Budai Tamás, Csillag Gábor: A Bakony természettudományi kutatásának eredményei 22. - A Balaton-felvidék középső részének földtana (Zirc, 1998)

GEOLOGY OF THE CENTRAL PART OF THE BALATON HIGHLAND (TRANSDANUBIAN RANGE, HUNGARY)

The Triassic of the Balaton Highland can be subdivided into cycles which are characterised by different sedimentary and geodynamic features. These are the followings: The first cycle lasted from the Permian/Triassic boundary till the end of the Early Triassic. The significant transgression at the beginning of the Early Scythian resulted in the establishment of a wide ramp. It was characterised by mixed siliciclastic and carbonate sedimentation of shallow subtidal to peritidal facies („Werfen Group"). - The 100 m thick Köveskál Formation (lowermost Scythian) consists of grey, porous dolomites, dolomitic siltstones and thin-bedded sandstones. - The 50 m thick siliciclastic Zánka Member (Middle Scythian) is characterised by laminated structure, i.e. alternation of red and grey siltstones. Bioturbation is common. Reddish brown limestone interlayers („gastropod oolite") also occur. It is overlain by well bedded, light grey or yellowish dolomites, silty dolomites (Hidegkút Member) is 20^10 m in thickness. Shrinkage cracks and bird's-eye structures are common. - The 200 m thick Csopak Formation (Upper Scythian) consists of grey marls (lower member), red silty marls and siltstones (middle member), and silty marls with limestone, sandstone and dolomite interbeds. Crinoidal and ooidic limestone interbeds are common in the whole sequence. Lower Triassic formations crop out near Zánka and Tótvázsony in the study area. At the beginning of the Middle Triassic a broad and shallow lagoon, surrounded by tidal flats developed. Sedimentologic features refer to arid climate and sabkha environment (Aszófő Dolomite) which subsequently transformed to restricted lagoon under more humid climatic conditions (Iszkahegy Limestone). Shallowing of inner shelf basin at the end of the Early Anisian led to regeneration of the carbonate ramp (Megyehegy Dolomite). - The 200 m thick Aszófő Dolomite is made up by thin bedded, platy laminated, yellowish-white or light grey, „cellular" dolomites and dolomarls. Bird's eye lamination and mud cracks are also common. - The 250-300 m thick Iszkahegy Limestone begins with dark grey laminites, grading upward into bedded, bioturbated limestones. - The Megyehegy Formation consists of light grey, yellowish or lilac, bedded or thick bedded, usually strongly recrystallized dolosparite. Its lower unit tens of metres in thickness, is widespread in the whole Balaton Highland. It is made up by marly, bituminous dolomite. Lower Anisian carbonates crop out between Zánka and Balatonudvari in the southern part of the study area and in the surroundings of Tótvázsony in the northern one. The next cycle, lasted from the Middle Anisian till the Early Carnian. It shows much more dynamism, than the previous ones. Gradual evolution of the Balaton Highland was suddenly and drastically disturbed by synsedimentary tectonic movements in the Middle Anisian. The shallow marine carbonate ramp was broken into blocks along normal faults (Fig. 29.) and relatively narrow, more or less restricted basins were formed in the subsided areas (Felsőörs Limestone). On the uplifted blocks carbonate platforms developed (Tagyon Fm.). - The Felsőörs Formation is made up by bituminous laminites (Fig. 5.), nodular (cherty) limestones and allodapic sediments redeposited from the coeval platforms (brachiopodalcrinoidal limestones). Its basin facies is characterised by ammonites (Balatonites, Paraceratites, Ptychites, Norites etc.) and pelagic bivalves. The formation can be followed in the northern belt of the Balaton Highland along the Liter overthrust, whereas it is substituted by coeval platform carbonates in the southern range between Monoszló and Aszófő (Fig. 30,1). Its thickness varies between 20 and 150 m.