Characteristics and Significances of the Geological Boundary SB21 in the Zhujiang Formation of the Liwan Sag,Pearl River Mouth Basin

Liwan sag is an ultra deep water sag in the southernmost of the Pearl River Mouth Basin(PRMB),whose structural characteristics and sedimentary process is the important part of passive margin evolution.Using seismic stratigraphy,structures and sedimentation analysis were carried out on high-resolution 2D/3D seismic data in the Liwan Sag of the PRMB.A significant geological boundary with obvious characteristics in the Zhujiang Formation could be identified and traced—SB21.Above the sequence boundary,four types of deposition were identified:the denudation zone in the north,the sediment passing zone in the central-north,the superimposition zone of sediment waves over gulliesin the central-south,the sediment accumulation zone in the South.It's suggested that the various depositional pattern has a close relationship with the tectonic movement—Baiyun Movement—which happened in 23.8Ma.The tectonic movement caused the differential subsidence of Liwan Sag.Accelerated subsidence happened in the central and south of the sag,and the relative uplift in the north and east.The former sedimentation equilibrium was destroyed and led to redistribution of the sediments.Sediments migrate from the northern denudation zone andmigrated along the passing zone and finally deposited at the accumulation zone.We inferred that the gullies which move from east to the west were related to the differential subsidence during this period.The differential subsidence caused topographic gradient change between the NS-trending uplift and the sag center.The study of the sedimentary reflection characteristics at the boundary SB21 reveals the complicated sedimentary process and its driving mechanism in ultra deep water area,which will make great contibution to the tectonic sedimentary process in larger areas.