中国南海北部珠江口盆地早渐新世末破裂不整合特征及其地质意义

珠江口盆地破裂不整合(T7)是南海北部边缘海盆地内一系列重要不整合界面之一,界面上下的物源与沉积环境等都有显著变化,其形成与盆地的动力学演化、海平面变化及油气聚集具有密切的联系。本研究依托地震数据和井资料,对研究区早渐新世末期T7(32~30 Ma)不整合的分布与结构特征,以及不整合分布进行了描述,对破裂不整合形成机制以及其体现的盆地演化过程进行了探讨。研究表明,研究区T7界面是具有破裂不整合面的明显特征,大部分断裂在T7终止活动,或经过T7界面后断距大幅度减小;而不整合面下伏的恩平组整体呈楔状,地层厚度明显受控于断裂活动。T7不整合在珠江口盆地范围内的分布划分为3个区带,分别是北部的角度不整合带、局部不整合带和上超带以及整合带,并与强剥蚀带,弱剥蚀带和无剥蚀带相对应。3个区带内的不整合具有不同的结构特点,角度不整合带主要以断块旋转、肩部削蚀为特征,局部不整合和上超带以珠海组上超和局部剥蚀为特征,且局部剥蚀面积与北部角度不整合带相比明显减少,整合带(南部隆起带以南)主要以整合和上超特征为主。这种展布模式说明不整合的分布样式与不同区带的构造作用和古地貌的差异有关。北部角度不整合带主要受控于断裂旋转和抬升作用,而在局部不整合和上超带内,剥蚀由底辟或局部隆起导致,整合带位于当时凹陷中心的水下环境。这样的模式反映破裂不整合面的形成与裂谷期盆地边缘的肩部隆起削蚀有关。

Characteristics and geological significance of unconformities at the late Early Oligocene in the Pearl River Mouth Basin,northern South China Sea

The widespread unconformity(T₇) in the Pearl River Mouth Basin(PRMB), the origin of which is closely related to dynamic basin evolution, eustatic variation and hydrocarbon accumulation, is one of the most important unconformities of the basins located at the northern margin of South China Sea.The provenances and sedimentary environments above and below the interface show significant differences.Based on seismic and logging data, we documented the distribution and structural features of the unconformity in the PRMB during the late Early Oligocene T₇(32~30 Ma), and discussed the mechanism of the unconformity and the basinal evolution features indicated herein.Results show that the T₇ boundary is a stage boundary unconformity with most faulting terminating at T₇ or the fault displacement diminishing markedly across the unconformity, while the underlying Enping Formation formed in the shape of a wedge with its stratum thickness obviously controlled by faulting.The T₇ unconformity can be divided into three zones:the angular unconformity zone, local unconformity and onlap zone, and conformity zone in the north, corresponding to the strong erosion zone, weak erosion zone, and non-erosion zone respectively.The angular unconformity zone is characterized by truncation at the shoulder part due to fault block rotation; the local unconformity and onlap zone shows onlap features and local denudation that is significantly gentle compared with angular unconformity zone; the conformity zone is located on the south of the southern uplift in PRMB and characterized by onlapping at the depression margin and conformity contact in the depression center.The distribution pattern of the stage boundary unconformity in this study area is controlled by tectonism and palaeogeomorphology in different zones of the basin.The angular unconformity zone in the north is mainly controlled by fault rotation and uplifting, but in local unconformity and onlap zone, the erosion is mainly caused by diapir or local uplifts.The conformity zone was located under paleowater level and at the depression center.The pattern indicates that the formation mechanism of the stage boundary unconformity is related to the erosion on shoulder uplift structure at the basin margin at the rifting stage.