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

THE NORTHERN BAKONY MOUNTAINS: A GEOLOGICAL AND GEOMORPHOLOGICAL DESCRIPTION

Land form As a consequence of dismemberment into horsts, the Cretaceous peneplain ceased to exist by the end of the Cretaceous. During the Cenozoic, the dismembered fragments were buried and (resulting from the oscillating movements of blocks) exhumed at various dates and to various extent. Regarding the cover sediments and present altitudes, the following types of horsts are identified: - threshold surfaces are constituted of Triassic carbonate rocks and have altitudes below 300-400 m. Pedimentation deprived them from their sediment cover and they acquired their present positions through subsidence. - Cryptopeneplains in basins have altitudes between 300 and 400 m. During the Cenozoic they were in low positions and, therefore, the basement is covered by well-preserved Cenozoic sediments. - The altitude of horsts in summit position ranges between 400 (occasionally 300) m to 550 m. In the Cenozoic they may have suffered repeated subsidence and uplift. There are several varieties, including buried horsts in summit position, where Middle Eocene limestone covers the Mesozoic basement, semi-exhumed horsts in summit position, where Cenozoic cover sediments are only preserved in patches and exhumed horsts in summit position, where Cenozoic sediments have been entirely removed. - Some horsts in summit position rise ca 550 m above sea level and are built up of Triassic and Jurassic limestones. There are no cover sediments since the blocks remained in higher position to escape inundation or the cover sediments were removed by subsequent erosion. The loss of cover sediments (the Csatka Gravel Formation) is usually associated with the process of pedimentation. It is assumed that during the process the landforms of the inselberg type karst were destroyed. Author has the opinion that some have probably survived. His arguement is that some surfaces were not buried under unconsolidated cover sediments or they (like the Middle Eocene limestone) showed a large degree of heterogeneity in the first place. In the absence of nummulitic limestone, the removal of unconsolidated cover deposits did not involve the destruction, only the truncation of landforms of limestone much more resistent to erosion. Examples are found on horsts of summit position with Middle Cretaceous Limestone exposed and inselberg type karst landforms developed. Similar landforms could also develop in nummulitic limestone (a cover sediment). In this case the tropical karst features could only have been destroyed if the nummulitic limestone cover had been entirely removed. The decisive elements of the mountain landscape are mountains, grabens, basins and the escarpments encircling them - all formed by tectonic movements (uplift and tilting). The summit levels are constituted of portions of the late Cretaceous peneplain varying in size, altitude and degree of burial or exhumation with spots of various size where remnants of the inselberg karst, eroded differentially, also occur. Older pediments developed almost everywhere in the mountains, while more recent ones are restricted to the marginal zone or edges of the individual blocks. Abrasional platforms and raised beaches are typical of the margins. Valleys (of both derasional and fluvial type) are also marked in the landscape. Fluvial valleys formed in grabens or along tilted structures (or occasionally along the axes of anticlines). Transversal valleys are more common than longitudinal ones. There are many regressional valleys but superimposed valleys even more often occur. Indicating that during their formation they had a meandering river mechanism, most of the latter show incised meanders. Among valleys with incised meanders asymmetric cross-sections are common.