Sedimentary Architecture of Abandoned Channel Fills

Channel fills are common elements of Holocene river systems and older fluvial sequences, but surprisingly little is known about formation and their sedimentary build-up. Abandoned channels result from channel shifting processes at various scales, including meander cut-off and channel belt avulsion. Channel fill sequences are of importance as containers of palaeoenvironmental proxy-records and materials that allow dating of abandonment events. This paper intends to make channel fill sequences more useful as recorders of channel abandonment processes and as recorders of palaeofloods, for which improved understanding is needed of the internal build-up of channel fills. We propose a sedimentary-architecture descriptive scheme that distinguishes elements from two stages of channel-fill development; (i) the abandoning stage with initial proximal fill, and (ii) the resultant fully abandoned palaeochannel that collects distal fill. We review oxbow lake infilling along meandering rivers and supplement it with details on two selected field examples from the apex-region of the Netherlands’ Rhine delta. We integrate this with knowledge on channel abandonment dynamics in meander cut-off situations and in bifurcating river situatuations in low-gradient valley and delta plain settings, including insights from recent numerical modeling work. Improving our understanding of channel fills requires to distinguish between infilling modes of relatively rapidly cut off meanders (the outcome of neck and chute cut-offs) and infilling modes of relatively slowly abandoning bifurcation channels (proceeding avulsion), respectively resulting in oxbow lake fills and residual channel fills. The geometrical and physical insights are important for studies that harvest proxy data from channel fills and for those that want to reconstruct palaeochannel dynamics from the fill sequence, e.g. to derive palaeoflood series. Integrated knowledge on the dynamic nature of channel abandonment and resultant sedimentary recording is a necessity for comparing and collating records from series of abandoned channel fills.