The concept, characteristics and significance of fluvial fans

ZHANG Yuanfu,DAI Xin,WANG Min,LI Xinxin

Table 1 Differences between fluvial fans and related sedimentary bodies.

Type External geometric morphology Morphology of channels inside the fan Development position Development scale Hydrodynamic condition Sediment transport mechanism Fan surface slope Termination type Lithology and matrix Typical sedimentary unit Typical sedimentary sequence Fluvial fan Fan shape on the plane and wedge shape on section Straight channels, bending channels, divaricating channels, wandering channels Edges of basins, mountainous areas, open depressions adjacent to uplands, and regions fluvial systems pass Greater than 10 km in radius Strong to moderate Dominated by tractive current <1° Converging into seas (lakes) and other stable water bodies, flowing into stable rivers or wet lands, terminates at the end Mainly pebbly sandstone, sandstone, argillaceous siltstone etc., dark mudstone in local parts behind bank, with high content of matrix and unstable components Abandoned channels, and aeolian sediments Positive and reverse rhythms Alluvial fan Fan shape on the plane and wedge shape on section Unstable short-term channels Edges and outlets of basins and mountains Less than 30 km in radius Strong (fast attenuation) Dominated by gravity flow and sheet flow >3° Terminates at end, or forms stable river Mainly conglomerate, with sand, silt and clay clasts filling between gravels, with high contents of matrix and unstable components Debris flow and cribriform sediments Positive and reverse rhythms Fluvial system Long axis and stripe shapes on the plane and lenticular or wedge shapes on cross-section Straight channels, meandering channels, braided channels Any position along the continent or continental margin From dozens of meters to thousands of meters in width Strong in denuded zone, and weak- medium in sedimentary zone Tractive current Flowing into seas (lakes) and other stable water bodies, or terminates at end Mainly sandstone and siltstone, followed by clay, with high contents of matrix and unstable components Channel filling sediments Positive rhythm, river-binary structure Delta Bird foot or stripe shape on the plane and lenticular or wedge shape on cross- section Straight channels, meandering channels, braided channels Edges of seas (lakes) and other stable water bodies From dozens to hundreds of kilometers in radius Medium- weak Dominated by tractive current, and reworked by waves and lakes/seas. Meandering river and braided river deltas<1° Flowing into lakes (seas) and other stable water bodies Mainly sandstone and dark mudstone, with low contents of matrix and unstable components Overwater/ underwater distributive channel sediments Reverse rhythm, composite rhythm