARY STUDY OF THE FORMATION OF SHIDAO ISLAND, XISHA ISLANDS

Shidao Island is the highest one in the Xisha Islands. It lies on the same coral reef flat together with Yongxing Island. Shidao Island is covered totally by a kind of biogenie calcareous limestone. The studies on components and sedimentary characteristics have shown that these biogenie calcareous limestones were the sedimentary bodies of the sand cay or bap on the reef flat. The present Shidao Island is the southeastern part of the fossil sand cay formed in the past. Due to the secular effect of NE monsoon, the northern reef flat and sediments of ancient Shidao Island have been warn out. Whereas, a new reef flat and a present sand cay, the Yongxing Island, have been grown at its southern part. The most of ¹⁴C dates from 29 samples of the biogenie calcareous limestone are ranged from 23,000-12,0000 BP. However, these apparent ages are much more younger than their real ones. The reason is that the ¹⁴C samples have encountered the alteration of early diagenesnis. We deduce that these biogenie calcareous limestones probably formed in a phase about 125,000 BP when a high sea level appeared in the world, but it cannot rule out the possible forming in a certain phase of the relative higher sealevels between 105,000 a,nd 40,000 BP.