Syn- and post-depositional sand bodies in lignite – the role of coal analysis in their recognition. A study from the Frimmersdorf Seam, Garzweiler open-cast mine, western Germany

Abstract

The Lower Rhine Basin is a Cenozoic-age rift basin at the NW end of the European Cenozoic Rift Basin. In Middle Miocene times, fluctuating sea-levels in the North Sea and the repeated spreading of paralic mires resulted in the deposition of marine sand units (Frimmersdorf and Neurath sands), alternating with thick lignite beds (Morken, Frimmersdorf and Garzweiler seams). Within the Frimmersdorf Seam, the presence of sand bodies has been interpreted both in terms of syn-depositional emplacement in an estuarine setting, as well as post-depositional sand injection and soft-sediment deformation partly linked to tectonic activity within the Lower Rhine Basin. While understanding of the causal mechanisms has improved, early recognition of sand concentrations in future mine area remains difficult. Therefore, organic petrological and inorganic geochemical analyses of sand and lignite samples were carried out in order to assess their efficacy as proxies for such studies. The results reflect the complexity of the depositional environment, which was influenced by both fresh and marine waters. Ash yields of the lignite samples are variable and sulphur contents as well as Ca/Mg ratios depend on the distance from the overlying marine Neurath Sand. Humic substance values of the lignites are dependent on the sampling position within the seam, and may be related to sand body deposition. Moderate contents of gelified macerals and the small size of huminite particles reflect the depositional environment during peat formation, but are not influenced by sand body deposition, not even when in close proximity to the sands. While the geochemical and petrological methods provide much information on the depositional environment of the Miocene-age Lower Rhine Basin, and also on marine versus freshwater deposition of the sands, they do not facilitate the differentiation of syn- and post-depositional sand emplacement in the Frimmersdorf Seam.