Geostatistical analysis of overbank deposits of anastomosing and meandering fluvial systems; Rhine-Meuse delta, The Netherlands

Abstract

The architecture of overbank deposits of anastomosing and meandering rivers in the central Rhine-Meuse delta (The Netherlands) is analysed by geostatistical investigation of three detailed cross sections in two study areas. The thickness of the overbank deposits in the cross sections were examined by semivariogram analysis. The cross sections show that two different types of overbank deposits are found at different depths at the same location. At greater depths, lithologically complex overbank deposits with many crevasse splays and a varying thickness are found. At shallow depths lithologically uniform overbank deposits with a more or less constant thickness are found. The semivariogram analysis serves as a tool to distinguish between these two types of overbank deposits. Semivariograms of the lithologically complex overbank thicknesses strongly differ from the semivariograms of the more uniform overbank deposits. The former show large nugget values combined with large sill values and a small range. The latter show small nugget values and a clear trend and no sill is reached. The complex overbank deposits are thought to be deposited by anastomosing rivers, the uniform overbank deposits by meandering rivers. Maximum boring intervals for mapping of the overbank deposits can be derived from the semivariograms. If a complete understanding of the fluvial architecture in the study areas is needed, e.g. for reservoir characterization, a sampling grid of 25 m perpendicular and 100 m parallel to main flow direction is necessary.