珠江口盆地荔湾凹陷珠江组关键地质界面SB21的识别及地质意义

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

荔湾凹陷是珠江口盆地最南部的一个超深水凹陷,其结构特点、沉积过程研究是认识被动陆缘演化的重要内容.运用地震地层学方法,基于高分辨率2D/3D地震资料的沉积追踪与对比,一个特征明显且全区可追踪的地质界面——SB21界面被识别出.该界面上下揭示出多种沉积现象:北部剥蚀区、中北部的沉积物过路区、中南部冲沟和沉积物波叠置区、南部的沉积物堆积区.推测这种沉积样式的多样性与发生在23.8Ma前后的白云运动有密切关系,该构造运动造成荔湾凹陷的差异性沉降,主要表现为荔湾凹陷中南部的快速沉降,导致凹陷北部及中东部的相对抬升.该期构造运动使原有的沉积平衡发生改变,造成沉积物的再分配,沉积物从北部剥蚀区经过路区向中南部堆积区运移.自东向西发育的冲沟,推测与该时期差异沉降导致中东部近南北走向的相对隆起与西侧沉降区的差异沉降,以及有来自凹陷东北角的物源叠加作用有关.SB21界面的沉积反射特征研究,揭示了洋陆边界复杂多变的沉积过程及其驱动机制,对我们认识更大区域的构造沉积过程具有一定的帮助.

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, struc- tures and sedimentation analysis were carried out on high-resolution 2D/3D seismic data in the Liwan Sag of the PRMB. A sig- nificant 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. 8 Ma. 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 and migrated along the passing zone and finally deposited at the accumulation zone. We inferred that the gullies which move from east to 1he 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 uhra deep water area. which will make greal contibution to the tectonic sedimentary process in larger areas.