4D flow pattern of the longest cave in the Eastern Alps (Schönberg-Höhlensystem, Totes Gebirge)

The Schönberg-Höhlensystem (SBH) is not only the longest cave system in the Eastern Alps (length 156 km, depth 1061 m), but a significant proportion of the passages have developed on or just below two surfaces that dip 1.7° to the NE. These so-called "speleogenetic phases" are rarely developed in caves of the Northern Calcareous Alps and have not yet been confirmed by detailed morphological mapping. Furthermore, the deep parts of the cave offer the possibility to study the active epiphreatic zone for a distance of 1.6 km. Detailed morphological mapping shows that the main level at about 1500 m a.s.l. and a second 140 m higher developed as distinct speleogenetic phases, and the dip of the planes to the NE is consistent with palaeo-flow. Isolated vadose trenches formed at saddle points are further evidence. Correlations with other caves at similar altitudes suggest an Upper Miocene to Lower Pliocene age. With few exceptions, a northeastern palaeo-flow can be observed down to about 1050 m a.s.l. However, 100 m above this there appears to have been a late phase with an opposite palaeo-flow direction, and below this altitude this SW palaeo-flow direction prevails. These morphological observations allow us to propose a complex model. During low and moderate flow there is drainage to the north. During floods, a restriction in the north causes backwater and an overflow threshold to the SW is exceeded, and then most of the water flows in this direction. This observation of dual flow behaviour can help to better understand the behaviour of the karst water table and to interpret complex results from tracer experiments. The almost ubiquitous paragenetic features in the SBH are due to sediments derived from the local Upper Jurassic Radiolarite Group. For the evolution of karst plateaus in the Northern Calcareous Alps, these observations support a local and rather radial palaeo-drainage of the Totes Gebirge, as opposed to a through-flow of allogenic waters as is likely for other karst massifs.