TOPOGRAPHY AND ITS CONTROL OVER DEEPWATER SEDIMENTATION IN THE QIONGDONGNAN BASIN

The Qiongdongnan Basin is located in the northwest of the South China Sea,and the deepwater area of the basin is a new favorable prospect for hydrocarbon exploration.Based on seismic and bathymetric data,the authors focused in this paper on the deep water topography of the Basin and the control of topography over the deepwater sedimentation.Various landform types were distinguished and their distribution pattern described.Discussed are also the depositional mechanism of the gravity deposits and their types,distribution and lithological characteristics,so as to reveal the control of topography on the deepwater depositional systems.Results show that the submarine topography of the Qiongdongnan Basin can be divided into three units of the continental shelf,the continental slope and the deep-sea plain.Under the control of topography,there are 6 types of sediments in the Qiongdongnan continental slope system: slope canyon,turbidite fan,slumping block,debris flow deposits,sliding mass and sedimentary waves.Our results prove that the types of sedimentary facies are closely related to the gradient of the slope.The gradient not only controls the distribution of types of gravity flow deposits,but also the adjustment of slope system.In addition,the gravity flow may change from one to another due to the change of landforms,for example,change from turbidity-current to sediment waves,or from slumping and/or debris flow to turbidity current.