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.

Bifurcation instability and chute cutoff development in meandering gravel-bed rivers

“…In the exper-iment, lateral channel migration and adaptation of the width-depth ratios lead to water level rise and overbank flow across the point bar. The division of flow and sediment results in sedimentation in the inner channel, here the main channel (in agreement with Toonen et al, 2012;Dieras et al, 2013). Sediment mobility in the numerical model is higher compared to experiments, so that a chute channel is excavated before the main channel is closed.…”

“…Because of sediment transport limitation in the inner channel, a plug bar forms that closes the channel so that inevitably the outer channel wins, as shown in the 1D model. The development of a plug bar was also observed for residual channels in the field (Toonen et al, 2012;Dieras et al, 2013) and in flume experiments (Braudrick et al, 2009;van Dijk et al, 2012).…”

“…Following a successful chute cutoff, the reduced sediment transport capacity in the abandoned branch is smaller than the supply that leads to closure by a sandy plug bar (Constantine et al, 2010a;van Dijk et al, 2012). Later, the residual channel is slowly filled by overbank flow and deposition of fine material (Toonen et al, 2012;Dieras et al, 2013).…”

Geometry and compartmentalization of fluvial meander-belt reservoirs at the bar-form scale: Quantitative insight from outcrop, modern and subsurface analogues

“…Muddy and organic-rich deposits dominate the infill of abandoned channels that have been isolated from river flow by processes of meander cut-off or meander-belt avulsion, becoming sites at which low-energy deposition prevails (cf. Fisk 1947;Guccione et al 1999;Toonen et al 2012;and references therein). Where these mud-prone abandoned channel fills border genetically related sand-rich macroforms, they may act as barriers or baffles to hydrocarbon flow, and, as a result, they can compartmentalize meander-belt reservoirs ( Figure 1).…”

Early Middle Pleistocene sediments at Sidestrand, northeast Norfolk, yield the most extensive preglacial cold stage beetle assemblage from Britain

“…Likewise, at face value, the sedimentology of Unit 3E is indicative of low-energy suspension-settling within vegetated wetland terrestrial environment (e.g. an oxbow lake) with deposition punctuated by hiatuses and phases of soil development (Toonen et al, 2012). However, the truncating relationship of both units to Unit 3C, the flame-like margins of Unit 3D, the presence of attenuated sediment inclusions and detachment surfaces towards the base of Unit 3E is not suggestive of a sedimentary mode of emplacement.…”