Veress Márton: A Bakony természettudományi kutatásának eredményei 23. - Covered karst evolution... (Zirc, 2000)

KARSTIFICATION

out). An evidence for this is that there is no difference between the activity of passages aligned in the channels and of partial depressions; consequently, sites of bathycapture does not show a retreat. Human activities also exert an indirect influence on the development of chimneys and the surface karst depressions above them (VERESS 1984a). In numerous depressions intensive accumulation caused by human intervention is observed (around Hárskút, on the Tés Plateau). Accumulation is a result of forest clearing and cultivation, particularly on large scale. The influences are not simultaneous and may repeatedly occur for the same depression. Accumulation increases the rate of corrosional development since water seepage in the sediment fill slows down. Corrosion of the chimneys is also favoured by the treeless environment of karst depressions filled by snow. This way not only more water reaches the chimneys but also the duration of solution can grow if snow fill lasts long. Origin by solution and its evidence On allogenic karsts ponors (JAKUCS 1971a) and on covered karst terrains covered karst dolines were identified (QUINLAN 1972). Ponors form on rock boundaries through the erosional transformation of cavities in the flowing karst ware zone by the water-course. On covered karsts a depression forms in the cover sediments owing to the matter deficit in the karstic basement resulting from karstification. According to BULL (1977), the origin and morphology of covered karst landforms depends on the thickness and type of cover sediments and on the nature of karstification of the basement. The more cohesive is the cover sediment and the larger cavities are formed by karstification, the steeper landforms are created in the cover sediment and caving is more important in their origin. If the cover sediment is less cohesive and karstification involves surface solution, the slopes of the karst landform will be gentle and subsidence will be the decisive process. It is to be noted that some authors (BÁRÁNY-JAKUCS 1984) do not refer depressions in cover sediments to landforms of karstic origin. This view reflects the concept that cover sediments do not influence the karstification of the carbonate basement; they are only passive objects of the process. In reality, cover sediments and carbonate basement are interactive systems during the process of karstification. The cover sediments are only passive if the limestone is a closed inactive mass and early developed cavities cave in to induce collapses in cover sediments. Various authors (QUINLAN 1972; JENNINGS 1975, 1985) identify steep-sided subjacent dolines, gentle-sided subsidence and alluvial streamsink dolines on covered karsts (Fig. 10). The generation of subjacent dolines is preceded by the caving of limestone cavities, which is also spreading to the cohesive cover sediments. According to BULL (1977), the development of various subjacent dolines is allowed by differential cavitation between the surface and a deeper horizontal cavern (Fig. 11). Subsidence dolines form in unconsolidated cover sediments. In the case of subsidence dolines, the matter deficit resulting from the karstification of the basement results from the development of passages reaching to the surface of the carbonate rock (TRUDGILL 1985; JENNINGS 1985), while others (BÁRÁNY-JAKUCS 1984; HEVESI 1987) explain it with the surface solution of the karstic rock. During the generation of alluvial streamsink dolines, the cover sediment is transported by water from the sides of the developing depression into the passage system of the karst.