NEOGENE SEQUENCES IN THE PANNONIAN BASIN, HUNGARY

The rifting in the extensional Pannonian basin occurred in the Middle Miocene, the subbasins within the Pannonian system formed under the co ntrol of expansional tectonics during this time. Sequence stratigraphi c analysis on well logs revealed a single third-order synrift sediment ary sequence developed due to tectonically enhanced eustatic sea-level change in shallow-marine environments. The Pannonian basin became an isolated lake by its postrift phase during the Middle-Late Miocene bou ndary. The first postrift sequence formed under peculiar conditions, w hen the basin lost its marine connections and simultaneously a conside rable differential subsidence began. The large volume of freshwater in put supplied by rivers and the rapid subsidence of the basement caused the basin to be a deep freshwater lake. The lacustrine postrift infil l was studied by sequence stratigraphy using seismic reflection, well log and magnetostratigraphic data. Numerous unconformities can be reve aled which represent relative lake-level fluctuations of different mag nitudes. The water level and volume of the large inland lake was very sensitive to different effects, the most important ones are lake-water budget, sediment influx and differential subsidence of the basement. The style of extension and sedimentation show differences with respect to passive continental margins that are reflected in the sedimentary architecture. A significant third-order water-level change has been di scovered in the eastern part of the Pannonian basin, while the western side did not experience such a great change in lake level. This fact gives credit to the view of different interplay of lake-level-influenc ing effects in different areas of the basin. This major third-order la ke-level change has been age dated using magnetostratigraphy and a ver y close match was found with the age of the Mediterranean salinity cri sis. The Pannonian basin offers the possibility to study the infilling style and stratigraphic evolution of an inland rifted basin. Detailed sequence analysis allows the recording of the apparent sea/lake-level motion which can be compared to those observed in the surrounding sea s. Further tectono-stratigraphic evaluations of the Pannonian basin ma y reveal hidden facts on the nature of lake-level fluctuations and con tribute new data to the discipline of sequence stratigraphy.