Sedimentary architecture of abandoned channel fills

ABSTRACT 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 cutoff and channel‐belt avulsion. Channel‐fill sequences are of importance as containers of palaeoenvironmental proxy‐records, can be used to reconstruct palaeochannel dynamics and derive palaeoflood records, and contain materials that allow dating the abandonment. Integrated knowledge on the dynamic nature (geometrical and physical insights) of channel abandonment and resultant sedimentary recording is a necessity for comparing and collating records from a series of abandoned channel fills. This paper intends to make channel‐fill sedimentological sequences more useful recorders of channel abandonment processes and palaeofloods, for which improved understanding is needed of the internal build‐up of channel fills. We review oxbow lake infilling along meandering rivers, and supplement this with highly detailed descriptions of two selected field examples of channel fills from the apex‐region of the Netherlands' Rhine delta. From these examples it becomes clear that regional setting and type of abandonment result in different channel‐fill end‐members; oxbow cutoffs generally produce thick laminated clayey fills as the channel entrance is plugged rapidly, avulsion‐abandoned channels are filled with coarse (proximal) deposits as a result of a maintained open river connection. Field examples of channel fills are integrated with knowledge on channel abandonment dynamics in meander cutoff and bifurcating river situations, including insights from recent numerical modelling. We propose a sedimentary‐architecture descriptive scheme that distinguishes elements from two stages of channel‐fill development; (i) the abandonment stage with initial proximal fill, and (ii) the subsequent fully abandoned palaeochannel that collects distal fill. Copyright © 2011 John Wiley & Sons, Ltd.