The facies model of a meandering fluvial fan and its application
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    Abstract:

    Distributive fluvial systems may form the bulk of the continental fluvial record and may also promote systematical classification of alluvial fan systems and quantification of source—sink analysis. The Okavango Fan provides an excellent member to Distributive Fluvial Systems dominated by meandering and low sinuosity rivers. The Okavango fan may possess many unusual hydrological, sedimentological, and geomorphological characteristics. These are: (1)the dominant singlethread channel bifurcates with distance downstream, creating smaller channels with varying sinuosity,and the fluvial system is composed of a radial network of channels and associated deposits dispersed below an apex where a river emerges from valley confinement and enters a sedimentary basin;(2)The channel bifurcation increases with increasing distance from the apex ,meandering and low sinuosity rivers, both of which are distinctly bedload in character, are mainly because of a restricted source of clastic sediment, and the lowest overall channel gradient measured between apex and toe and the discharge reducing with distance downstream, controls meandering and low sinuosity geometries. Potential control variables of channel pattern transitions are gradient, discharge, sediment size, and bank strength. Channel widths, Water depths, grain sizes, and bankline vegetation vary systematically but not significantly downstream, contributing to an overall downstream decrease in discharge;(3)Our field observations, combined with the results from previous studies, suggest that the fan can be subdivided into four subenvironments. These are: (a) the Panhandle or feeder valley characterized by high sinuosity river belts and scroll bars: (b) the proximal fan characterized by meander belts diverging from the fan apex, comprising peatconfined meandering channels, with interchannel swamps forming thick peats; (c) the middle fan with low to high sinuosity rivers and less common peats ; and (d) the distal fan in which annual floods from relatively unconfined channels spread over the fan surface and interact with vegetation islands;(4)The fan sediments are unconsolidated quartz sands, derived chiefly from Aeolian sands of the Tertiary to Recent Kalahari Basin, so that the supply of fine sediments onto the system is small. Sediment analyses indicate that the most abundant constituent in all the sediment examined is well rounded quartz sand ,and fine matrix are mainly produced biologically and include diatoms, phytoliths and organic matter, and authigenic calcite and silica are also present. Large ‘inland deltas’ in Ordos basin contain river systems that are controlled by similarly channel processes. The explanations for downstream channel adjustments in the meandering fluvial fans thus may be more widely applicable. The fan in Ordos basin is a large shallowly sloping, highly vegetated subaerial fan, which can be subdivided into three subfacies: the proximal fan , the middle fan ,and the distal fan. The proximal fan is characterized by fluvial gravel and sand deposits possibly formed by alluvial fan where water is carried in channels that form the major primary distributaries. Swamps develop in the interchannel areas which sustain permanent wetlands in the upper portion of the fan. The middle fan is characterized by meander belts diverging from the fan apex, comprising meandering channels, with interchannel swamps and fines forming thick floodplain deposits. the dominant channel bifurcates downstream, creating smaller channels with varying sinuosity concomitant with the arrival of the seasonal flood .Largescale avulsion and bifurcation is evident but not prevalent. An increase in sinuosity is formed by lateral migration and point bar deposition. As gradient decreases, channels develop chute and neckmeander cutoffs in areas where complete channel abandonment becomes apparent, thus promoting outofchannel flow and the scouring of new channels or reoccupation of abandoned channels. The lower fan is characterized by unconstrained shallow channel deposits and interacts with preexisting alluvial plain and occasionally lacustrine deposits. The floodplain is intensively cultivated, and the fines are dominated. Comparable ancient examples of the fans dominated by meandering and low sinuosity rivers are recognizable, many giant gas formations in China have depositional processes associated with meandering and low sinuosity rivers.

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ZHANG Jinliang.2022. The facies model of a meandering fluvial fan and its application[J]. Geological Review,68(1):2022010017.

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History
  • Received:April 30,2021
  • Revised:November 28,2021
  • Adopted:
  • Online: January 19,2022
  • Published: January 15,2022