南海南部海域南薇西盆地新生代沉积特征

CENOZOIC SEDIMENTATION OF NANWEIXI BASIN,THE SOUTHERN SOUTH CHINA SEA

南薇西盆地主体奠基在南沙地块上,发育古新世-第四纪地层,最大沉积厚度为11000m.通过分析区域地质背景和盆地沉积演化史,认为南薇西盆地是在南海北部被动大陆边缘上裂解形成的.成盆初期(初始裂陷期)与华南大陆相连,其沉积特征与南海北部新生代盆地类似,为陆相沉积环境.晚始新世以后,随着南海海底扩张,南沙地块裂离华南大陆并向南漂移,发生大规模海侵,盆地进入主裂解扩张期;至早渐新世,盆地渐次由海陆交互相过渡为海相;晚渐新世-中中新世,盆地逐渐由断陷转为断拗,沉积范围不断扩大,整体处于海相沉积环境.晚中新世以后,盆地进入浅海-半深海沉积环境的裂后热沉降期.通过对比解释大量地震资料,在盆地内划分出3个超层序、6个层序,结合地震相的分析,对上新统以下的4个层序进行了沉积相推断解析.

Nanweixi basin is developed on Nansha block and has a sedimentation history from Paleocene to Quaternary, with a maximum sediment thickness up to 11,000 m. Analyses of the regional geological setting and the basin sedimentation history suggest that rifting of the passive continental margin along the northern South China Sea formed Nanweixi basin. At the early stage of basin evolution, the area was still connected with South China continent, with a nonmarine sedimentation environment similar to the Cenozoic basins in the northern South China Sea. As the result of spreading of South China Sea since Late Eocene, Nansha block was separated from the continent and drift southward, occurring a wide transgression and a main stage of basin rifting and extending. In Early Oligocene, an alternate continental_marine environment gradually became a marine environment. During the period from Late Oligocene to Middle Miocene, the basin changed from rifting to depression, and the depositional area continued to expand in the marine environment. After Late Miocene, the basin got into a post_rift thermal subsidence stage in a shallow to semi_deep sea environment. By interpretation of a lot of seismic data in Nanweixi basin, three super sequences and six sequences are identified, and in combination with analyses of seismic facies, the sedimentary facies are concluded for the four sequences below Pliocene in the basin.