琼东南盆地晚中新世以来底流流向及沉积侵蚀特征

Direction and deposition/erosion characteristics of the bottom currents in the QDNB, northwestern South China Sea

底流(等深流)在珠江口盆地的流动方向、起始时间和沉积物已有大量研究,而琼东南盆地鲜有报道.本文依据测深数据、高分辨率二维、三维地震等,首次以整个琼东南盆地为研究对象,对底流进行了详细的研究.研究表明:琼东南盆地西南端广乐—西沙海槽是该盆地西部唯一的深水海槽,为该盆地提供底流水源,其底流流动方向为SSE-NNW,从广乐—西沙海槽的底流流入琼东南盆地北部后受海南岛隆起的遮挡,向NE向偏转.该盆地底形呈南北高中间低、东深(>2000 m)西浅(<800 m)的台阶状,不同深度(375～1850 m)的南海北部底流(与中层水相关)可能均朝NE向流动,发育时间至少可追踪至晚中新世早期(11.6 Ma).盆地东部和中部都发育一个海山地貌,海山南北侧发育了近东西向环槽地貌,环槽的规模与底流的沉积与侵蚀作用有关.东部海山北侧环槽底流受科氏力影响规模大于南侧环槽;中部海山环槽规模较小,环槽内底流速度较低,以沉积为主,受科氏力影响,北侧环槽内速度较大,沉积速率较小,致北侧环槽内小丘漂积体比南侧环槽小.北礁地区朝SE向迁移的单向侵蚀水道、朝SW向迁移的底流沉积物波和北部单向迁移峡谷都指示了不同深度的底流朝NE向流动.该研究成果第一次系统探讨琼东南盆地底流流向及沉积侵蚀特征,对南海西北部的古海洋、古气候、古环境具有重要意义.

Although the orientation,initial geological period/era, and types of drifts of the bottom currents in Pear River Mouth Basin (PRMB)adjacent to Qiongdongnan Basin (QDNB)have been researched in detail and been well-known,these of the bottom currents were few documented and they are poorly understood in the QDNB.Based on the multibeam bathymetrie data,high-resolution 2D and 3D seismic data,and marine remote sensing technology,the authors first focused on these of the bottom currents in detail in the whole QDNB.The results show that Guangle-Xisha gateway (GXG) on the southwestern QDNB is the only narrow deep water passway, which provide the NNW oriented bottom currents as the source of the bottom currents in the QDNB.The unique geomorphology of the QDNB (i.e.,north and south higher,middle lower,gradually lower from west to east,overall the NE dip of"syncline"of geomorphology) obliged the bottom currents related to the intermediate water to flow from SW to NE in the different depth of QDNB.The scale of northern meat on the north side of seamount with erosion is obviously larger than that on the south side of seamount with progradation in the eastern QDNB,which implies bottom currents have flown from SW to NE since the early Late Miocene(11.6Ma).Small mounded drift within the southern moat on the south side of seamount is slightly larger than that on the north side of seamount with truncation in the middle QDNB,which also indicated bottom currents have flown from SW to NE since the early Late Miocene.The characteristics of SE oriented unidirectionally migrating bottom currents channels and SW oriented migration bottom current sediment waves directly indicate the same features of bottom currents in Beijiao sag.Integrated analysis of the bottom currents from different water depth suggest that they would be related to the intermediate water and their occurrence depth was possibly more than 1850 m.NNW oriented bottom currents through GXG enter into the QDNB,and were deflecteds to NE orientation by the obstruction of Hainan island uplift,which can be confirmed by marine remote sensing data.The results are of great significance to the paleocean,paleoclimate and paleoenvironment of the northwestern South China Sea.