砾质辫状河型冲积扇沉积微相及沉积模式:以准噶尔盆地西北缘现代白杨河冲积扇为例*

冲积扇砂砾岩储集层具有相变快、连续性差等特点,显示冲积扇内部沉积环境的复杂性。本次研究以准噶尔盆地西北缘现代白杨河冲积扇为例,在丰富的野外露头资料基础上,结合区域水文资料以及冲积扇文献资料,对白杨河冲积扇沉积演化特征及沉积模式进行探讨。按其沉积机制,白杨河冲积扇属于辫状河型冲积扇,具有规模大(扇体总面积约327.6km²)、坡度平缓(约4‰~7‰)、沉积粒度粗和漫洪细粒沉积物不发育的典型特征。在白杨河冲积扇内共可识别出16种岩相类型,并根据岩相形成的流体动力差异划归为5类成因,即重力流成因、高流态牵引流成因、低流态牵引流成因、静水沉积成因以及风成沉积成因。白杨河冲积扇扇体建造过程可划分为洪水期和间洪期2个时期,洪水期以沉积作用为主,从扇根至扇缘依次发育扇根补给水道沉积、扇根片流沉积、扇中片流沉积、扇中辫状水道沉积、扇缘径流水道沉积和扇缘湿地沉积6种沉积微相类型;间洪期以改造作用为主,从扇根至扇缘依次发育扇根主槽沉积、扇中辫状沟槽沉积和扇缘湿地沉积3种沉积微相类型。最后,依据白杨河冲积扇建立了砾质辫状河型冲积扇洪水期和间洪期的沉积模式。

Microfacies and sedimentary models of gravelly braided-river alluvial fan: A case study of modern Baiyanghe-river alluvial fan in northwestern margin of Junggar Basin

Alluvial fan coarse-grained reservoirs are characterized by fast facies change and poor continuity,which shows the complexity of the depositional environments. In this study,a modern Baiyanghe-river alluvial fan is located at the northwestern margin of the Junggar Basin,and it is studied to explore the complex sedimentary characteristics and sedimentary model of alluvial fan,by using abundant outcrop data,hydrological data and alluvial fan publications. According to the depositional mechanism,the Baiyanghe-river alluvial fan belongs to a braided river alluvial fan,with a large scale of fan area(~327.6 km²),gentle slope(4‰~7‰),abundant coarse-grained sediments and little flooding fine-grained sediments. Sixteen lithofacies are recognized in the Baiyanghe-river alluvial fan. According to the hydrodynamic differences,the forming mechanism of lithofacies are classified into five genetic types,which are debris flow,high-flow traction current,low-flow traction current,hydrostatic deposition and aeolian deposition. The constructional process of the Baiyanghe-river alluvial fan can be divided into two periods,flood period and inter-flood period. During the flood period,deposition plays a major role. From proximal to distal part of alluvial fan,six types of microfacies can be recognized,followed as feeder-channel and sheetflood deposition in proximal part,sheetflood and braided-stream deposition in intermediate part,channel and wetland deposition in distal part. However,during the inter-flood period,reworking is the major process. Three microfacies are developed form proximal part to distal part,including main-channel deposition in proximal part,braided-trench deposition in intermediate,and wetland deposition in distal. Each microfacies is in transitional contact,without obvious interface. Sedimentary models of gravelly braided-river alluvial fan were finally established based on the data of Baiyanghe-river alluvial fan.