Late miocene evolution of the Paleo-Danube Delta (Vienna Basin, Austria)

The first deltas of the Paleo-Danube formed around 11.5 Ma ago along the northwestern margin of the Vienna Basin, during a lowstand of Lake Pannon. We present a detailed description of the depositional architecture of five individual lobes of these deltas in the Austrian part of the central Vienna Basin based on the integration of 3D seismic surveys and well-log data, covering an area of about 600 km2. Based on geometry, steep clinoforms and the development of beach ridges, the delta is classified as mostly river-dominated with significant influence of wave-reworking processes. The development and shifts of individual lobes suggest a stepwise northward movement of the delta system, which is discussed in relation to a stepwise activity of regional fault systems. True vertical depth measurements between topset and bottomset deposits document a decrease of lake water depth from about 180 m during the early phase of delta formation at 11.5 Ma down to about 100 m during its mature phase. A major lake level rise of Lake Pannon around 11.3 Ma caused the flooding of the margins of the Vienna Basin, resulting in back stepping of fluvial deposits into the North Alpine Foreland Basin, which led to the termination of delta deposition in the study area. The Paleo-Danube delta of the Vienna Basin is comparable in many ways to the Holocene to modern Danube delta, located in the Black Sea. Size, sediment volume and speed of progradation of the lobes, however, indicate, that this primeval delta was distinctly smaller than its modern successor and had a much smaller sediment load.