珠江口盆地始新统“源-渠-汇”分析——以阳江东凹和开平凹陷为例

Eocene “provenance-channel-sink” analysis of Pearl River Mouth Basin:A case study of eastern Yangjiang Sag and Kaiping Sag

以阳江东凹及开平凹陷为研究对象,运用碎屑锆石U-Pb定年、地震剖面渠汇分析及沉积相分析等手段,系统分析文昌组和恩平组沉积期沉积物来源、输送渠道以及沉积形式,将“源-渠-汇”耦合的思想运用到珠江口盆地始新统沉积演化中。碎屑锆石U-Pb定年结果表明,研究区物源在时空上均表现出不均一性,文昌组阳江东凹碎屑锆石年龄以燕山期为主,年龄谱图表现为单峰值,开平凹陷则存在燕山期和印支期两个峰值,锆石自形程度均较好,指示盆内近源沉积;恩平组碎屑锆石年龄谱图均具有多峰值特征,且古生代和前寒武纪的锆石数量增多,锆石磨圆度高,说明开始接受盆外物源,为盆内和盆外双重供源。基于大量的地震剖面,在研究区分别识别出断槽、古沟谷及构造转换带等物源运输通道,阳江-一统暗沙断裂带的走滑作用对研究区始新统沉积物沿上述通道输送起到了重要作用。整体来讲,文昌组沉积期主要发育滨浅湖相、半深湖相和扇三角洲相,恩平组沉积期则主要发育辫状河三角洲相和滨浅湖相。本研究对于认识阳江东凹和开平凹陷始新统“源-渠-汇”过程以及阳江-一统暗沙断裂带在始新世的表现形式具有重要意义。

Taking the Eastern Yangjiang Sag and the Kaiping Sag as the research objects,we systematically analyzed the provenance,transport channels,and sedimentary facies of the Wenchang and Enping depositional periods by means of detrital zircon U-Pb dating and seismic profiles analysis,and then applies the idea of "provenance-channel-sink" to the Eocene sedimentary evolution of the Pearl River Mouth Basin.The U-Pb dating results of detrital zircons show that the provenance in the study area is heterogeneous in time and space.For the ages of detrital zircons from Wenchang Formation,the ones in the Eastern Yangjiang Sag mainly show a Yanshanian unimodal peak,while those in the Kaiping Sag display Yanshanian and Indosinian bimodal distributions.The zircons have good euhedral morphology,indicating the near-source deposition in the basin.The age spectra of detrital zircons from the Enping Formation show multimodal distributions.The number of older(Paleozoic and Precambrian) zircon grains increased and they are in higher roundness,indicating that the basin provenances had changed from near-source to far-sources.Based on a large number of seismic profiles,the transport channels such as fault trough,ancient gully,and structural transition zone were identified,which indicated that the strike slip of the Yangjiang-Yitong Fault Zone played an important role in transporting Eocene sediments in the study area.In addition,mainly littoral-shallow lacustrine facies,semi-deep lacustrine facies,and fan delta facies developed in the Wenchang depositional period,while braided river delta facies and littoral-shallow lake facies are more commonly observed during the Enping depositional period.This study is of great significance for understanding the Eocene "Provenance-Channel-Sink" process in the Eastern Yangjiang Sag and the Kaiping Sag,and the manifestations of the Yangjiang-Yitong fault zone in the Eocene.