Stratigraphy
The total thickness of molasse deposits in the basin can be up to 6 kilometers. Lithostratigraphically, this molasse is treated as a group that is divided into four formations. This division is made on weather the sedimentary facies is continental or marine.
The lowermost formation is the Lower Marine Molasse (in German: Untere Meeresmolasse). Its age is Rupelian (Early Oligocene, 34 to 28 million years old) and it consists of shallow marine sand, clay and marl. On top of this is the Lower Freshwater Molasse (German: Untere Süsswassermolasse) of Chattian and Aquitanian age (28 to 22 million years). Due to an eustatic drop of the sea level combined by tectonic uplift, the basin was now above sea level. This second formation therefore consists of fluviatile sands and clays and huge alluvial fan systems (conglomerates and breccias) originating from the Alps to the south.
By 22 million years ago, the Alpine foreland was flooded again due to tectonic subsidence. A shallow (intertidal) marine environment formed from Lyon to Vienna. In this environment the third formation, the Upper Marine Molasse, was formed. It consists of marine sands, clays and marls and new fan conglomerates and is of Burdigalian to Langhian age (22 to 16 million years old). More tectonic uplift caused the sea to retreat one final time and between 16 and 5 million years ago (during the Serravallian, Tortonian and Messinian/Pontian ages), the basin was in a continental facies again. The fluviatile sands and clays and fan conglomerates of this time form the Upper Freshwater Molasse, the topmost molasse formation.
Around 5 million years ago a phase of uplift occurred in the Alps. During this phase the Molasse basin ceased to be an area of net sedimentation.
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