乍得Bongor盆地北部斜坡带P组沉积特征分析

Study of sedimentary characteristics of the P Formation in the north slope of Bongor Basin,the Republic of Chad

乍得西南部的Bongor盆地是受中非剪切带影响发育的中—新生代被动裂谷盆地。在P组沉积时期,Bongor盆地处于裂陷湖盆初始发育阶段,总体处于持续水进扩张过程,发育陆相扇三角洲-湖相沉积。岩心观察结果表明,P组沉积物具有近源、深水、粗屑、重力流为主以及持续水进等特征。结合岩心、录井、测井及地震资料分析,明确Bongor盆地发育扇三角洲相、湖泊相及重力流沉积,其中扇三角洲相发育扇三角洲前缘和前扇三角洲亚相,扇三角洲前缘亚相发育水下分流河道、河口坝、水下分流河道间和席状砂微相。以Bongor盆地北部斜坡带Baobab北次凹为例,进行构造-沉积耦合分析,认为其存在2大物源体系:长轴方向发育三角洲相沉积,短轴方向存在2种受构造控制的沉积模式,分别为在构造调节带(断层生长连接点)发育扇三角洲相沉积,在非构造调节带发育重力流沉积。

Bongor Basin,located in the south-west of the Republic of Chad,is a Mesozoic-Cenozoic passive rift basin evolved under the influence of shear zone of Central Africa Rift.Bongor Basin was in the beginning of a rift lacustrine basin development and always kept expansion through a continuous transgression progress during the sedimentation of the P For-mation,during which terrestrial fan delta-lacustrine deposited.The results of core observation show that the P Formation are characterized by near-provenance,deep-water,coarse-grain,gravity-flow and consecutive transgression sediments.Combined with the indication of well-logging and seismic data,it is determined that fan delta,lacustrine and gravity flow develop in the study area.The fan delta consists of fan delta front and pre-fan delta sub-facies,and the fan delta front con-sists of submarine distributary channel,estuary bar,interdistributary and sand sheet.Taking the sub-sag in the north slope of Bongor Basin as an example,it is concluded that two main provenance systems exist through coupling analysis of struc-ture and sedimentation,where delta deposits along the long axis.There are two structure-defined sedimentary modes built along the short axis:fan delta grows in the structure-controlled(fault-connected)zone,while gravity flow grows in the nonstructure-controlled zone.