Changes in Eastern Alpine Miocene Ecosystems and their Geodynamic Control
Fonds zur Förderung der wissenschaftlichen Forschung (FWF), Vienna, Austria
Coordinator: Werner E. Piller
Institute for Geology and Palaeontology, Karl-Franzens-University Graz
Distinct changes of fossil communities - bio-events - occurred repeatedly during the Miocene in the East Alpine region. On the basis of these distinct events, early studies in the last century discriminated between a I. and II. Mediterranean Stage based mainly on the marine molluscan fauna. These 2 stages were correlated with the Mediterranean stratigraphic scheme. In the 1960s, however, this correlation was detected to be ill-founded and a regional stratigraphic framework for the (Central) Paratethys was established. Some of the stages included in this framework, particularly those of the Early and Middle Miocene, were inferred from marine sequences in Austria (Eggenburgian, Ottnangian, Karpatian, Badenian). Most of the stage boundaries are founded on changes in molluscan fauna.
The causes for these distinct changes are thought to have a palaeoecological background. However, we have reason to believe that tectonic activities on a local, regional or a global scale had a major influence on the development of these distinct changes. The possible and highly probable link of these biological parameters with geodynamic forces is not known in any detail as well as a possible coincidence between bio-events and geodynamic events. This hypothesis is the basis for this project cluster. Below, we describe why we believe that tectonic and geodynamic processes influenced the development of changes in the fossil communities and individual project goals that will document this.
Palaeobiogeographic and tectonic frame
The East Alpine - Carpathian - Pannonian region suffered from tectonic activities on a large scale during the Miocene. Particularly during the Ottnangian/Karpatian the principal tectonic system changed from overthrusting to lateral extrusion. The later means eastward movement of tectonic blocks of the central Eastern Alps along prominent transform faults. The escape of these blocks was linked with distinct E-W directed extension (approx. 50 % in the central Eastern Alps), crustal thinning and tectonic exhumation of metamorphic domes (e. g., Tauern window, Rechnitz). This development terminated the subsidence of foreland basins (Molasse Basin) in the Ottnangian/Karpatian and led to the development of new sedimentary basins on top of the Alpine/Carpathian overthrust wedge. Some of these newly-created basins were formed by a pull - apart mechanism along newly-developed lateral faults (e. g., Norian trough, Vienna Basin, basin of the Lavant valley). Some are rift basins over a distinctly thinned crust at the transition to the Pannonian Basin (Styrian Basin).
During the Miocene the topography of the Alps evolved from a terrestrial plain to a medium and high mountain chain. Synchronous with this uplift thick rock sequences were eroded. On top of the Alpine orogenic wedge fluvial systems developed preferentially following active transform faults (Mur/Mürz/Vienna Basin, Lavant Valley). These systems transported the eroded clastic materials into the adjacent marine basins. Additionally, this development of topographic relief directly influenced the regional climate in the Eastern Alpine region.
The reconstruction of the time schedule of these geodynamic processes and their relationship with ecosystems needs exact dating which can be performed only by well-founded biostratigraphic data in connection with palaeomagnetic and stable isotope data. An integration of data from aquatic and terrestrial systems is also an indispensable necessary requirement.
In the Eastern Alpine region these geodynamic changes primarily led to substantial change of the marine environments and ecosystems by changing basin geometries, subsidence, water depth and salinity, input of terrigenous material, and climate. With increasing uplift of the Alps the marine basins disappeared and terrestrial systems evolved which also changed with the constantly changing environment driven by geodynamic processes.
These geodynamic processes, however, did not only affect this restricted area but widely influenced also the general palaeogeographic development of the Paratethys. Linked with this palaeogeographic evolution are also climatic changes, which are currently reconstructed as follows:
During the Early Eggenburgian a subtropical climate is reconstructed due to the marine fossils. A cooling phase followed caused by the closure of the Indopacific seaway and the opening of a passage to the west resulting in cooler water influence. In the Late Ottnangian the marine connection over the Alpine foredeep was interrupted leading to a brackish interval and continental sedimentation. A new connection to the Western Mediterranean was established at the intersection of the Periadriatic fault/Idria-Transform fault/Lavant valley fault (Trans-Tethyan-Trench Corridor). Marine sedimentation was restricted to the Pannonian System and the Western Carpathian foredeep. Only with the Middle Miocene transgression, which coincided with a period of global warming, during the Early Badenian a marine reflooding occurred. At the same time subsidence of the Vienna Basin started. In the Middle Badenian a regressional phase occurred leading to thick evaporites in Poland and Rumania. In the Late Badenian a transgression followed, the Trans-Tethyan-Trench-Corridor, however, was closed and a re-connection with the Indo-Pacific has to be considered. During the Early Sarmatian salinity probably decreased and/or alkalinity increased and euhaline organisms disappeared from the Central Paratethys during the Late Sarmatian. In the Late Miocene (Pannonian) the aquatic realm was reduced to the inner-Carpathian basins and mostly continental conditions prevailed in the Eastern Alpine Basins with a rich mammal fauna and continental vegetation.
Goals of the project cluster
The above portrayed palaeoecologic, climatic, and palaeogeographic development reflects very rough ideas which still lack well-founded data on the different fossil groups as well as a profound stratigraphic frame. Therefore, as many palaeoecologically important fossil groups as possible have to be investigated parallel to the study of sedimentology, palaeomagnetism, stable isotopes, and the structural evolution of the East Alpine area in order to reconstruct its development during the Miocene.
Within the proposed project cluster, 9 individual projects are proposed which will investigate 9 different time slices of about 11 million years of the Miocene:
"Temporal and spatial changes of microfossil associations and ichnofacies in the Austrian marine Miocene (J. Hohenegger)". This project deals with micropalaeontological groups, particularly with foraminifera, but also with ostracods. Besides the palaeoecological focus, within this project a biostratigraphic base for all other projects of the marine phase will be provided.
"Evolution versus migration: Changes in Austrian Miocene molluscan paleocommunities (M. Zuschin)". This project will provide data on changes in molluscan assemblages which may be caused by migration during periods of open seaways and by local evolution during periods of isolation. This project deals not only with marine environments but includes also ecosystems with reduced salinities (Sarmatian - Pannonian).
"Miocene terrestrial environments reflected in leaf assemblages (J. Eder)". On the base of various leaf-floras it is attempted to reconstruct palaeoecological parameters, particularly palaeoclimate.
"Extracting palaeoecological and palaeoclimatic data from dispersed fruits and seeds (diaspores) in Miocene terrestrial ecosystems of eastern Austria (D.K. Ferguson)". This project will gain palaeoecological information relying on fruits and seeds.
"Use of dispersed palynomorphs and organic facies for the reconstruction of terrestrial ecosystems in eastern Austria (C.-C. Hofmann)". Similar to the two above projects this project focuses on terrestrial ecosystems. In contrast to the last 2 projects, this project will use pollen and spores as tools for palaeoecological interpretations. The advantage of this project is the combination of microfloras and organic facies and sedimentology. In addition, this project is supposed to supply the project cluster with biostratigraphic information for the non-marine environments. This information can produce a good data base for correlating marine and non-marine (Eggenburgian - Sarmatian) as well as various aquatic and terrestrial environments (Sarmatian - Pannonian).
"Stable Isotopes and changing Miocene palaeoenvironments in the Eastern Alpine region (W. E. Piller)". This project will produce palaeoenvironmental proxy data, e.g. for salinity, temperature, water mass distribution, and productivity in studying stable isotope (oxygen, carbon) composition of skeletal material, particularly of foraminifera and echinoids. For echinoids, prior to isotope study, a modern taxonomy has to be established and methodological questions on isotopy of this group have to be addressed. A continuous oxygen isotope curve for the studied time interval of marine sediments is supposed to be established.
"Paleomagnetic reconstruction of geodynamic events and climate changes in the Eastern Alpine Miocene (R. Scholger)". Using palaeomagnetic information rotations and latitudinal movements in different Miocene basins in the Eastern Alps will be studied. This information will provide further information on changing seaways and trans- and regressiv developments. In addition, magnetostratigraphic data will support litho- and biostratigraphic correlations in the studied intervals.
"Stratigraphic simulation in Neogene basins of Austria: the influence of synsedimentary tectonics on stratigraphic sequences (M. Wagreich)". Within this project the role of tectonic movements, sediment supply and eustatic changes of sea level on basin development will be studied using computer modeling and simulation.
last updated: 30.04.2007