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Morphogenesis of the Postojna Basin karst periphery : dissertation
Astrid Švara, 2023, doctoral dissertation

Abstract: This thesis represents a karstological study on the drainage of the Postojna Basin. It focuses on a multidisciplinary approach, based on fieldwork, computer analyses and dating. The northern study area has the highest: doline density of 108 dolines/km2, number of contact karst features (i.e. 9), and number of collapse dolines (i.e. 19). It has three catchments. The Lokva cuts into the flysch ridges and sinks in the Predjama Cave System at the lowest point of the basin (i.e. 474 m a.s.l.). Between 37 selected caves, 14 were longer than 150 m. The Predjama Cave System was developed in 3 levels and the Postojna Cave System was developed in 2 levels. The vertical passages connecting them are deepest in the Predjama cave, where the vadose zone is up to 250 m deep. By the favourable ponor steepheads and 140 m deep vadose passages, the Hrušica Plateau shows a major uplift phase. The Postojna Cave System, Risovec blind valley and 17 collapse dolines indicate significant past hydrological changes. First, Nanoščica formed the Risovec blind valley, followed by Otoška jama, Tartarus, Male jame, and Artificial tunnel toward E/NE, forming the Vodni dol. At the same time, the ponor of Pivka shifted in the Postojnska jama (at present “Stara jama”), flowing towards N, forming Jeršanove doline. The successive subsidence of the erosional base and the Postojna karst uplift redirected the Nanoščica in Pivka. Now they sink into active parts of the Postojna cave at 511 m a.s.l. The eastern study area has the Unroofed cave Kriva dolina as a former ponor of Pivka. At present the area has springs and favourable (sub)vertical passages. The southern study area has the highest variety of contact karst features (i.e. 5). The Prestranški ravnik represents an aquifer with ponors and springs. It is separated from the Slavinski ravnik, by a flysch belt. In the southern study area, 10 caves were longer than 150 m among 49 selected caves. The Loza Cave System, a case study in Slavinski ravnik, is developed in 3 epiphreatic levels that followed two phases of tectonic uplift with a successive formation of diversion routes through 30-40 m deep vadose passages. The oldest upper cave level has sediments from at least the Gilbert Chron with CW rotations. U-Th dates from speleothems prove vadose speleogenesis before the cave ceiling denudation and collapse from 210 ka to 550 ka (≤1.2 Ma). The middle cave level has epiphreatic sediments at least in the Gauss Chron (i.e. 2.59-3.59 Ma) and reveals 35-38° CCW rotations. The allogenic sediments in the lower cave level show Brunhes and Matuyama Chrons (i.e. <0.78-2.58 Ma). We generally discussed ponor steepheads that develop on steepest slopes on the thrust/fault contact, while blind valleys and border depressions develop on normal stratigraphic contact with mildest slopes. The speleogenesis was mainly driven by relatively quick tectonic uplift, followed by vadose speleogenesis, with intermediate speleogenesis in the epiphreatic zone. Speleogenesis and contact karst features follow the subsidence of the water table, evidenced by cave levels, and active and relict features. Allogenic sediments are followed by speleothems as shift of caves between hydrological zones. The sequence of events repeated 2-3 times. The main local source of allogenic sediments is in the Postojna Basin, represented by the erosion of flysch rocks and alluvium. The mineralogical composition between catchments is similar with no significant change. The regional compressional-tectonic regime has significantly influenced the changes in the drainage of the Postojna Basin during the last 7 Ma, with different uplifts and drop of the karst water table. The major uplift was reflected by the change in the Nanoščica course from the Slavinski ravnik to the Postojna karst from S to N, presumably between 3.59 Ma and >1.77 Ma and represented the last important general shift in the drainage of the Postojna Basin.
Keywords: karst, contact karst, regional tectonic uplift, cave levels, shift from epiphreatic to vadose speleogenesis, Loza Cave System, Postojna drainage basin
Published in RUNG: 05.12.2023; Views: 954; Downloads: 57
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