Abstract
The objective of this thesis is to analyze and explain the architecture, facies distribution, age and origin of coarse-grained overbank deposits, with special attention for organic-clastic lake fills, and organics in the distal Holocene Rhine-Meuse delta plain. In order to depict the influence of lakes and their fills on the
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development and architecture of fluvial systems, I reconstructed the development in the Angstel-Vecht area. To obtain insight in the facies composition, architecture and formation of organic-clastic lake fills, I investigated these deposits in the central delta and in an area adjacent to the central delta. It was shown that delta-plain lakes affect downstream architecture and facies composition of fluvial deposits. Owing to trapping of the coarsest sediment grains in lakes, channel deposits downstream of lakes are relatively fine-grained and are accompanied by relatively thin natural-levee deposits. The large proportion of sand that is contained in organic-clastic lake fills, in addition, provides weak channel banks compared to the clay and peat usually found in distal delta plains. Channels that traverse organic-clastic lake fills, therefore, have a tendency to meander instead of being straight. Additionally, a synthesizing conceptual model is presented that describes the development of these deposits. The sedimentary and botanical variability of the basal-peat layer was described, first, by designing a key for classifying organics. The classification key for organics enabled identification of four types of sedimentary organic facies (algal, detrital, calcareous and siderite gyttja) and four botanical-organic facies (wood, reed, sedge and oligotrophic peat). Second, application of this key on archived borehole descriptions resulted in determination of the distribution of organic facies in the basal-peat layer in the distal part of the Rhine-Meuse delta in ~4000 cores. These data were interpolated, which resulted in a distribution map of basal-peat organic facies. It was feasible to obtain the spatial distribution and spatial proportion of reed peat (63%), wood peat (8%) and gyttja (29%). Organic facies variability in the basal-peat layer indicates the presence of three main hydrological domains that characterized the onset of Holocene aggradation in the Holocene Rhine-Meuse delta: marine-dominated, fluvial-dominated and seepage-dominated. I used existing valley-wide cross sections to determine the delta-scale spatial and temporal distribution and lithofacies composition of coarse-grained overbank deposits. The results show that both narrow and wide floodbasins tend to have relatively low proportions of crevasse-splay deposits whereas intermediate floodbasin widths, 3.1-3.6 km in the Rhine-Meuse delta, yield optimum crevasse-splay deposit proportions. The lithofacies composition of crevasse-splay deposits and organic-clastic lake fills is found to be controlled by channel planform, superelevation and by the composition of the substratum. Coarse-grained overbank deposits along anastomosing river channels contain more sand relative to those associated with meandering river systems. Finally, we found that coarse-grained overbank deposits have a positive effect on reservoir volumes and connectedness ratios. This study shows that the ubiquity of organics and the lakes that are associated with peat-forming wetlands have had a great influence on fluvial processes and thereby affected facies composition and architecture of fluvial deposits in distal delta plains
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