Case Studies from the Dinaric Karst of Slovenia

ly several dozen meters deep. Shafts that canreach great depths in steps often occur on highplateaus. A typical example is the 218 m deepStrmadna shaft on the Nanos plateau.High karst water tables that remainedstable over a long period enabled the formationof large horizontal caves. This is how largeponors and spring caves such as the Postojnskajama and Planinska jama and the caves inRakov Škocjan park formed in the karst riverbasin of the Ljubljanica river and caves such asthe Škocjanske jame and Kačna jama along theunderground flow of the Reka river.The greater part of Dinaric karst territoryin Slovenia is drained by the Reka river in theKras region and by the Ljubljanica river in theNotranjska region. These rivers formed largecave systems that are partly accessible. TheReka sinks into the Škocjanske jame and canbe followed as far as the siphon. After a shortbreak, we can follow its course again in thecave Kačna jama, then in the Jama v Kanjeducah,and finally in Jama v Stršinkini dolini. Altogetheralmost twenty kilometres of cave passagesare accessible along the undergroundcourse of the Reka river.Due to denudation, cave passages get evercloser to the surface. Finally they are left withoutceilings and roofless caves become part ofthe morphology of the karst surface. As thesurface continues lowering, the remains ofcave passages and sediments disappear completely.The evolution of the caves in Slovenia(from the start up to their total destruction bydenudation) took part within one karstificationperiod, which began with the regressionof Eocene sea and exposing of limestones atthe surface within complicated overthrustedstructure, which formed principally during theOligocene to early Miocene. The interpretationof palaeomagnetic data (Zupan Hajna etal. 2008a), with some support from palaeontologicalfinds, indicates that karst developedin pulses tightly linked with tectonic evolutionand changes of the geodynamic regime. Individualpulses were not sharply limited, however,and therefore cannot be tied to preciselydefined karst phases. Moreover, the complicatedgeological structure and tectonic/geomorphicevolution makes the picture less cleardue to the differing tectonic evolution of individualmorpho-structural units, which oftenhave also quite different histories of evolutionof the relief and karst.Postojnska jamaPostojnska jama is developed in Postojnskikras where the surface is at 600 to 650 m a.s.l.The evolution of the Pivka basin (Eocene flyschrocks) is defined by the altitudes of the ponorsof Pivka River that drain into this cave. Thegentle fluvial surface of the basin itself standsout in sharp contrast to the karst lands abovethe cave and to other higher karst plateaus,where there are no traces of fluvial valleys orother elements of the early fluvial relief today.These surfaces are dissected with numerousdolines. Sixteen large collapse dolines developedabove some parts of Postojnska jama,blocking certain passages. The thickness ofbedrock overburden above the cave is 60 to120 m.The cave was formed by the Pivka river. Itsmodern ponor is at 511 m a.s.l. and the terminalsump in Pivka jama is at 477 m a.s.l. Thereare still more than 2,200 m of unexplored galleriesbefore the river re-appears in Planinskajama at 460 m a.s.l.The historical entrance at 529 m a.s.l. islocated above the modern ponor. Other entrancesand parts of the system, i.e. Otoškajama, Magdalena jama, Črna jama and Pivkajama, are scattered on the surface above thecave. All these caves are interconnected andform a cave system 20.5 km in length, the longestin Slovenia.The entrance to Postojnska jama is situatednear the contact between the Eoceneflysch and the Upper Cretaceous limestones(Buser et al. 1967). The entire cave system isdeveloped in an 800 m thick sequence of the60