南海西沙西南海域表层沉积物特征

FEATURES OF THE SURFICIAL SEDIMENTS AT THE SOUTHWESTERN XISHA,SOUTH CHINA SEA

西沙西南海域表层沉积物样品矿物成分、地球化学、微体古生物分析测试结果表明：沉积物可分为７种类型，沉积环境主要是陆坡，部分为深海平原，海洋生物、海洋化学以及火山物质的沉积起到了积极的作用，陆源物质的影响较小，它们主要来源于北部大陆和南部岛礁等物源区。

Based on the various data from the tested 30 spot samples of the surficial sediments at the southwestern Xisha sea area,obtained by the Guangzhou Marine Geological Survey,MGMR,1993,types,mineral compositions,geochemistry and micropaleontology of the sediments are expounded in this paper in order to provide valuable information for marine geological researches of the South China Sea. The sampling spots are almost located at the 2000—3000m water depth area so that the grain sizes of the sediments are fine.The surficial sediments are mainly composed of silty clay or clayey silt and the amount of the biogenic component is more than that of biogenic detritus.They are mainly calcareous organism (including foraminifera,calcareous nannofossil,etc.)and siliceous organism(including diatom,radiolaria,spicula,etc.).The microfossil type and content and the granularity of biogenic detritus are all quantitated in sediment classification and nomenclature.As a result,there are seven types such as calcareous organism silty clay and siliceous organism bearing silty clay,etc.The mineral compositions are divided into detrital and clay minerals.The fragment contains light and placer minerals from the source in Indo China Peninsula,marine authigenic minerals like glauconite and a few weathering minerals.The content of the light and placer minerals is higher in the middle of the study region;the glauconite,in the north;and the weathering minerals,in the middle and the south.The illite,kaolinite,chlorite and montmorillonite are the important compositions of clay minerals and the content of the illite is the highest.The source of the clay minerals is largely terrigenous weathering products from Indo China Peninsula,but may contain the substances transported from the northern continent and the southern archipelago,according to their compositional proportion and distribution.The activities of submarine volcanos have also affected the composition and content of the clay minerals.The oxides,microelements,calcium carbnate and organic carbon are applied to discussion of geochemistry.By the correlation and quantitative analysis,it is known that the marine chemical sedimentation plays much larger role in the feature of geochemistry in this region than the terrigenous does.The foraminifera is paid more attention in micropaleontology analysis.Large quantities of planktonic foraminifera of various species grew in the tropic zone,tropic subtropic zone,tropic wet tropic zone and/or polar region wet subtropic zone.The amount of the foraminifera decreases with the water depth in the study region.