Using a 148km long cross-section of Holocene deposits, to study natural subsidence along the Dutch coastal plain

Abstract

This two-part research into subsidence over the Dutch coastal plain uses a 147.7 km long cross-section to study differences in subsidence between the south and the north of the Netherlands. The focus lies on splitting the total natural subsidence into three components: Glacio-isostatic adjustment due to the collapse of the peripheral forebulge when the Scandinavian-British ice sheet started to melt; Tectonic subsidence, caused by the lowering of the North Sea depocentre; And, compaction of Middle-Holocene peat layers, due to peat compression and oxidation. In the first part, a vertical profile is created that describes lithogenetic units along the full cross-section, with a focus on Noord-Holland where lithological descriptions are scarce. The lithological relations from the crosssection, in combination with 14C samples, are used to create isochrones that span the whole study area. The depths of the isochrones are measured at 5km intervals. These values are used to calculate spatial isochrone depth differences, which provides a quantification of differential subsidence. The results from this exercise were compared to interpolated subsidence values from parallel research by De Wit et al., (in prep.) and used to quantify differences in reconstructed and interpolated isochrones characteristics. Comparing differential subsidence along the cross-section shows that in the north isochrones lie up to two meters deeper compared to the south. This difference is largely described by subsidence due to glacio-isostatic adjustment and tectonic subsidence. However, subsidence due to peat compaction also causes large differences in isochrone depths. Peat compaction is a likely cause to why the reconstructed isochrones are found deeper compared to the interpolated isochrones from De Wit et al., (in prep.), which excludes this effect of compaction.