阵发性洪水条件下辫状河型冲积扇构型研究:以准噶尔盆地西北缘现代白杨河冲积扇为例*

白杨河冲积扇是由阵发性洪水期的碎屑流沉积物与间洪期辫状河沉积物组成的复合型冲积扇,其构型与碎屑流扇的构型和河流型扇的构型有很大的不同。本研究选取准噶尔盆地西北缘干旱气候下发育的现代白杨河冲积扇作为解剖实例,对103个天然剖面和9个人工大型探槽进行了详细测量,在沉积微相及不同级别的沉积构型的观测和分析的基础上,分析了阵发性洪水条件下间歇性辫状河型冲积扇体的地貌单元的演化过程和沉积构型特征,研究不同流态的阵发性洪水条件下各种建造和改造机制,明确受阵发性洪水控制的间歇性辫状河型冲积扇的沉积特征,建立了其沉积构型模式。认为: 在洪水期,阵发性洪流(碎屑流)形成席状化的片流(或片洪)沉积,在洪退期,随着洪水强度的减弱,又转变为辫流沉积,而在间洪期,仍有持续的辫状流体(牵引流)在限制性的水道中流动,并对洪水期的碎屑流沉积物进行改造,形成了限制性的(条带状的)辫状河道沉积,2个时期的沉积物在时空上频繁叠置,形成了一种更加复杂的沉积构型。该模式对油田地下冲积扇砂砾储集层的成因识别、预测及对比具有一定的参考价值。

Architecture of braided river alluvial fan under episodic flood conditions: a case study of modern Baiyang River alluvial fan in northwestern margin of Junggar Basin

The Baiyang River alluvial fan is a composite alluvial fan composed of the debris-flow deposits during the episodic flood period and the braided river sediments during the intermittent flood period. Its architectures are very different from that of debris-flow fans and fluvial fans. The current study selected the modern Baiyang River alluvial fan developed in the arid climate of the northwestern margin of the Junggar Basin as an anatomical example,and conducted detailed measurements on 103 natural profiles and 8 artificial large-scale exploration trenches. On the basis of the observation and analysis,the evolutionary process and sedimentary configuration characteristics of the geomorphic unit of the intermittent braided river alluvial fan under the condition of paroxysmal flood were analyzed,and the paroxysmal flood of different flow states was studied. Various construction and reconstruction mechanisms under different flow regime were analyzed,and the sedimentary characteristics of intermittent braided river alluvial fans controlled by episodic floods were clarified,and their sedimentary architecture models were established. Observations from the current study suggest that in the flood period,the episodic flood formed sheet-like sheet flow(or sheet flood)deposition,and that during the flood retreat period,with the weakening of the flood intensity,it was transformed into braided flow deposition,and that during the inter-flood period,there are still continuous braided fluids(braided rivers)flowing in the restricted channels,resulting in restricted(banded)braided channel deposits. A more complex depositional architectures were seen to occurrence in which the sediments of the two periods were frequently superimposed in space and time. This model has a certain reference value for genetic identification,prediction and comparison of alluvial fan sand-gravel reservoirs in oilfields.