The Rhenish Massif (Rheinisches Schiefergebirge)

The Rhenish Massif as part of the Variscan mountain chain is a classical region of Palaeozoic successions, particularly Devonian and Carboniferous rocks, which have a wide distribution. Palaeozoic sedimentary rocks are well known for their rich faunas from different facies settings providing the background to define international biostratigraphic units and regional stages. The Rhenish Massif is a classical fold-and-thrust belt and is interpreted to be largely part of the Avalonia Terrane. Beside Middle-Palaeozoic rocks, which dominate the geological record, also Late Palaeozoic rocks occur in the Rhenish Massif, which provide insights into the Variscan Orogeny. Much progress has been made over the last two centuries in terms of biostratigraphy and facies research, and more recently by the application of analytical methods such as isotope analysis, geochemistry and provenance analysis in order to get a better understanding of complex facies settings, tectonic framework, and age-determinations of non-fossiliferous units. This led to new plate-tectonic interpretations particularly in the southern and eastern Rhenish Massif (Taunus, Lahn-Dill area, Kellerwald) where numerous allochthonous units occur. Nevertheless, research over the last 200 years focussed primarily on thick sedimentary rock piles of Devonian and Carboniferous successions representing a huge number of different depositional facies along the southern shelf of Laurussia (Old Red Continent) located at low southern latitudes. Lower Devonian rocks exhibit very diverse neritic facies settings and siliciclastic rocks accumulated in different depocenters onto the shelf up to several thousand metres thickness. During the Middle Devonian, the palaeogeographic position favoured the development of widely distributed coral/stromatoporoid reefs and carbonate platforms. East of the river Rhine (Lahn-Dill area), most reefs are associated with volcanic build-ups, which triggered reef growth at that time, whereas west of the river Rhine platform carbonates with bioherms and mud mounds prevail. In the northeastern Rhenish Massif (Sauerland and Bergisches Land), volcanic build-ups are less frequent, and reefs grew on the inner shelf. Extinction of reefs within the Rhenish Massif followed the global pattern, which means extinction generally occurred well before the global end-Frasnian Kellwasser Crisis, with one exception. The youngest reef of the Rhenish Massif is the Rohdenhaus/Wülfrath Reef, which went extinct during the Upper Kellwasser Event. Drowning and final extinction in different reef settings did not happen isochronically. Late Devonian (Famennian) sedimentation is characterized by mixed siliciclastic-carbonate deposits in more deeper-water facies settings and cyclic sedimentation in the east, whereas more shallow-marine sedimentary successions occur in the west, which are also exposed in northern France and eastern Belgium. The Mississippian successions in the Rhenish Massif are mainly composed of deep-water sediments but shallow-water facies also occur, such as the Kohlenkalk facies near Aachen, Ratingen and Velbert. Carbonates are less prominent, but organic-rich shales, cherts and greywackes represent characteristic marine sediments. During the Pennsylvanian, the marine realm was affected by the prograding Variscan Orogeny, and molasse-type sediments occur in a number of sedimentary basins (e.g., Ruhr Basin), which have been of high economic importance due to paralic coal occurrences. The entire Devonian and Carboniferous sedimentary rock pile within the Rhenish Massif is accompanied by subduction-related and continental intraplate volcanism. Volcanic rocks are documented from the Silurian to Carboniferous. The Rhenish Massif underwent folding and thrusting during the Stephanian, which led to—on average—50% crustal shortening.