Before the development of the Molasse basin, in the
Mesozoic era, the region was covered by a shallow sea that was the northern margin of the Alpine part of the
Tethys Ocean (
Valais Ocean in the west, Tethys Ocean proper in the east). The Molasse basin was formed when the orogenic wedge of the forming Alps was pushed north over the European
continental margin, due to the
convergent movement of the
European and
Apulian plates during the
Paleogene period. The weight of the orogenic wedge made the European plate bend downward, forming a deep marine foredeep. In the
Eocene epoch (55 to 34 million years ago) the basin became deeper until north of the developing orogen it formed a small
oceanic trench, in which
flysch sediments were deposited. The huge amounts of sediments eroded from the forming mountain chain filled the basin and made it shallower. During the Oligocene and Miocene epochs (more exactly, between 10 and 30 million years ago), shallow marine to continental molasse was deposited in the basin. Around 10 to 5 million years ago, tectonic uplift had raised the basin so high that net sedimentation stopped. From the south, the molasse deposits were overthrust about 10 kilometers by the
Helvetic nappes, which caused the deformation in the Subalpine Molasse zone. The
Jura mountains, a
fold and thrust belt along the present Swiss-French border, also originated from this tectonic phase. In some places in the Jura mountains, molasse deposits were folded together with older Mesozoic
limestones. In present-day central Switzerland, however, the molasse formed a thick competent mass that was thrust northward in one piece over a
decollement horizon at the base of the Mesozoic, in
Triassic evaporites. Deformation instead localized further north, thus forming the relatively flat Swiss Mittelland between the Alps and the Jura Mountains. The Swiss part of the Molasse basin is now between the Alps and the Jura mountains, as a large
piggyback basin. In the
Eastern Alps an external mountain range such as the Jura Mountains never developed. Due to the last phase of tectonic uplift around 5 million years ago, the molasse in the
Swiss Plateau, the South Bavarian plain and Eastern Austria is now 350 to 400 meters above
sea level at its northern rim, slowly rises southwards and can reach more than 1,000 m at its contact with the Alps. sands and red
silty
marls of the
Lower Freshwater Molasse at
Wallenried,
Switzerland. ==Stratigraphy==