Speleogenetic factors and processes in the karst conduits of Zagorska Mrežnica Spring Cave (Croatia)Petra Kovač Konrad
, 2018, doctoral dissertation
Abstract: Zagorska Mrežnica spring cave is in the Desmerice village, 7.59 km SW from the town of Ogulin. The coordinates of the entrance are X: 399563 m, Y: 5006974 m, and Z: 314 m (HTRS96). The cave system is positioned in the contact zone of Jurassic limestone and the thrust front composed of Triassic Dolomite. The karst drainage system has elements of point recharge through a set of ponors in the hinterland karst poljes and diffuse infiltration through numerous dolines on Velika Kapela Mountain. The karst of Ogulinsko Zagorje area and its epiphreatic and phreatic cave systems have been intensively explored for the last eight years, resulting in the mapping of 1134 m of submerged passages of the Zagorska Mrežnica spring cave. Until 2014 it was the longest mapped submerged system in Croatia explored by cave diving techniques only. Altogether, over 3.5 km of submerged passages in seven caves of Ogulinsko Zagorje (Zagorska Mrežnica spring cave, Spring of Rupečica, Ponor of Rupečica, Cave system Pećine-Veliko vrelo, Spring of Bistrac, Cave Zagorska Peć and Pit Klisura) were explored during 500 hours of diving over a ten-year period. The goal of the research was to determine speleogenetic factors and processes in phreatic conditions. A new methodology for mapping of cave cross-sections, microrelief forms, and structural elements was developed, sediment and petrographic analysis were done, hydrological analysis before and after the building of the accumulation lake Sabljaci, 48 cross-sections of the cave passaged and a 3D model of the cave system was created, water chemistry as well as geomorphological analysis was carried out. In the end, a simplified (modified) vulnerability assessment was also done.
The results of my research showed that the distribution of the cave passages is a result of complex tectonic activities that are reflected in the orientation of cave passages. The general orientation of the measured fissures, measured during cave diving, show a dominance of a NWW-SEE direction but also the pattern of cave passages shows a significant dominance of the NE-SW orientation indicating the existence of multiple secondary traverse faults originating from main NW-SE faults. The morphology of the cave passages shows a transition between epiphreatic (possibly vadose) and phreatic phases as well as the presence of paragenetic developments in the ceiling. This indicates the existence of several epiphreatic (possibly vadose) and phreatic speleogenetic phases. The initial shape of the cross-sections was identified by sub-horizontal beds and bedding planes. The mapping of microrelief forms showed that the mechanical erosion was a very intensive process shaping the morphology of the cave passages.
The sediment mineralogy coincides with the lithology of the catchment area, and the sub- angular grains reveal relatively short transport distances. Further analysis of the catchment area’s geomorphology and its relevance for the speleogenesis of Zagorska Mrežnica cave show influence of relief structures that have a Dinaric orientation (NW-SE) with cave structures and passage orientations, great relative relief, steep slopes on Velika Kapela, and flat areas of karst poljes dictate the allogenic and autogenic hydrological regime of the cave. A 3D model of the cave system reveals a relationship between the cave system and the landscape, as well as a possible connection with the nearby Zagorska Peć cave. The modified karst vulnerability assessment shows that the catchment area of Zagorska Mrežnica spring cave has a high vulnerability ranking, mainly due to high doline density and the potential large diffuse capacity of infiltration of pollutants.
Found in: ključnih besedah
Keywords: Ogulin Zagorje, karst, spring, speleogenesis, Zagorska Mrežnica spring cave
Published: 02.03.2018; Views: 2028; Downloads: 96
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