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

KARSTIFICATION

Sedimentation in karst depressions The sediments of covered karst depressions indicate the relationship between the depression and the background area, the events occurred in these areas and the stage of development of the depression. The sediments of covered karst depressions are original cover sediments or those produced by karst processes. The latter are either locally deposited or transported into the depression. Transport takes place by water, mass movements, eolian porcesses or by human activities. Sediments deposited by water derive directly from water inflow or indirectly from flood ponds. Sedimentation from flood ponds From a given pond only those sediments deposit the grains of which have a higher speed of subsidence than the rate of dropping of the pond water table. Grains with a lower speed are transported to the karst with the outflowing water of the pond. The dip of the beds of accumulation is adjusted to the slope of the floor during sedimentation. With permanent conditions of sediment influx the sequence produced by an activity phase thickens towards the centre of the depression if water table dropping is reducing. With reducing water table dropping finer grains also deposit. Therefore, the sequences with poorer sorting in thicker parts were deposited fro ponds with reducing rate of water table dropping during emptying. At the same time, it is to be noted that a thickening of the sequence (in exposures decreasing dips on series boundaries) may be caused - independent of the rate of water table dropping - by increased sediment influx. It may also occur that the dip of series is growing upwards. It indicates that the subsidence of the floor due to karstification exceeded the rate of accumulation. The main characteristics of recent pond sedimentation in covered karst depressions are the following (VERESS 1987a, Fig. 25). - In short-term ponds the dropping of water table may be so rapid that even the coarsest grained sediment is partly or completely transported into the karst. The finer sediment and the plant detritus may accumulate - as a consequence of the slow outflow of the emptying pond - on the floor or in its environs (Pict. 29) or even on the uneven surrface of the conduit. - In ponds of longer existence fine-grained sediments may accumulate. In depressions where such ponds develop, a series of or with plant detritus (plant detritus coated or mixed with sediment) is deposited (Pict. 30). The accumulation of a sediment series with plant detritus is explained by the rate of subsidence of plant detritus exceeding that of water table dropping in the intermittent pond. (The subsidence of plant detritus is mainly caused by the adhesion of sediment grains on detritus fragments.) If on the margins of the pond a zone of plant detritus takes shape (Pict. 31), while towards the centre a series of plant detritus forms, the dropping of water table may be interrupted for a time. (Along the pond margins plant detritus adheres to the floor without subsiding, while in areas with deeper water it happens after subsiding.) In the resulting series with plant detritus the amount of plant detritus may decrease laterally or even disappear completely if temporarily the dropping rate of water table increases. The series with plant detritus may be of variable composition. The zonal appearance of the series with plant detritus is typical in depressions where the rate of water table droping fluctuates. In case of a uniform series with plant detritus, the water table of the pond is reduced at a uniform and slow rate (Pict. 30). Variation in plant