南沙岛礁区中西部海底岩石拖网和发现早第三纪半深海沉积的地质意义

PHOSPHATE AND OTHER ROCKS DREDGEDFROM CENTRAL AND WESTERN NANSHA BLOCK,SOUTHERN SOUTH CHINA SEA

在南沙岛礁区中西部实施海底岩石拖网，有3个站获得海底岩石样品。其中在巽它陆架东北缘水深200多米的NS99－17站见深灰色淤泥中有大量铁褐色含泥钙质结核，说明该区（可能在末次冰期时）处于滨海或海滩氧化环境形成结核，之后海平面上升了至少 150m。在南威岛西北方康泰滩的 NS99－57站 1360m水深的海底获强溶蚀的白垩状多孔灰岩，含底栖有孔虫化石，指示康泰滩是从早──中中新世或更早开始生长的生物礁。在永暑礁东北约55km处的NS99－52站新命名了“东生海山”，其西北坡获含第四纪钙质超微化石的礁灰岩，以及由抱球虫类等生物碎屑和胶磷矿组成的钙质磷块岩和含磷生物屑灰岩滚石，指示半深海环境，其中的钙质超微化石大部分时代为中始新世──渐新世，以始新世可能较大，小部分属第四纪。这次海底岩石拖网为研究南沙岛礁区中部地质构造演变和油气资源评价提供了重要的“硬证据”，尤其是发现早第三纪半深海相钙质磷块岩离南海西南次海盆很近，不但证实了南沙海域广布下第三系海相沉积的推断，而且为判断南海的张开时间提供了补充依据。

Rocks were dredged from 3 stations in central and western Nansha block,southern South China Sea. Large quantities of brown--colored argillaceous and calcareousnodules in dark--grey mud were recovered from the station NS99--l7 at the NE edge of theSunda shelf, indicating the sea level there has raised for more than 150m after the for-mation of the nodules at oxidizing 1ittora1--beach environment possibly during the lastglaciation. At the station NS99--57 of the Kangtai Shoal, highly alterated and porouschalky limestone contalns benthic foraminifera, suggesting an early--middle Miocenereefal buildup. At the station NS99--52 along the slope of a seamount about 55 km NE ofthe Yongshu Reef, reefal limestone with Quaternary calcareous nannofossils and phos-phorite pebbles composed of globigerina and other bioclasts and collophane were collect-ed. Calcareous nannofossils in the pehbles have ages most of Mid Eocene to Oligoceneand subordinary Quaternary. Their phosphoric composition indicates bathyal to pelagicenvironment. These dredged rock samples provide important ' hard evidence' for thestudy of geological history and for the assessment of hydrocarbon potential. In particu-lar, the obtained Paleogene bathyal to pelagic phosphorite near the border of the SWsubbasin of the South China Sea not only verify the widespread existence of Paleogenemarine sediments in the Nansha block, but also provide supplemental evidence for datingthe opening of the South China Sea.