珠江口盆地陆丰凹陷恩平组内部构造—沉积转换面识别及意义

Characteristics and significances of the tectono-sedimentary transitional unconformity in Enping Formation of Lufeng Sag,Pearl River Mouth Basin

晚始新世珠江口盆地由裂陷阶段逐渐过渡到裂后阶段,同期沉积的恩平组记录了构造—沉积转换的构造变革和沉积演化过程,备受关注。以陆丰凹陷为例,基于层序地层学、沉积学理论与技术,结合地震相特征、岩电特征、沉积旋回特征,识别出恩平组内部存在一个全区可追踪对比的构造沉积转换面——T72界面,探讨了该界面的地质意义。结果表明:①研究区在恩平组裂陷幕开始发生构造—沉积转换,以T72为分界面,断裂活动由南向北规律性迁移,沉降中心、断陷的萎缩与消亡具有自南向北逐步变新、逐步演化的特征;②沉积环境由半封闭逐渐转变为开放状态,沉积物源供给方式由盆内多物源近距离供给向盆外大水系远距离供给转变;③惠州11洼为下恩平期的沉降中心,发育滨浅湖—半深湖相烃源岩,是潜在的富烃洼陷。

During the Late Eocene,the Pearl River Mouth Basin was in transition from rifting to post-rifting.Evolution of tectonic and sediments at the tectono-sedimentary transitional signature were recorded at the Enping Formation.Based on the sequence stratigraphy and sedimentology,the seismic facies,well log,and sedimentary cycles were used to identify the T72 unconformity that is widely recognized at the Lufeng Sag.Three conclusions can be drawn:Firstly,the T72 unconformity in Enping Formation defines the boundary between rift and post-rift sediments.Fault activity and the subsiding center in the study area migrated from the southern margin to the northern margin,and,the depressions are regularly shrinked and demised.Secondly,the sediment environment varied from semi-enclosed environment to open environment.During rifting,the Lufeng Sag was supplied by the multiple proximal sources.Once rifting ceased,it was dominated by the mega-fluvial sediment supply.Finally,the Huizhou11 sag formed the subsidence during the depositional period of the lower Enping Formation,and developed shore-shallow,middle-deep lacustrine source rocks.Source rocks are rich in the Huizhou11 Sag.