Hypoxic Events on a Middle Miocene Carbonate Platform of the Central Paratethys (Austria, Badenian, 14 Ma)

During the Middle Miocene Badenian the Eisenstadt-Sopron Basin was structured by the Leitha and the Fertörákos carbonate platforms, comprising different lithological types of the Leitha Limestone. In the quarry "Kummer" near St. Margarethen (Burgenland, Austria) three lithofacies of the Leitha Limestone can be observed. A short historical review, data on regional geology and sedimentology, along with detailed autecological and synecological analyses of the fauna are presented. The section is characterised by the intercalation of bright yellow calcarenites (corallinacean debris facies) and olive green marls (laminated marl facies). This fining and thinning upward sequence is interrupted by rudstones, chaotically sedimented in a broad channel. The mainly cross-bedded calcarenites bear corallinacean debris, rhodolithes, oysters, pectinids and fragmented echinoids. The most striking feature of the laminated marl facies is the extraordinary preservation of the fauna, which consists of a large number of various fishes, regular and irregular echinoids, bryozoan colonies (in situ), pectinids, and brachiopods. In addition, rich foraminiferal faunas and calcareous nannoplankton are investigated. In small lagoon-like structures with reduced water energy and turbulence, marl sedimentation took place. These 30 m to 60 m deep depressions were surrounded by carbonate sand build-ups. Storm events caused sedimentation of coarser-grained material via rip currents from the shoals into the subtidal level of the depressions. The channelised rudstones (rudstone facies) are deposits of multiple-phase, seismic events. Therefore the section represents tempestites with at least one observable deepening upward sequence. The remarkable preservation of the fauna is interpreted to be caused by two different processes: In the case of the benthic animals, such as the bryozoans and the echinoids, this preservation is mainly related to the lack of agitation. In small populations of echinoids whose spines are still attached, on the other hand, occasional hypoxic events killed off the benthic fauna and created conditions that preserve even complete fishes, by precluding scavenging and deterioration. Hypoxic conditions are further indicated by the microfauna, the nannoplankton and by the occurrence of iron-sulfides (altered to iron-oxides) within the marl layers. For the first time the bivalves "Chlamys" rakosensis Csep.-Mez. and Flexopecten scissus (Favre), the brachiopod Discinisca cf. leop