Tihany Formation in the surroundings of Lake Balaton: type locality, depositional setting and stratigraphy

Abstract

Revisiting the Tihany, Fehérpart section, overviewing archive data, comparison with successions of nearby wells, well-logs, stratigraphic data and results of the high-resolution seismic surveys on Lake Balaton resulted a coherent picture on the depositional environment, age, stratigraphic correlation and palaeogeographic connections of the Tihany Formation. In addition to former analyses of grain-size distributions, carbonate and clay content, the sedimentary structures were investigated, a pilot study of gamma-ray measurements on the field was carried out, and several orders of cyclicity were demonstrated. Palaeontological data from earlier studies were collected and analyzed, and magnetic polarity of the rocks was measured. The Tihany, Fehérpart section is correlated with the Spiniferites tihanyensis dinoflagellate, the MN11 micromammal and the Lymnocardium decorum littoral mollusc biozones. With the exception of the lowermost few metres, it shows normal magnetic polarity. It is underlain by open lacustrine, reverse polarity shales of the Congeria praerhomboidea zone, and is overlain by layers indicative of the Prosodacnomya zone. The latter is well definied by the radiometric age (7.9 Ma) of the overlying volcanosedimentary suite. Therefore the Fehérpart section was deposited either 8.1–8.0 Ma (C4n.2n) or 8.3–8.2 Ma (C4r.1n chron) ago.
The Tihany Formation was deposited in a variety of palaeoenvironments related to deltas entering Lake Pannon. It consists of parasequences, i.e. shallowing up successions from below wave base to lake level, generated by sediment
accumulation. Parasequences were formed on the delta front or in inter-distributary bays to delta-plain swamps and distributary channels. Beyond the high frequency lake-level and partly autocyclic environmental fluctuations, most likely
climatically induced fourth-order lake-level changes of about 15–30 m amplitude occurred, resulting in minor transgressions followed by repeated progradation of deltaic lobes.
Although the Tihany (as well as the very alike Somló) Formation is currently found along the rim of the hills, it was originally deposited in the same way as the Újfalu Formation known only from the subsurface of deep basins. The
dynamics of deltaic settings feeding to Lake Pannon can be understood by studying the Tihany Formation in outcrops. The only difference between the two formations might be in the number of overlying delta cycles and their thickness; both
were determined by rate of subsidence, being smaller at basement highs where Tihany Formation accumulated than at basin areas where Újfalu Formation was defined. It is suggested here to include the Tihany (and Somló) beds as members
of the Újfalu Formation. Fourth-order sequence boundaries were recognized between the overlying progradational deltaic bodies. In the vicinity of Tihany no evidences of lake-level drops were revealed, but elsewhere small incised-valley fills point to minor lake-level drops. The overall regression, interrupted by transgressive events, continued in the study area until the shelf edge of Lake Pannon shifted as far to the south as 50–60 km, i.e. at about 8 Ma ago. After that, flooding events became rare and small in amplitude, then the area became a terrestrial plain. Fluvial deposits are not known from the direct vicinity, but travertines formed in small freshwater ponds fed by karst springs. The transition from lacustrine to
terrestrial palaeo-environments is part of the overall normal regression as a result of high sediment input to Lake Pannon. Large incised valleys or other evidences of recurring terrestrial conditions which could be related to thirdorder sequence boundaries mappable all over the Pannonian Basin were identified neither in Tihany nor in Újfalu Formation.