Case Studies from the Dinaric Karst of Slovenia

Upper Cenomanian and Turonian to Senonianlimestones. The cave passages are formed inthe Postojna anticline, which is oriented NW–SE (Gospodarič 1976), most of channels beingin its steeper south-western flank. The cavesystem is in the tectonic block confined by twodistinctive dextral strike-slip fault zones in theDinaric trend, the Idrija and Predjama faults(Buser et al. 1967; Placer 1996). Habič (1982)explained different features, such as collapsesand sumps in the cave, as the results of neotectonicmovements. A geological survey ofthe cave passages was made by Gospodarič(1964, 1976) and Šebela (1994, 1998a, b) addedstructural mapping in detail. The cave andthe Pivka underground river have a generalN–S trend.The known passages were formed attwo main levels. The upper level is between529 m a.s.l., at the main entrance to the caveand 520 m a.s.l. in the Črna jama. This levelis composed of large passages, generally up to10 m high and wide. Their profiles are morerounded and show also traces of paragenesis(levelled ceilings, side notches on the walls andscallops on the walls and ceiling). There arealso remnants of cave fills indicating repeatedfillings of the cave and successive erosion ofthe sediments. Speleothems were depositedin different phases above clastic sediments.The natural floor of the cave was modified forthe construction of a railway during openingfor tourists.The second level is about 18 m below theupper one, where the modern undergroundPivka river flows from its entrance. The riverbed has a low gradient and, except for somecollapses and sumps, there are no natural barriers.It leaves the system through a terminalsump. The active river passages are mostlyFig. 45: Postojnska jama - the longest and the most touristic cave of Dinaric karst (photo: N. Zupan Hajna).61