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

THE NORTHERN BAKONY MOUNTAINS: A GEOLOGICAL AND GEOMORPHOLOGICAL DESCRIPTION

hippuritic limestone. Into Cretaceous limestone sequences marl and clay strata (eg. turrilitic marl or munierian clay) are locally intercalated. At the end of the Lower Cretaceous and particularly in the Upper Cretaceous the area was mainland with bauxite accumulation. The Middle Eocene nummulitic limestone (Szőc Limestone) is a characteristic formation over a relatively large area. Its thickness, however, is moderate (100-300 m) and intercalated with marl beds. In many cases, it immediately overlies Triassic Hauptdolomit or covers the bauxite deposits, occurring in patches in the mountains (primarily in karstic hollows). In the late Eocene and early Oligocène clays (eg. foraminiferic clay) formed. In spite of the Tertiary denudation, a Middle Oligocène to Lower Miocene fluvial sequence of mostly gravelly material with abundant clay and silicified tree trunk remnants, the Csatka Gravel Formation, is still wide-spread. Its thickness ranges between 50-300 m. In various periods of the Miocene, abrasional gravels accumulated on the lower marginal blocks of the mountains. (Abrasional gravels are also known from earlier times, eg. from Lower Eocene). Sporadic occurrence is typical for Miocene marine limestone (Leitha Limestone) and Pliocene travertines (eg. around Várpalota). The latter are associated with former karst springs. Among formations of small (some metre) thickness Pleistocene blown sands (around Fenyőfő), travertines and - in the valleys - fluvial deposits can be mentioned. Loess has to be underlined since it occurs over an extensive area although in moderate thickness. Redeposited varieties are equally found over higher-lying terrain and in the valleys and karstic depressions of older surfaces. There are loess patches on the dissected surfaces of limited extension, while over undissected and extensive surfaces (Tés Plateau) and in lowerlying basins it has a uniform appearance. Out of the listed formations Dachstein Limestone, Dachstein-type Liassic Limestone, requienian and nummulitic limestones are liable to karstification. Their distribution marks the portions of the mountains where surface karstification is possible. On younger Jurassic limestones there is virtually no karstification. It may also be missing on dolomite or occur in a characteristic manner not be detailed in this book. Cavernation, however, is typical of both rocks. Thus, regarding karst development as a whole, the occurrence of the latter rock types also has to be taken into account when delimiting karst areas. It can be mentioned that marl and clay interbeddings in carbonate rocks cannot only modify cavern formation (for instance, storeyed karst water levels may form and promote cavernation at several levels simultaneously) but they may be exposed over large areas during denudation. At these exposures there is no surface karstification either. Two rocks have to be cited under which carbonate rocks are buried: the Csatka Gravel Formation and loess. The former is virtually impermeable and, therefore, runoff from its surfaces flows over carbonate surfaces (or leaves the mountains) and thus the water influx into karst increases. Under these cover deposits only buried karst can develop. In areas, however, where limestone is overlain by loess, permeability allows the development of cryptokarsts. Tectonics Folding is only found scattered in the mountains, while faulted structure are much more common. Fractures and faults of NE-SW strike primarily formed by compressive strain (joints, paraclases, inverse faults and imbrications). (Extensional joints favour karst water movement along the strike of the mountains.)