西沙群岛现代海滩岩岩石学初见

引言我国美丽富饶的西沙群岛位于海南岛东南180海里南海西北部台阶式大陆坡的西沙台阶之上,由近40个岛洲礁滩组成。其中岛屿部分,除高尖石是由火山岩构成的以外,其余均为珊瑚礁岛屿。

PRELIMINARY STUDY OF PETROLOGY ON RECENT BEACHROCKS IN THE XISHA ISLANDS, GUANGDONG PROVINCE

Xisha Islands are mostly built up by reefs. In the intertidal zone of the Islands sands and gravels consist of calcareous skeletal fragments of organism, including corals, coralline algae, foraminifers, bivalves, green algae, echinoderms, gastropods etc., which are cemented and lithified in different degrees to form recent beachrocks. Within beachrocks grains amout to 50-85%, cement or matrix about 5%, porosity 10-45%.The sorting coefficient (So) of loose sediments at intertidal zone is generally between 1-1.58, and in a few samples > 1.58. The mean diameter (Md) of grains at windward side of the Islands is larger than that at the leeside. This sorting is clearly effected by prevailing winds.The intertial zone is marked by high energy conditions with periodic alternation of dry and wet. The cementation of beachrocks took place in warm sea water. Three types of cementation may often be observed: (1) aragonite mud (micrite) and needles (scicular aragonite) rim; (2) aragonite needles (inner) and carbonate mud (mainly aragonite) mixed with bioclastic silt (outer); and (3), as opposed to (2), carbonate mixed with bioclastic silt mud and aragonite needles.No pillar and micritic high-Mg calcite cements were found in beachrocks of Xisha Islands. The cements of beachrocks evidently differ from those of granular calcite (sparry calcite) which are subaerially exposed under the effect of meteoric water.