Abstract
Avulsion is a principal process in the formation of fluvial-deltaic successions and a primary control on deltaic architecture. It determines the distribution of sediment and water and hence influences which location in the delta receives clastic sedimentation in what amounts for what time. It also determines channel network configuration and
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recurrence interval of channel belts in the sedimentary succession. Starting from an overview of the depositional history and (quantified) boundary conditions of the Holocene Rhine–Meuse delta, The Netherlands, this paper discusses the implications of avulsions for fluvial-deltaic architecture based on extensive field data, highlighting the geometric properties, channel-deposit proportion (CDP), channelbelt connectedness (CR) and channel-belt width to thickness ratios (W/T). Our study area stretches over 150 km downstream distance and covers fluvial, lagoonal, and estuarine reaches of the Holocene delta. Delta width and thickness increase considerably in downstream direction, from 15 km x 4 m at the apex to 60 km x 20 m at the river mouth. Overall, the architecture and avulsion history is governed by relative base-level rise until 3000 years ago and by human-increased sediment delivery thereafter. Channel-belt CDP, CR, and W/T ratios decrease downstream within the wedge-shaped fluviodeltaic sequence. Their spatial variation is due to differences in provided accommodation space and inherited floodplain geometry, and due to distribution of stream power over branches. Initial high avulsion frequencies decreased with dropping rates of sea-level rise since 9000 years until ; 3000 years ago. Thereafter avulsion frequency increased again due to increased delivery of fine sediment. This multi-millennial trend in avulsion frequency is overprinted with an ; 600 year periodicity,
attributed to intrinsic avulsive behavior occurring in the distributary network. The highest avulsion frequencies occurred in the areas with highest CDP. Differential tectonics between the upstream and downstream delta interrupt overall downstream architectural trends, with a local minimum in W/T and a maximum in CDP and CR over the relative upthrown block. This downstream change favored nodal avulsion immediately downstream of the block. Amalgamated channel belts formed along the sandy margins of the wide backfilled paleo-valley. In contrast, channels of an anastomosed river system in the western–central part of the delta formed isolated channel belts, encased in cohesive floodplain sediments until it was abandoned ; 4000 years ago. In the last 3000 years, when the situation in the delta changed dramatically, major channel belts became established due to major avulsions in the apex region, leading to a substantial increase in channel-deposit proportion. Seven sequences of avulsions, shifting upstream along a feeder branch, have been recognized. Depending on the mechanism creating these sequences, the sequences may influence fluvial-deltaic architecture. Based on the observations in the Rhine–Meuse delta, these sequences seem to be caused mainly by the backwater effect after an avulsion occurred downstream and not by continued growth of alluvial ridges as was suggested by Mackey and Bridge
(1995). Therefore, the high CDP and CR in the upper Rhine–Meuse delta is not related to a particular avulsion-sequence-driving mechanism being dominant.
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