Alluvial architecture of the Holocene Rhine-Meuse delta (The Netherlands) and the Lower Mississippi Valley (U.S.A.)

Abstract

Alluvial architecture describes the geometry, proportion, and spatial distribution of different types of fluvial deposits in an alluvial succession. Alluvial architecture is frequently subject of study, because natural resources commonly occur in ancient fluvial sequences. The ability of models to simulate alluvial architecture realistically is largely unknown due to a lack of natural data to test the models. Generating high-resolution datasets describing alluvial architecture of natural fluvial systems would help the development of better-performing models of fluvial stratigraphy. This Ph.D.-thesis describes the alluvial architecture of two Holocene fluvio-deltaic settings: the Rhine-Meuse delta (The Netherlands) and the Lower Mississippi Valley (U.S.A.). The research goals were: 1) to quantify alongstream variability of channel-belt dimensions; 2) to assess the relative importance of various external controls for the formation of fluvio-deltaic wedges; 3) to quantify and compare alluvial architecture of both study areas. The alluvial-architecture parameters explored include sand-body geometry (width, thickness, and width/thickness ratio), channel-belt deposit proportion (CDP), and connectedness ratio (CR). Two particular aspects of alluvial architecture are highlighted: 1) channel-belt geometry; 2) valley-wide patterns in alluvial architecture. Channel-belt geometry was determined using cross sections across eight channel belts in the Rhine-Meuse delta and one channel belt in the Lower Mississippi Valley. It was found that the width of all channel belts encased in cohesive deposits decreases by a factor of 4 to 6.5 in a downstream direction. The width/thickness ratio decreases by a factor of 2.5 to 5. These observations are related to variations in bank erodibility and stream power that influence lateral migration rates of channels. Valley-wide patterns in alluvial architecture were quantified using detailed cross-valley sections. For each cross section, the alluvial-architecture parameters were quantified. Alluvial architecture of both fluvio-deltaic systems appears to be similar and varies both spatially as well as temporally. For example, CDP and CR are twice as high in the upstream part than in the downstream part of both study areas. The observed trends in alluvial architecture are related to variations in aggradation rate, floodplain geometry, and channel-belt size. These in turn are related to changing external forcings, but also to variations in intrinsic fluvial processes. Tectonic movements affect alluvial architecture locally. The similarities in alluvial architecture suggest common controls, operating in both study areas in a similar manner. Factors that controlled alluvial architecture of areas include: river gradient, bank erodibility, floodplain width, base-level rise, aggradation, differential subsidence and neotectonics, and sediment supply. Despite the new results of this study, more work needs to be done with regard to characterisation of fluvio-deltaic successions in distal areas of deltas, assessing three-dimensional architecture, and implementation of architectural field-data into alluvial-architecture models. With regard to alluvial-architecture modelling, it is imperative to define realistic channel-belt geometries, because of the importance for alluvial architecture. The Rhine-Meuse delta is a suitable test case for alluvial-architecture simulation, because customary model parameters such as aggradation rates, avulsion frequency, channel gradient, channel width and depth, floodplain width and length, and channel-belt geometry are well-constrained.