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The carbonate complexes in the Rhenish trough of the Variscan geosyncline (central Europe) range from late Givetian to early Frasnian and are restricted to the external and internal shelves and isolated submarine volcanic rises. On the western part of the external shelf (southeast margin of the Old Red continent), the carbonates form a widespread shelf-lagoon facies on deltalike clastic deposits (Belgium, Aachen, Eifel); in the eastern part they are isolated reef complexes and shelf-margin reefs on locally higher exposed platforms (Bergisches Land, Sauerland). On the internal shelf (northwest margin of the "Mitteldeutsche Schwell") the carbonates overlie crystalline rocks (borehole Sarr 1) or clastic Devonian strata (Giessen). The Middle to Upper Devonian carbonates gener lly are 350-400 m thick, and at Balve (Sauerland) they are more than 1,000 m. In the internal part of the trough the carbonates form isolated submarine volcanic rises on submarine ophiolites (Lahn-Dill syncline, Elbingerode in the Harz Mountains).
Carbonate sedimentation starts everywhere with a widespread carbonate bank (Schwelm facies). This bank is the foundation for the subsequent younger true reefs. The well-bedded bank carbonates are commonly dark and fine grained. The potential reef builders--stromatoporoids and tabulate and rugose corals--built flat, widespread biostromal structures in a muddy environment rather than wave-resistant structures. Within the bank, 8 subfacies can be distinguished.
Overlying the bank, isolated and locally restricted true reefs (Dorp facies) show mostly atoll-like features. At the western margin of the Old Red continent
a now-eroded, oblong reef rim probably existed. Within the reefs, 7 forereef, 2 reef-core, and 12 backreef subtypes can be distinguished. Transgressions and regressions of the sea resulted in cyclic sedimentation on the flat, widespread shelf-lagoon. Locally the transgressive cycles start with black marls, whereas the regressive cycles terminate with laminites and erosion features. The topmost parts of the subsiding reefs are built of convex limestone caps (Iberg facies), tens of meters thick; there is no backreef lagoon facies. Two facies subtypes can be recognized within these very fossiliferous limestone caps--biodetrital limestones with a high original interframe porosity, and micritic limestones with "stromatactis" (so-called still-water bioherms).
The interreef basins between the isolated reef complexes are characterized by black bituminous shale (so-called "Flinz" facies). Also in the geosynclinal trough, dark, pelagic shales are present. Limestone turbidites are continuous from the outer forereef flanks into the adjacent deeper basins.
Dolomitization occurs mainly in the fine-grained bank types and the micritic backreef subtypes, whereas the reef-core and the sparry-cemented, forereef subtypes are less dolomitized. The dolomitization is preponderantly epigenetic (bound to joints, faults, bedding planes, or schistosity planes). No economic discoveries of oil or gas have been made in the Devonian carbonate complexes in central Europe. Origin and source of asphaltite in the isolated small Iberg-Winterberg reef, Harz Mountains, are still unresolved.
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