Bongor盆地北部斜坡带次级凹陷裂陷期粗粒扇体沉积特征、差异充填模式及控制因素

Sedimentary characteristics, differential filling pattern and controlling factors of coarse-grained fans during the rift period in the secondary depression of Northern Slope Zone, Bongor Basin

摘要

以非洲中部典型裂谷Bongor盆地北部斜坡带为例,基于岩心、测井和地震资料,识别盆地北部斜坡带次级凹陷下白垩统粗粒扇体沉积特征,明确层序格架下不同成因扇体的沉积过程和差异充填模式,分析盆地充填的控制因素。结果表明:研究区发育扇三角洲和近岸水下扇两类粗粒扇形沉积体,分别沿盆缘边界断裂和盆内基底凸起分布,表现为牵引流搬运成因的水下分流河道-河口坝和碎屑流搬运成因的碎屑流朵体-分支水道特征。扇三角洲呈长帚状,以前积式结构、进积叠置为主;近岸水下扇呈裙带状,表现为加积式结构、垂向叠置。在完整层序格架内,由低位域期、湖侵域期到高位域期,扇三角洲较近岸水下扇对湖平面的变化响应更明显。裂陷期活跃构造为两类粗粒扇体的发育提供背景,盆内凸起和盆缘边界的相对陡-缓地形主导两类粗粒扇体的沉积搬运成因差异,较强的盆外物源供给使得扇三角洲展布规模大于盆内物源供给的近岸水下扇的。该结果为裂谷盆地粗粒扇体沉积充填过程研究及有利勘探目标预测提供指导。

Abstract

Taking the subdepression of the northern slope zone of the Bongor Basin, a typical rift in Central Africa, as an example. Based on core, logging, and seismic data, the coarse-grained sedimentary characteristics of the Lower Cretaceous in the Northern Slope Zone of the basin are identified. The sedimentary processes and differential filling model of different genesis fans under the sequence framework are clarified. The controlling factors of basin filling are also elucidated. The results showed that two types of coarse-grained fan bodies, namely, fan delta and nearshore subaqueous fan, were identified. They developed along boundary faults of the basin edge and the basement uplift in the basin, respectively. They respectively consist of underwater channels-mouth bar caused by traction flow and lobe-branching channels caused by debris flow. The fan delta is long and broom-like, mainly characterized by a progradational structure and progradational stacking. The nearshore subaqueous fan is a skirt-like feature, exhibiting an aggradation structure and vertical stacking. Within the complete sequence framework, from the low system tract, transgression system tract to high system tract, the response of fan delta to changes in the lake level is more pronounced than that of nearshore subaqueous fans. The active tectonics during the rifting period provided the background for the development of coarse-grained fans, and the relatively steep/gentle topography of the basin's uplift and boundary dominated the differences in sedimentary transport genesis between the two types of fans. The strong external source supply promotes the distribution scale of fan delta to be larger than that of nearshore subaqueous fans with internal source supply. The research results are of great guiding significance for understanding the sedimentary filling process of coarse-grained fan bodies in rift basins and predicting favorable exploration targets.

关键词

粗粒扇体 / 沉积特征 / 盆地充填 / 控制因素 / 下白垩统 / 北部斜坡带 / Bongor盆地

Key words

coarse-grained fan / sedimentary characteristics / basin filling / controlling factors / Lower Cretaceous / Northern Slope Zone / Bongor Basin

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基金资助

国家自然科学基金项目(92255302)