Seismic stratigraphy of the Late Miocene sequence beneath Lake Balaton, Pannonian basin, Hungary

This study is based on the interpretation of about 190 km of high-resolution, single-channel reflection seismic profiles acquired on Lake Balaton in June of 1993. The seismic survey investigated the lake subsurface at maximum depth of about 200 msec down to the acoustic basement whic is represented by Sarmatian strata. Seismic interpretation showed that Lake Balaton lacustrine deposits (Holocene) rest over a subhorizontal unconformity which dramatically truncates the underlying Late Neogene (Pannonian) sequence. This unconformity marks a major stratigraphic gap that practically encompasses the whole Pliocene and Pleistocene. The Pannonian sequence in turn, unconformably overlies Sarmatian (Late Serravallian) strata at depth of 50 to 200 m. Sarmatian strata are truncated at the top by a mature (policiclyc) erosional surface that marks a stratigraphic gap which spans over a few million years (about 12 to 9 Ma) and includes amalgamation of two 3rd-order sequence boundaries (Sar 1 + Pan 1). Incised valleys, fluvial terraces and channel fills are also recognized from the seismic record. The lowermost part of the Pannonian sequence directly overlying the Sarmatian "basement" is represented by Szák formation, interpreted as a Transgressive System Tract (TST). The maximum flooding surface at the top of this TST corresponds, in the area of lake Balaton, to the top of Congeria czjzeki bearing marls and can be dated at 9.0 Ma. by correlation with magnetostratigraphic age dating at Iharosbereny-I well. Then follow Somló and Tihany formations which are interpreted as Highstand System Tract (HST). Architecture of the Highstand deposits shows typical downlapping strata onto the top of the transgressive deposits (ca. 9.0 Ma), with local development of small coarse-grained prograding deltas. A subtle 3rd-order sequence boundary (Pan 2) was also detected towards the upper part of the Tihany formation. It is related with a significant lowering of base level of erosion in the Pannonian Lake and possibly also associated with tectonic and volcanic activity. Pan-2 sequence boundary crops out at the top of Tihany Peninsula. Its occurrence (ca. 8.6 Ma) predates the onset of basaltic eruption of the Tihany Volcano (ca. 7.8 Ma, Balogh, 1995). The Pannonian sequence in the area of Lake Balaton is affected by intense tectonic deformation. Tectonics mostly postdates Late Miocene and consists of SW-NE strike-slip faulting possibly associated with gentle folding. Evidence of minor tectonic activity exists at abut Pan 2 sequence bondary (ca. 8.6 Ma). This suggests causal relations between tectonics and volcanic activity in the area of Lake Balaton and also offers an alternative solution to explain 3rd-order changes of base level of erosion within the Pannonian Lake, other than purely climatic. Significant tilting of the entire Pannonian sequence towards SE is also seen on seismic profiles that consistently show ESE dipping strata which are erosionally truncated at the top. This fits earlier interpretation which showed that Pannonian Basin underwent significant late stage tectonic inversion and uplift with consequent subaerial erosion of Latest Neogene strata in Transdanubia and Northern Hungary.