Flow regime evolution of a major cave system in the Eastern Alps (Hirlatzhöhle, Dachstein)

The 116 km-long and 1560 m-deep Hirlatzhöhle is one of the major cave systems in the Northern Calcareous Alps (NCA; Austria). It is located in the NW part of the Dachstein, an extensive karst massif encompassing 576 km² with its highest point at 2995 m a.s.l. In contrast to most other caves in the NCA, Hirlatzhöhle comprises old (epi)phreatic passages located up to 1 km above the base level as well as two modern major drainage systems. The aim of this study is to define the palaeo- and the active flow conditions in combination with speleogenesis, and the age of the cave levels of Hirlatzhöhle. We use morphological as well as sedimentological studies and correlations with other caves. Another difference from cave systems in the NCA is that the majority of passages in Hirlatzhöhle are not developed within the so-called Giant Cave Level between 1200 and 1800 m a.s.l., but deeper between 800 and 1300 m. Most parts of Hirlatzhöhle developed under epiphreatic conditions as indicated by rills and condensation corrosion cupolas, which is the case for much other cave systems in the NCA. In contrast, paragenetic features like canyons and ceiling channels are relatively rare as are insoluble sediments. Elongated scallops indicate that flow velocities were high and abrasive sediments were abundant. Opposite to the nearby Dachstein-Mammuthöhle and other caves east of it that show a west-directed palaeo-flow, scallops in Hirlatzhöhle indicate a NE-directed palaeo-flow and an autogenic recharge. Modern drainage is autogenic and N- to NE-directed as well. Even though burial age dating did not give reasonable results for Hirlatzhöhle yet, the correlation with other adjacent caves suggests a Late Pliocene to Early Pleistocene age of the main palaeo-phreatic level. This is supported by an infinite U-Th age (>0.6 Ma) of a flowstone.