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The urbanised peat-rich coastal-deltaic plain of the Netherlands is severely subsiding due to human-induced phreatic groundwater level lowering, as this causes peat layers to compress and oxidise. To determine the potential susceptibility of this area to future subsidence by peat compression and oxidation, the effects of lowering present-day phreatic groundwater levels were quantitatively evaluated using a subsidence model. Input were a 3D geological subsurface voxel-model, modelled phreatic groundwater levels, and functions for peat compression and oxidation. Phreatic groundwater levels were lowered by 0.25 and 0.5m, and the resulting peat compression and oxidation over periods of 15 and 30 years were determined. The model area comprised the major cities Amsterdam and Rotterdam, and their surrounding agricultural lands.
The results revealed that for these scenarios agricultural areas may subside between 0.3 and 0.8m; potential subsidence in Amsterdam and Rotterdam is considerably lower, less than 0.4m. This is due to the presence of several metres thick anthropogenic brought-up soils overlying the peat below the urban areas, which has already compressed the peat to a depth below groundwater level, and thus minimises further compression and oxidation. In agricultural areas peat is often situated near the surface, and is therefore highly compressible and prone to oxidation. The averaged subsidence rates for the scenarios range between 7 and 13mma−1, which is corresponds to present-day rates of subsidence in the peat areas of the Netherlands. These results contrast with the trend of coastal-deltaic subsidence in other deltas, with cities subsiding faster than agricultural areas. This difference is explained by the driver of subsidence: in other deltas, subsidence of urban areas is mainly due to deep aquifer extraction, whereas in the Netherlands subsidence is due to phreatic groundwater level lowering.
The present paper, dedicated to the legacy of local geologist–engineer Peter Jozef (Sjeuf) Felder, who died in 2009, confirms his view that bioclasts constitute a valuable tool in the correlation of outcrops and borehole cores across the type area of the Maastrichtian Stage in the vicinity of Maastricht. His approach of interpreting changes in bioclast contents as having been influenced by Milankovitch cyclicity has here been applied successfully to the entire sedimentary complex of Maastrichtian (latest Cretaceous) age in the study area. In the present approach, results are corroborated by index fossils, mainly dinoflagellate cysts but also calcareous nannofossils, which allow correlation with the Stevns-1 core reference section in eastern Denmark. With the exception of local remnants of Belemnella obtusa Zone age, the Maastrichtian Stage in its type area encompasses the last 4.6 Ma of the Cretaceous Period (i.e. the Belemnella sumensis/Acanthoscaphites tridens Zone up to the K/Pg boundary). P.J. Felder's bioclast analyses have enabled the detection of twelve 400 kyr eccentricity cycles of Milankovitch cyclicity in the area. However, the section is not continuous; there is a hiatus of c.700 kyr between the Gulpen and Maastricht formations at the ENCI-HeidelbergCement Group quarry. In addition, smaller hiatuses, usually in the range of several 20 kyr cycles, have been detected in the upper Maastricht Formation.
Due to canal-digging activities in 2011 and 2014, two small and one large temporary exposure, all ranging from 4 to 5 m in depth, were studied with respect to the sedimentology and structural geology, in the glacial ridge of Midwolda, Groningen, the Netherlands. The lowermost unit consists of clay of Elsterian age and is composed of glaciolacustrine and turbiditic deposits (Peelo Formation). These show synsedimentary deformations due to loading, as well as post-sedimentary Saalian glaciotectonic deformations, consisting of folding, and faulting structures. The overlying Saalian till sequence consists of two main units. The lower unit, with clear features of a subglacial deformation zone (e.g. lateral heterogeneity), has a local origin and strongly resembles the underlying Elsterian clay. Glacial tectonic and morphological observations indicate a primary NE–SW ice-flow direction. The second till layer has a sandy texture and high crystalline gravel content, while glacial-tectonic indicators point to a NW–SE ice-flow direction. The deformation of the till layers has caused a repetition and mixing of till layers, due to the last ice movement. The NW–SE ice movement is supported by the morphology as well as data from erratic gravel counts. Correlation with geological cross-sections strongly suggests regional subsurface control on ice-sheet behaviour.