HOLOCENE RELATIVE SEA-LEVEL CHANGE AND CRUSTAL MOVEMENT IN THE SOUTHWESTERN NETHERLANDS

Sea-level data collected by Jelgersma (1961) from Zeeland in the southwestern Netherlands show an anomalously high time-depth position with respect to sea-level curves established for the rest of the Netherlands. Until now, it was not clear whether this was related to differential crustal movements, a higher tidal range and/or to groundwater-influenced peat growth above contemporaneous sea level. New radiocarbon dates from the base of the basal peat in Zeeland and the adjacent estuarine flood plain of the River Schelde in Belgium have a considerably lower time-depth position than the original data of Jelgersma (1961). Comparison of the lowest mean sea-level (MSL) time-depth points from the study area with a reference MSL error band for the western Netherlands yields a maximum crustal uplift of Zeeland relative to the western Netherlands of about 0.17 m/1000 yr over the last 8000 yrs cal B.P. A regional MSL envelope for the southwestern Netherlands has been constructed by correcting the reference MSL error band with this maximum rate of relative uplift. The new regional maximum MSL error band plots significantly below the Zeeland curve and the Zeeland data points of Jelgersma (1961, 1966) but is still higher than MSL curves established for the rest of the Netherlands. The remaining vertical distance of the data of Jelgersma to the new Zeeland MSL error band is attributed to early basal peat growth above contemporaneous MSL, primarily due to a groundwater-gradient effect on the gently inclined Pleistocene subsoil. The sea-level index-points with the lowest time-depth position all come from locations close to or within the Late Pleistocene palaeovalley of the River Schelde. This was probably due to the slope of the regional groundwater table towards the palaeovalley, resulting in better drainage conditions on the valley sidewalls. Beat could only develop under the direct influence of the river-water level, which was controlled by sea-level rise. This suggests that comparable palaeovalleys on other coastlines of the North Sea or elsewhere offer considerable potential for sea-level research, if the topography of the compaction-free substratum is well known.