河流决口扇沉积模式及演化规律——以信江府前村决口扇为例

Sedimentary model and evolutionary process of crevasse splays:a case of crevasse splays around Fuqiancun village along Xinjiang River

信江府前村周边发育多个典型的决口扇。决口扇沉积厚度0.3～2m,剖面呈反粒序,可分为内扇和外扇;内扇进一步分为水道和水道间滩2个沉积单元。决口扇演化包括早期、中期、晚期3个阶段：①早期,主河道决口形成单体扇,决口扇表面的水道短而浅,位置不稳定,沉积剖面具有多种形态;②中期,决口扇进积呈树枝状,水道深而长,成为决口扇的骨架,沉积剖面表现为多个单体扇叠置形成的反粒序;③晚期,大部分单体扇废弃成为泛滥平原,少数水道继续下切并延伸,水道两侧形成天然堤,水道内发育边滩等沉积单元,水道进入河漫湖泊可形成决口三角洲。卫星照片表明,决口扇早期生长快,中、晚期生长慢。水道-主河道距离、河道宽度是控制水道分汊角度的主要因素。可以通过沉积规模、沉积物特征、测井曲线的幅度变化、旋回变化、砂体平面形态、剖面形态、与河道共生关系等特征识别古代决口扇。

Four typical crevasse splays（CS）have developed around Fuqiancun village along Xinjiang River.CS present sedimentary thickness of 0.3-2 m and reverse graded profiles.A CS includes inner fans and outer fans.An inner fan can be divided into crevasse channels and interchannel beaches.A CS evolves in three stages：the formation stage,the extension stage and the channelized stage.In the formation stage,a single fan is formed due to burst of levee;channels on CS surface are short and shallow with instable locations;the sedimentary profile shows multiple morphologies.In the extension stage,CS progradation is formed in arborization pattern;channels are deep and long,and become the framework of a CS;the sedimentary profile shows the reverse grading structure formed by the superposition of multiple single fans.In the channelized stage,most single fans are abandoned to be flood plains,and a small number of channels continue to incise and extend headward.Natural levees are formed on both sides,and sedimentary units such as point bar appears.In the stage,crevasse deltas can be formed when the channels enter alluvial lakes.Satellite images indicate that a CS grows fast during formation stage while slowly during extension and channelized stage.Paleo-CS can be identified by sedimentary scale,sediments characteristics,amplitude change of logging curve,cycle change,sand plane form,profile morphology,the coexistence with river channels and other characteristics.