神狐—一统暗沙隆起中部新生代地层层序划分及沉积演化

Sequence Division and Sedimentary Evolution of Cenozoic in the Middle Shenhu-Yitong Ansha Uplift

利用中德合作“南海地球科学联合研究”SO 4 9航次所采集的地震测线 ,对南海北部陆缘神狐—一统暗沙隆起中部的新生代地层层序进行了划分 ,并与北部的珠江口盆地的层序进行了对比。共识别出SB1、SB2 、SB3 、SB4和Tg5个主要反射界面 ,对比表明 :Tg反射面为新生界基底反射 ,SB1、SB2 、SB3 、SB4相当于珠江口盆地的二级层序界面 ,层序界面年龄分别为 :10 .5Ma、2 1.0Ma、2 5 .0Ma、39.4Ma。在此基础上 ,对隆起区新生代沉积环境及其演化进行了分析 ,认为隆起区新生代沉积经历了断陷湖泊—滨浅海—深海相的演变过程 ,隆起区主要隆升时期为晚渐新世—早中新世。

Based on the seismic profiles acquired by cruise SO 49 of 'China and Germany Joint Study on Marine Geosciences in the South China Sea', and correlated with those of Pearl River Mouth Basin, sequence division and correlation of Cenozoic in the middle Shenhu Yitong Ansha Uplift were carried out. 5 seismic reflectors, including SB 1, SB 2, SB 3, SB 4?and T g , have been recognized and their ages have also been assigned according to their seismic reflection characteristics and correlated to their counterparts in the Pearl River Mouth Basin. SB 1 is the boundary between middle Miocene and Upper Miocene, being the response of Dongsha Movement in northern shelf of South China Sea, which age is 10.5 Ma. SB 2 with truncation, incised valley filling and onlap represents the lowest sea level stage during Miocene, and can be easily traced in the northern shelf basins of South China Sea; its age is 21.0 Ma . SB 3 with truncation and onlap on the massif is the boundary between Eogene and Neogene, its age is 25.0 Ma . On the uplift SB 3 is represented by a hiatus between late Oligocene and lower Miocene where being lack of sequence SB 3 SB 2 on the high, which may be correlated to the hiatus spanning from 24～27.0～27.5 Ma in site 1 148 of ODP Leg 184. SB 4 is the boundary between upper and middle Eocene, representing the second stage of Zhuqiong Movement in northern shelf basins of South China Sea, which age is 39.4 Ma. Tg is the boundary between Cenozoic and pre Cenozoic, which can be easily traced. The sedimentary environments and evolution of different sequences stated above have also been analyzed according to their seismic characteristics and combining with those of the Pearl River Mouth Basin. Sequence T g SB 4 and SB 4 SB 3 were mainly controlled by separated half grabens, developing fan delta, fluvial, shallow lake and swamp sediments, similar to those in the other areas in northern shelf of South China Sea, but the subsidence rate on the uplift was much lower, resulting in the forming of shallower lake and coarser sediments. After uplift and erosion stage during late Oligocene to early Miocene, transgress started and sequence SB 3 SB 2 onlapped towards the uplift, developing coastal and shallow marine sediments, but the uplift high remained eroded and acted as a barrier between the Pearl River Mouth Basin and central sea basin. SB 2 represents a regional sea level fall and the paleo shelf break probably shifted to modern lower slope. After that, sequence SB 2 SB 1 onlapped to northern shelf and uplift, which had subsided under sea level, thus the modern framework of shelf slope central basin formed, and the uplift was covered by shelf slope sediments. Sequence SB 1 sea floor is dominated by slope sediments similar to modern sedimentary environment.