Palaeobathymetry and palaeogeography of the Bakony Eocene Basin in western Hungary
Abstract The authors studied two new outcrops and the cores of 10 new bore holes drilled through the marine Eocene of the Bakony Hills. The main purpose was to recognize palaeoenvironments and their changes in the Bakony Eocene Basin. Earlier research rather neglected this aspect. Palaeoecologic analyses of different groups of fossils (nannoplankton, foraminifera, molluscs, ostracods) and to a lesser extent, sedimentologic observations and field studies served as tools for the reconstruction of bathymetric conditions and subsidence history, by studying the trends in lateral facies changes. The reconstruction of paleaogeographic patterns was made possible. Vertical facies successions are clear, due to the excellent core material and the rather simple tectonic structure. Accurate datings have been possible, using the sophisticated stratigraphy worked out for the Bakony Eocene by a combination of magnetostratigraphic and planktonic zonations, published elsewhere. In part 1 the authors present the description and palaeoecologic analyses of the studied sections. The vertical succession of the main facies units in the SW Bakony Basin is as follows (bottom to top): coastal sandy pelites, pebbly sands, biogenic shelf limestone, glauconitic calcareous marl, hemipelagic globigerina marl with turbiditic sandstone intercalations. The same sequence in the NE Bakony Basin is as follows: alluvial-coastal sediments with coal seams, nummulitic shelly sands, glauconitic sands and marls, laminated clay, globigerina marl and bathyal clays. The authors discuss palaeonvironments and their evolution in part II: The coastal facies in the SW Bakony Basin is a few million years older than in the NE. Nevertheless, this facies indicates in both areas flat coastal plains and seashores probably covered by mangrove. A carbonate shelf developed in the SW Bakony Basin with very low sedimentation and subsidence rates. The shelf period was much shorter in the NE. The shelves were influenced by intense currents that were active in the whole basin. Since Mid Bartonian times, retrogradation of the shelves is demonstrated by the sequence. First, sediments of shelf break and upper basin-slope environment overlie the shallow marine deposits. Strong glauconitization indicates current activity. The glauconitic facies is overlain by a globigerina marl and silts, described earlier as shelf sediments! The authors set out to prove that the maximum depth of its deposition has been at least 800 m. Turbidites are intercalating with the globigerina marl in the SW. In part III mainly palaeogeographic conclusions are discussed. The authors recognized on the basis of lateral facies changes that shallow marine conditions prevailed to the east and south of the Bakony Basin during the entire Bartonian Priabonian. The position of erosional source areas and the direction of dense turbidity currents suggest the same conditions. The correlation with the Late Eocene-Early Oligocene Zala-Velence andesitic arc is also discussed in some detail and is based on tephra layers , found in the globigerina marl. The authors recognized a remarkable, Middle Bartonian coincidence of several different events, such as increasing andesitic volcanism, sudden acceleration of basin subsidence, increase of sea depth, beginning of turbidity current activity. This may indicate a coeval increase in tectonism. The transgressions are younging towards the NE . The basin generation shows a similar shift. A complete palaeogeographic and probably a tectonic “inversion” took place in the Priabonian. The Southern alpine-Dinaric character of the Eocene fauna and lithofacies changed gradually to an Eastern Alpine-Carpathian one during the Priabonian. From the beginning of the Oligocene, only the latter influence did exist. Also during the Priabonian, a new marine basin was formed in Hungary to the NE of the Balkony Hills.