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

Opis: 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.
Ključne besede: karst, contact karst, regional tectonic uplift, cave levels, shift from epiphreatic to vadose speleogenesis, Loza Cave System, Postojna drainage basin
Objavljeno v RUNG: 05.12.2023; Ogledov: 904; Prenosov: 57
.pdf Celotno besedilo (29,89 MB)

Geomorphological characteristics of karst on contact between limestone and dolomite in Slovenia
Petra Gostinčar, 2016, doktorska disertacija

Opis: Contact karst is a type of karst formed where allogenic waters from the surface influence the karst geomorphic system. Contact karst may be considered in both a strict sense and in a wide sense. In a strict sense, contact karst is the karst phenomena and forms influenced by the contact between a karstifiable rock and a non-karstifiable rock. In a wide sense, contact karst may also be the karst phenomena and forms influenced by the contact between two different karstifiable rocks, for example limestone and dolomite. This thesis focuses on the geomorphological characteristics of contact karst on limestone-dolomite contacts in Slovenia. The purpose of the research was to determine which processes contribute to the development of contact karst on the contact between limestone and dolomite, to define their dynamics, and to identify which surface and underground landforms are developed. The spatial distribution of contacts between limestone and dolomite in Slovenia was determined in a GIS. Using existing lithological data as a data layer, the extent of carbonate rock cover in Slovenia was calculated. Carbonate rocks cover 47 % of Slovenia’s territory (27 % limestone, 14 % dolomite, and 6 % clastic carbonate or impure carbonate rocks). And, there are 1,353 limestone-dolomite contact lines in the country, totalling a length of 2,625 km. Study areas were selected based on GIS analysis of the limestone-dolomite contacts. A total of 17 areas in Slovenia were studied in detail. Fieldwork at the study areas consisted of the collection and analysis of rock, sediment, and water samples, allowing each study area to be geomorphologically mapped. General factors contributing to contact karst development on the lithological contact between limestone and dolomite were determined. The most important factor appears to be the characteristics of the inflow part, formed on the dolomite. Where dolomite functions as a karst rock, the water is dispersedly drained into the karst. In that case, the limestone-dolomite contact does not function as contact karst. Alternatively, where the dolomite functions as fluviokarst, a point recharge, or sinking stream, is formed. In that case, contact karst may be formed. The fluviokarstic character of the dolomite depends on its chemical and mechanical properties. The dolomite bedrock must be positioned at a higher elevation than the neighbouring limestone bedrock. To meet this requirement, dolomite beds, which in Slovenia are generally older than limestone and hence stratigraphically positioned below the limestone beds, need to be positioned above limestone by either folding that leads to inverse stratification, overthrusting, or by displacement along faults. Along faults, the dolomite is more prone to mechanical weathering due to tectonic crushing in addition to its chemical properties. Hence, contact karst is more likely to form at thrust contacts between thrust limestone and dolomite. Limestone-dolomite contact karst develops predominately at higher elevations due to increased precipitation (where allogenic inflow is higher) and greater frost action due to lower temperatures. Intense mechanical weathering of dolomite over limestone directly affects contact karst processes and significantly contributes to the spatial distribution of these types of surfaces. The location of the water table close to the surface is also a leading factor in limestone-dolomite contact karst formation due to enhanced border corrosion. Landforms typical of contact karst were identified in the study areas during geomorphological analyses. However, they are not as clearly recognizable as those on contact between carbonate and non-carbonate rocks. The reason for this is the fact that allogenic waters from dolomitic catchment areas are by far not as corrosive as those from non-carbonate catchment areas.
Ključne besede: carbonate rocks, dolomite, limestone, contact karst, allogenic water, karst geomorphology
Objavljeno v RUNG: 19.12.2016; Ogledov: 6829; Prenosov: 262
.pdf Celotno besedilo (126,22 MB)

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