Climate as main factor controlling the sequence development of two Pleistocene alluvial fans in the Vienna Basin (eastern Austria) — A numerical modelling approach

Tectonics, climate, base level and source lithology are the main controlling factors on alluvial fan development and evolution. In this study a numerical model approach was used to investigate climate-induced aggradation and degradation cycles, the influence of tectonic subsidence, and the impact of an axial main river on Austria's largest Pleistocene alluvial fan setting, the Mitterndorf Basin, eastern Austria. A simulation time frame of 25 ka was applied. Climatic variations were mainly modelled through variations in sediment supply, whereas the impact of subsidence was modelled through variations in slip rates along faults. Incision and aggradation tendencies of the axial main river were modelled by rising or lowering of the water level. The sequence development and geometry of Mitterndorf Basin's alluvial fans are mainly controlled by Pleistocene climate oscillations. Cold periods generally led to a decrease in vegetation cover and to an increase of periglacial weathering processes which is associated with abundant sediment release and aggradation on fan surface. In contrast, decreasing sediment supply and increasing precipitation during warmer periods led to fan degradation. Our models indicate that the alluvial fan development is primarily affected by the distinct sediment supply pulses during phases of short climatic perturbations. Subsidence becomes the determining factor again when models approach a steady-state behaviour. Similarly, the impact of the axial main river becomes more important during simulated warm periods when sediment release to the drainage basin is low. The overall basin slope additionally controls the evolution of fan and drainage. Variations in sediment supply and subsidence do also significantly control the long term evolution of the drainage network during simulated warm periods and thus control the sequence preservation potential of fans and therefore its long term sequence evolution.