Paleoenvironmental Changes during the Middle Miocene

Middle Miocene marine sediments (-16.4 to -11 million years ago) predominate in basins of Eastem Austria, especially the Vienna and Styrian Basin. During that geological time interval, which is divided into the stages Badenian, Sarmatian, and Pannonian, these basins belonged to the Paratethys Sea. The holostratotype of the Badenian was established at the former brickyard Baden/Sooß near the western border of the Southern Vienna Basin. The transition from Lower to Middle Miocene (Karpatian/Badenian) could be investigated in the Styrian Basin during the forerunning project an basis of environmental changes pictured in the microfauna, nannoflora, and sedimentology. This transition should now be investigated in the Vienna Basin. As the main patt of the project, the environmental development during the whole Badenian should be investigated in both basins. A 96m deep drill at the locality of the Badenian holostratotype, which was financially supported and processed during the forerunning project, should be investigated in detail, where changes in the microfauna (foraminifera and ostracods), nannoflora (coccoliths), supported by sedimentological and stable isotope investigations in cooperation with paleomagnetic measurements must be treated. Especially ichnological investigations using the concept of ichnofabrics allow the reconstruction of environmental conditions during the sedimentation of basin sediments, since ichnofossils strongly react to physical and biological changes and - as autochthonous elements - represent an undisturbed picture of sedimentary conditions. For the detection of astronomic cycles, minnte sampling of the drill is necessary. Similar investigations should be performed in the Styrian basin, where a thick section consisting of muddy to sandy sediments of Badenian age is exposed in the limestone quarry Retznei/Leibniz. The last aspect of the project should be the transition from the Badenian to the Sarmatian in both basins. Again, environmental changes are best pictured in assemblage compositions of the microfauna and nannoflora, supported by stable isotope and paleomagnetic analyses. Based an a lot of samples, where the process of preparation started in the forerunning project, this project could enlighten the development of marine basins in Eastem Austria during the Middle Miocene.

The main target of the project was the investigation of early Middle Miocene ( 16.3 to -12.7 my) sediments in Austria and the interpretation of environmental changes in the marine realm. In the western Styrian Basin, the transition from the Early Miocene (Karpatian) to Middle Miocene (Badenian) is preserved in marine sediments. An integrated research based on magneto-, litho- and biostratigraphical data showed that various sedimentation unconformities could be found marking larger time gaps. These are caused by tectonic events, the so-called Styrian Phase of the Alpine tectonics. The basal `Steirischer Schlier` of Karpatian age could be interpreted as sediments deposited at ~260m depth that are strongly influenced by slightly acidic conditions caused by volcanism. In the Wagna brickyard, an unconformity is marked on top of the `Schlier` through sedimentation changes (coarser silt to fine sand) and the index species of the early Langhian, Praeorbulina sicana. Therefore, this part belongs to the Middle Miocene despite the calcareous nannofossil Helicosphaera ampliaperta still indicates NN4. Thus the Karpatian/Badenian boundary does not coincide with the NN4/NN5 boundary. The latter is located above a coral patch reef and sandstones. The following sediments contain rich shallow water molluscs and are superpositioned by corallinacean limestones. According to nannoplankton zones, this belongs to NN5 as indicated by Helicosphaera waltrans. In the Retznei quarry, corallinacean limestones directly overlie the Karpatian `Steiririscher Schlier`; here, the Karpatian/Badenian boundary coincides with the NN4/NN5 boundary due to the large sedimentation gap. On top of these limestones, the sedimentation of silty sediments indicates deeper water environments, which is manifested by paleodepth estimation using benthic foraminifera starting on top of the corallinacean limestones at -170m and deepening to -300m. The comparison of biostratigraphic data with polarity changes in palaeomagnetics allowed the assignment of polarity changes to magnetochrons. Based on these data, large sedimentation gaps could be recognized spanning ~400 ky at the Karpatian/Badenian boundary (Styrian Phase) by elevating the basin to ~50m depth. After further 400 ky with interrupted sedimentation of shallow water sediments (coral patches), the second gap spans ~ 600 ky ending at the NN4/NN5 boundary (-14.74my). The following continuous sedimentation of corallinacean limestones followed by silty marls belongs to the early NN5-zone because of containing Helicosphaera waltrans. Multidisciplinary investigations were performed on a scientific core that was drilled near the stratotype of the Badenian at the former brickyard Baden/Sooss, Lower Austria. Biostratigraphically, the whole core is positioned above the lower part of NN5 as indicated by the lack of Helicosphaera waltrans, which is present in the Vienna Basin only in the deepest parts. Magnetic susceptibility, calcium carbonate and organic carbon content show periodic variations through the whole core that could be decomposed and paralleled with orbital cycles. The main influence seems to be through two 100 ky excentricity cycles spanning the time period of the core. Cross- correlation with astronomical cycles allowed an exact determination of the core age between -14.36 and -14.16 my. The foraminiferal fauna demonstrates a weak cooling that is confirmed by d 18O values of planktonic forms. The d13C measured on benthic foraminifera shows oscillations corresponding to the organic carbon content, which again pictures orbital obliquity (41 ky) cycles. Eutrophication is demonstrated by the foraminiferal fauna with increasing infaunal and decreasing oxiphilic species. Shallowing towards the top of the core could also be estimated (from - 260m to -240m) and the increasing terrestrial input is documented by magnetic susceptibility correlated with the ichnofauna composition (fossil traces).