The Rhenish Massif: More than 150 years of research in a Variscan mountain chain

The Rhenish Massif is a classical fold-and-thrust belt and is interpreted to be part of the Avalonia Terrane, which separated from Gondwana in the Early Ordovician (e.g. Romer and Hahne 2010; Linnemann et al. 2014). The later collisionwith Baltica and Laurentia led to the closure of the Iapetus Ocean and Laurussia (“Old Red Continent”, ORC) was formed (Kroner et al. 2007; Nance et al. 2010). The Rhenish Massif, as one of the structural units of the Variscan orogen, was subdivided by Kossmat (1927) into the Moldanubian, Saxothuringian, and Rhenohercynian zones as well as the Subvariscian. Kossmat`s (1927) model was later modified by Brinkmann (1948), who added the MidGermanCrystalline Zone, which is interpreted as a structural high between the Rhenohercynian and Saxothuringian zones. The Rhenish Massif represents a classical region of Palaeozoic successions, particularly Devonian and Carboniferous rocks, which have a wide distribution (Fig. 1). Palaeozoic sedimentary rocks are well known for their rich faunas, which were described from various facies settings (Fig. 2). Facies analysis and detailed knowledge of fossils are crucial to get a better understanding on complex interactions of palaeoecology and facies settings. This finally led to the intention to publish this special issue, which has a clear focus on palaeontology, facies analysis, biostratigraphy, and biogeography. Facies analysis and rich faunas from the Rhenish Massif provided the background to interpret various facies settings and fossils were used to define international biostratigraphic units and regional stages. The Wetteldorf section (Fig. 3) at the southern limb of the Prüm Syncline, Eifel region, is the Global Stratotype Section and Point (GSSP) of the Lower-Middle Devonian Boundary (Emsian-Eifelian Boundary). The base of the Eifelian stage is defined by the first occurrence of the conodont Polygnathus costatus partitus. Based on the wellestablished biostratigraphic record, the section was restudied in a cyclostratigraphic framework, which points to the imprint of the 18-kyr precession cycle (De Vleeschouwer et al. 2018). However, there is further potential to define biostratigraphic units and boundaries as is suggested in this issue. Lower Devonian rocks exhibit thick siliciclastic successions, which have been traditionally assigned to two facies settings, the “Rhenish” and the “Hercynian” facies (Erben 1962). As this “two facies” concept oversimplifies diverse neritic environments, new subdivisions have been proposed, based on benthic assemblages (see Jansen 2016, cum lit). Biostratigraphic correlation of neritic facies and more hemipelagic settings is a challenge and requires more future research. During the Midand early Late Devonian, the palaeogeographic position of the Rhenish Massif favoured the development of coral-stromatoporoid reefs and carbonate platforms, which are widely distributed. On a global scale, the Eifelian to Frasnian represents the time of maximum reef growth in Phanerozoic Earth history. Caused by lateral and vertical changes in palaeoecology, reef palaeotopography, synsedimentary tectonics, and/or active volcanism, the reef development varied greatly at a local scale. Whereas in the northern part of the Rhenish Massif, east of the river Rhine, laterally extensive and hundreds of metres thick reef complexes became established on the inner shelf (e.g. Hagen-BalveReef) or at its margin (e.g. Attendorn and Brilon reefs), in the Lahn-Dill area to the south, most reefs are associated with volcanic buildups within the outer shelf basin. West of the river Rhine platform carbonates with bioherms and mud-mounds prevail (e.g. Krebs 1974; Becker et al. 2016a, b; Königshof et al. 2010; Stichling et al., this issue). A number of This is the editorial to the special issue “The Rhenish Massif: More than 150 years of research in a Variscan mountain chain”.