Spatial and Temporal Distribution of Sand-Containing Basin Fills in the Holocene Rhine-Meuse

Abstract

The quantitative significance of coarse-grained deposits in the overbank realm, such as crevasse-splay deposits, has not been studied at the delta scale or at the Holocene timescale. Such knowledge would be beneficial for understanding and explaining sediment distribution in delta plains. This study addresses delta-scale distribution of sand-containing basin fills and their sand-body proportion variability, based on eight valleywide cross sections in the Holocene Rhine- Meuse delta in the Netherlands. We found that sand-containing basin fills form 7.1% of the fluviodeltaic wedge, of which splay deposits are most frequently observed midway between the delta apex and the coast. Organic-clastic lake fills and bay-head delta deposits, in contrast, are limited to the distal delta plain. Over four successive periods (between 9000 and 800 cal yr BP), the largest proportions of splay deposits remain at 50–150 km downstream of the upstream-shifting delta apex. We show that intermediate floodbasin widths (between 3.1 and 3.6 km in the Holocene Rhine-Meuse delta) yield the highest proportions of splay deposits. High rates of base-level rise and wide floodplains both facilitate the creation of accommodation, which in turn provides conditions for peat-forming wetlands in which organic-clastic lake fills can develop. The results show that sand bodies form 26%–30% of sand-containing basin fills. This proportion is shown to be controlled by, among other variables, channel planform and superelevation of the trunk channel and substrate composition. We conclude that potentially large volumes of nonchannel sand bodies exist in distal delta plains. They constitute up to 39% of the reservoir volume in the distal Rhine-Meuse delta and yield relatively high connectedness ratios.