渐新世至中新世南海北部IODP U1499站位年龄框架与沉积演化

AGE FRAMEWORK OF IODP SITE U1499 AND SEDIMENTARY EVOLUTION IN THE NORTHERN SOUTH CHINA SEA FROM THE OLIGOCENE TO MIOCENE

南海的沉积演化与构造和洋流活动关系密切,过去对沉积过程的讨论集中在陆坡区,对深水海盆的了解较少。本研究选用南海北部的国际大洋发现计划(IODP)367航次的U1499B钻孔761.86~933.17 m段,在精细的浮游有孔虫生物地层学基础上,结合Sr同位素地层年龄,建立了覆盖30–10 Ma的年龄模型。利用相邻站位同时期沉积物的沉积速率、岩芯物理参数进行对比,总结出南海北部渐新世至中新世的沉积演化历史。研究发现,南海北部陆坡和海盆的沉积演化过程并不相同。30–27 Ma海盆和陆坡大部分地区沉积速率较低,而海盆内沉积物多由浊流或泥石流搬运而来。27–25 Ma期间,洋脊跃迁导致海盆快速扩张并加深,陆地物质供应减少,同时底层流侵蚀作用强,沉积速率降至极低。海盆区域25–13 Ma沉积速率略有提高而后保持稳定,而陆坡地区在25–22 Ma期间构造沉降依然强烈, 22 Ma后才形成稳定的远洋沉积环境。27–13 Ma深水海盆内出现红层沉积, 13 Ma后,青藏高原快速隆升导致陆源输入迅速增加,南海海盆沉积速率加快致使红层消失。陆坡区域则是在15 Ma后红色沉积物增多,这是南海扩张结束后发生快速热沉降,陆坡底部下沉进入了深层水中氧化程度更高的区域所致。

The sedimentary evolution of the South China Sea(SCS) is closely related to tectonic and ocean current activities. The discussion on the sedimentary processes of the continental slope has been well discussed in the previous studies, but the deep ocean basin was rarely mentioned. The 761.86~933.17 m section of Hole U1499B of the International Ocean Discovery Program(IODP) expedition 367 in the northern SCS was selected in this study. On the basis of accurate planktonic foraminifera biostratigraphy, combined with the Sr isotopic stratigraphy and calcareous nannofossil biostratigraphy, an age model covering 30–10 Ma was established. The sedimentary evolution history from the Oligocene to Miocene in the northern SCS was summarized by comparing the sedimentation rates and core physical parameters of the adjacent cores in the same period. It is found that the sedimentary evolution of the continental slope and basin in the SCS is not the same. The terrigenous input was high, but the sedimentation rate was low from 30 to 27Ma in the deep basin. From 27 to 25 Ma, the ridge jump resulted in the rapid expansion and deepening of the ocean basin, terrestrial material supply decreased, with active bottom current and extremely low sedimentation rate. The deposition rate in the ocean basin slightly increased and then stayed steady from 25 to 13 Ma. In continental slope, the tectonic subsidence was still strong from 25 to 22 Ma, and the stable pelagic sedimentary environment was formed after 22 Ma. From 27 to 13 Ma, the red bed deposits formed in the deep-water basin, after 13 Ma, the rapid uplift of Tibet Plateau led to a large increase in terrigenous source supply, which resulted in the disappearance of red beds in the basin.The red sediment increased after 15 Ma on the continental slope, because the oxidation intensified with the deepening of the slope due to sudden thermal subsidence following the termination of the SCS spreading.