The Carboniferous of the Netherlands and surrounding areas; a basin analysis. Geologica Ultraiectina (294)

Abstract

This thesis deals with several aspects related to the Carboniferous of the Netherlands and surrounding areas. In the first part, the tectonic setting and sedimentary infill of the Northwest European Carboniferous Basin (NWECB, running from the United Kingdom to Poland) are illustrated by palaeogeographical maps and well correlation panels. As such, the reader is introduced into the other chapters which focus on more specific research questions related the Carboniferous in the study area. Due to deep burial, the Lower Carboniferous is very poorly known in the central part of the NWECB (northern part of the Netherlands and adjacent areas). Using high-quality 3D seismic data, it was now possible to show that Early Carboniferous carbonate platforms are present in the central part of the NWECB. A model is proposed in which carbonate platforms are limited to structural highs while in intervening graben areas deep-water sediments accummulated. In general, the Late Carboniferous is characterised by siliciclastic sedimentation. In northern England, it is proposed that sedimentation took place at varying basin-water salinities (marine-lacustrine). Cores obtained from this area have been sampled and analysed for major and trace elements. Based on these data, it is suggested that the basin essentially remained marine. The succession of Upper Carboniferous siliciclastic sediments is very thick in major parts of the NWECB. In this thesis, it is investigated which subsidence mechanisms were responsible for the accommodation of these sediments. Simple modelling experiments showed that the conventional mechanisms (flexural subsidence and thermal subsidence, following an Early Carboniferous rifting event) cannot account for the total observed subsidence. Some additional mechanisms (dynamic subsidence, intra-plate stress) or another phase of extension (rifting) are proposed. The Westphalian of the NWECB is characterised by fluvial-lacustrine deposits. Marine bands can be found intercalated in this succession. These marine bands are thin mudstone horizons (up to 30 m) deposited during marine flooding of the delta plain. Some of these marine bands are characterised by a high uranium-enrichment, which is detectable by gamma-ray devices. However, the marine bands found in the Netherlands do not show these enrichments. A relatively high siliciclastic sedimentation rate and the (periodic) absence of anoxic bottom-water conditions are probably the main factors limiting the enrichment of uranium and other trace elements in these marine bands. The NWECB was fragmented into a number of sub-basins during the Late Westphalian due to long-wavelength folding and faulting. Most of the gas reservoirs in Carboniferous rocks are found in thick fluvial sandstones deposited during this time. In the northeastern part of the Netherlands, the Coevorden Gasfield is characterised by 40m-thick fluvial sandstones of Late Westphalian age. In this thesis, a detailed reconstruction is presented of the alluvial architecture of the Coevorden Field