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

KARSTIFICATION

water is reduced and C0

2 is released by the system. Acidity increases when the accumulation of the series with charcoal and plant detritus stops. The appearance of lime concretions may indicate input of calcareous sediments or the lack of favourable conditions for carbonization (the depression is filled in or the influx of plant detritus stops). Evolution of karst depressions Covered karst depressions evolve from their development to their destruction. If the grain size distribution of sedimentinflux, the stages of their evolution is best reflected by the sediments produced (VERESS 1986, 1987b). From the actual morphological conditions the stage of evolution can also be estimated. Depressions in different stages, however, may present similar morphologies. The phases of evolution are the following (VERESS 1986, Fig. 26). With stable rate of sediment influx, the grain size of the fill of activating karst depression tends to grow coarser upwards since the life-time of ponds above the broadening passage (chimney) is ever shorter and thus the dropping of water table is more and more rapid. If beds are increasingly thicker upwards (their dip decreases), it can happen with increased sediment influx. With unchanged rate of sediment influx, the gain size of the fill of equilibrium karst depression shows no alteration. If the rate of sediment influx grows, the series also tend to become thicker in the centre of the fill at an increasing rate. In the fill of the inactivating karst depression (Fig. 27) an upward enrichment of finer sediment is observed as with the filling of the passage the water table dropping in intermittent ponds tends to slow down. Karst depressions of complex evolution (Fig. 28) with changeable activity may also occur. In their fill grain size may change rhythmically. From fossilized karst depressions (Fig. 29) no transfer of sediment and water into the karst takes place. The depression becomes inactive as ever finer sediments arrive from the background area of ever gentler slope. Depositing this sediment increases the life-time of the intermittent pond and thus causes the accumulation of more and finer sediments; the depression is filled in (VERESS 1995). At first semifossilized depressions develop. Here the original landform is almost completely filled but its remnants are still visible. (The depression may be completely filled but still detectable from the darker tone caused by the accumulated plant detritus.) Of the completely fossilized depression the development of a secondary depression is typical. Finally, in the area of destroying fossilized depressions erosion starts. Channels formed by rainwater or intermittent water-courses fed by groundwater develop in their area. As a consequence of erosion (indicated by surfaces of erosional unconformities in the fill), the destroyed depression may reactivate (postgenetic karstification). COVERED KARSTIFICATION DEPENDENT ON FLOWING KARST WATER This type of karstification is associated with cavernation in the zone of flowing karst water. Cavernation is through mixing corrosion evidenced by spherical cauldrons occurring in caves and on rock walls which remain after cave destruction. Cavitation is also influenced by nonkarstic rocks. On the one hand, the process takes place under impermeable rocks (hidden karstification). On the other, cavity formation is also promoted by the water seeping from watercourses which arrive onto the carbonate rocks of the valley floor from non-karstic terrains.