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

THE NORTHERN BAKONY MOUNTAINS: A GEOLOGICAL AND GEOMORPHOLOGICAL DESCRIPTION

The blocks bordered by faults constitute horsts, escarpments, grabens and basins. Thus, the mountain range is built up of a cluster of blocks of various elevation. In the basins the extension along NW-SE faults allowed differential subsidence of blocks, while tilted uplift was mostly due to compression in NE-SW direction. The individual blocks were affected by considerable relative displacement. Geological evolution The Triassic formations indicate a shallow marine environment, which was gradually replaced in the Jurassic by a deeper sea (ammonitic limestones). Towards the end of the Lower Cretaceous terrestrial conditions prevailed and in the Middle Cretaceous shallow marine sedimentation resumed. The area emerged as a land surface in the late Middle Cretaceous. The karstification under tropical climate transformed the landscape into a peneplain with isolated conical karst hills (inselberg type karst). At the end of the Cretaceous the peneplain was dissected into horsts, which underwent oscillatory movements during the Cenozoic. As a consequence of emergence from the sea, tropical karst processes affected the carbonate surfaces from the Lower Cretaceous to the Upper Eocene with interruptions of various length in the Middle Cretaceous and Middle Eocene. Interruptions are indicated by bauxite niveaus and varieties of inselberg type karst. Karstic planation probably did not extend over the whole mountains but it was restricted to portions where the time interval was sufficiently long for karstification. In certain parts the development of inselberg type karst came to halt at various stages and it is also manifested in landforms. As a result of oscillating block movements, the Middle Eocene transgression produced an archipelago sea. Considerable subsequent inundation followed in the Middle Oligocène and Lower Miocene. The Csatka Gravel Formation is a delta sequence accumulated by a river (or rivers) arriving from S or SW It is probable that cover deposits did not extend over the whole mountains but the highest elevations were not inundated. Until the Middle Miocene the area of the present mountains functioned alternately as a site of accumulation for sediments removed from the environs and, at least in part, a terrain of erosion. This erosion under warm or semiarid climate can be best described as pedimentation. The cover sediments (eg. the nummulitic limestone) were partly or entirely removed from the Mesozoic basement and extensive pediments formed. Pedimentation also involved the exhumation of the karstic peneplain as well as the truncation of the inselberg type karsts of various development (morphological) stage. The blocks of various elevation became buried to various degrees in a given interval and in another cover sediments were removed to variable extent. Over certain portions of the individual blocks pediments developed even in the Pliocene and Pleistocene. (Pedimentation could have repeated between the late Miocene and the Pleistocene.) Along the margins of some blocks abrasion platforms and raised beaches could have also developed in the Miocene and Pliocene. The present altitude and dissection of the mountains are due to late Pliocene and Quaternary differential vertical crustal movements. The present landscape is a mosaic of blocks of the late Cretaceous peneplains exhumed to various degrees and uplifted to various elevations.