在洪水期时，过量的洪水冲决天然堤，并在堤岸靠平原一方的斜坡上形成树枝状水系的舌状堆积物，称为决口扇沉积。沉积物主要为细砂及粉砂岩等细粒物质，但粒度比相邻的堤岸沉积要粗一些。可见各种小型交错层理，局部有中型交错层理，另见冲刷及充填构造，植物及其他化石是河水带来的。决口扇沉积呈舌状体，断面上星透镜状，厚度不大。
主要沉积微相为水下分流河道、河口坝、远砂坝、席状砂、天然堤、决口扇等。
The main sedimentary microfacies comprise distributary channel, mouth bar, distal bar, blanket sand, natural levee and crevasse splay.
黑石渡组属于向上变细层序，早期为冲积扇沉积，中期为洪泛平原、决口扇和扇三角洲沉积，晚期为深湖浊流沉积。
HsdF developed the fining upward sequences, and alluvial fan in the early stage, floodplain and crevasse splay and fan delta in the middle stage, and lacustrine turbidites in the late stage.
黄河决口扇前缘洼地沉积形成的鱼台组；
Yutai group which is formed by sedimentary of Huanghe cressave splay edge;
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