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

KARSTIFICATION

Postgenetic karstification denotes the situation when the chimney or the passage had formed before the associated depression. Postgenetic karstification is a local subsidence or caving in in cover sediments due to matter deficit which is caused by the removal of sediment fill from older passages. The surface depression forms partly (or entirely) in unconsolidated cover sediments. The conditions of postgenetic karstification are particularly favourable on the Tés Plateau. The evidence are summarised below. - Some of the covered karst landforms here developed in the cover sediments of valley floor sections without drainage, which were produced by the older stage of karstification (the resulting karst features were later buried). Therefore, those in their interior, in the sediment fill, must be products of more recent karstification. - Part of the ponor-like passages of the plateau are at least partly of erosional origin. Erosional genesis is evidenced by the extraordinarily great dimensions of passages as well as the fact that some of them are found in calcareous marl (eg. Alba Regia Cave - Figs. 12, 13). Since present-day cavitation does not occur on the plateau (in lack of sufficient abrading material as carbonate rocks are directly overlain by loess), the passages must have formed under conditions different from the actual ones. Older karstification is indicated by the existence of a range of superimposed valleys on the plateau (Pict. 18). By now they are lined with loess and there are a number of karst features, regarded postgenetic, on their floors. At rock boundaries in the superimposing valleys ponors formed during bathycapture. (Postgenetic karst landforms in valley floor position came about in their sediment fills.) Ponor formation and the cover sediment (probably Csatka Gravel Formation), in which valleys deepened, favoured the development of relatively spacious passages of erosional origin (Fig. 14). The cover sediment in which postgenetic landforms develop may or may not grow thicker above the passage. In the first case, there is a karstic feature filled by sediment on the surface which is associated with the karst passage and of the same age (there is a genetic relationship between the karst passsage and the surface feature.) In the latter case, there is no depression above the karst passage. (The surface depression which contributed to the formation of the karst passage has been truncated or erosion exposed the underground cavity.) The postgenetic karst landform develops in the cover sediment in its entirety. This type may also occur on dissected surfaces but then surface landforms do not influence their genesis. A surface postgenetic depression forms through the redeposition of the fill of the karst passages to the depth, into yet unfilled karst passages by processes of downward water percolation and collapses. For these reasons and possibly to the effect of the dissolution of lime and compaction, a matter deficit arises in the upper portion of the passage, from where sediments were removed. It is often a precondition of the clearing of the passage that the sediments covering it or filling the depression should thin out by way of surface erosion. (The degree of this thinning-out is not yet known.) Then more water can collect in the filled passage per time unit. The deficit can be further increased by the solution effect of infiltrating waters. Through further solution (and probably also by the collapses of solution remnants) the already existing passage can broaden. The opportunity for the partial or even complete removal of sediments from the upper section of the karst passage, close to the surface, increases. The subsidence or caving of sediment fill of the old karst depression fills in the emptied space. In the case of subsidence the postgenetic depression in cover sediments will have