Reoccupation of Channel Belts and its Influence on Alluvial Architecture in the Holocene Rhine-Meuse Delta, the Netherlands
Esther Stouthamer, 2005. "Reoccupation of Channel Belts and its Influence on Alluvial Architecture in the Holocene Rhine-Meuse Delta, the Netherlands", River Deltas–Concepts, Models, and Examples, Liviu Giosan, Janok P. Bhattacharya
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Avulsion was a principal process in the development of the Holocene Rhine-Meuse delta (The Netherlands). Two avulsion styles occurred: (1) regional avulsion: the new channel followed an entirely new course, and (2) avulsion leading to reoccupation. In the case of reoccupation, there are two possibilities: the new channel reoccupied (a) its previous channel (local avulsion) or (b) a preexisting channel. Over the Holocene time scale, avulsion style was related to aggradation rate and coastal evolution. Avulsion by diversion into flood basins (regional avulsion) was prevalent during the period of rapid sea-level rise, from 7300 cal yr BP until approximately 4700 cal yr BP, when the coast existed as a fragmented system of coastal beach barriers and rivers discharged into the North Sea at various locations. The number of reoccupations remained relatively low. Avulsion by channel reoccupation occurred more frequently after 4700 cal yr BP, and reached a maximum after ~ 3200 cal yr BP, during a period of slow relative sea-level rise (slow aggradation rate), and a “closed” coastal barrier with only two remaining tidal inlets. The closed coastal barrier system stimulated reoccupation of channel belts, because channels were forced to merge before discharging into the North Sea. Reoccupation was further enhanced by a combination of factors, including flood-plain morphology, low gradients, widespread formation of wood peat, and tectonic tilting, leading to reoccupation lengths (distance over which the new channel reoccupies an older channel belt) of over 60 km in the near-coastal part of the delta. Long reoccupation lengths lead to few channels in the near-coastal area of back-barrier deltaic plains, and thereby influence alluvial architecture.
Successive reoccupations can lead to multiple stacked channel-belt sand bodies with thick (> 3 m) natural-levee deposits and/or with overbank deposits at different stratigraphic levels. However, from the stratigraphy alone, reoccupied channel belts cannot be distinguished from channel belts that experienced discharge fluctuations. Average interoccupation time (time between the abandonment of the old channel and the beginning of reoccupation by the new channel) increased after 4700 cal yr BP, which is explained as a result of decreased regional aggradation rate.
Contrary to common belief, avulsion frequency increased during a period of low regional aggradation rates and reached a maximum after ~ 3200 cal yr BP. This is explained by increased local sedimentation rates (within-channel and/or on natural levees) after approximately 5000 cal yr BP (4500 14C yr BP).
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Deltas are amongst the most environmentally and economically important coastal sedimentary environments. Studies of deltas lag behind research in both fluvial and deep-water depositional systems, as well as more geomorphologically oriented land studies. This knowledge lag reflects both a reorientation of the petroleum industry in the last two decades toward deep-water systems, as well as the difficulty of working across the shoreline with the traditional tools used for oceanographic or land-based work. However, deltaic studies are experiencing a renewed focus, because of their global importance in environmental and other societal concerns. This volume stems from a special session: Deltas: Old and New, held at the Annual Geological Society of America conference in October 2002, that was convened to highlight these new directions in deltaic research.