Hypogene Caves in Germany, Geological and Geochemical Background

Abstract

Germany exhibits a very diverse geological history. Thus, a large number of stratigraphically, petrographically and tectonically different carbonate and sulfate rock deposits exist that have been subject to karstification. First, we discuss the possible “agents” (sensu Klimchouk) of hypogene karstification. Three principally different processes are identified: water rising because of buoyancy (either thermally or concentration induced), in situ oxidation of siderite or rising gases (CO2, CH4 or H2S). Next, German caves and karst are discussed in order of stratigraphy. Applying the most pertinent epigene versus hypogene morphological characteristics, it becomes evident that hypogene caves occur in many different areas, often side by side with clearly epigene caves. For many areas, the agents of hypogene speleogenesis still remain unclear. This applies for most caves in the Paleozoic limestones of the Rhenish Schist Massif. The largest of these caves is the >700 × 200 × 20 m measuring cavity at the bottom of the Wulfrath quarry, filled with Lower Cretaceous sand. Only the Iberg/Harz caves seem to be a clear case; here, the worldwide highest concentrations of siderite weathering-induced caves occur. In the South Harz, large cavities were discovered in anhydrite below the water level by mining for the underlying upper Permian Copper Shale. Apparently these were formed by water rising from underlying carbonate aquifers. The large cavities discovered recently in the Blauhohlen System and some of the deep pit-caves in Upper Jurassic limestone of the Swabian Alb may have their explanation in volcanic CO2, having emanated from some of the 355 pipes of the Swabian volcanic field. Most striking is the high concentration of hypogene caves in the central Franconian Alb that occur in a small area while the surrounding areas are almost devoid of larger caves. In the geological past, this region formed the center of a continental basin where artesian water was able to ascend along major fractures. This could explain the strange geographic cave distribution as well as suggest that the ascending waters carried sulfide or methane from below.