Emergence
The Zwischenahner Meer is located above a salt dome as a remnant of the Zechstein Sea from the Permian era about 250 million years ago. It was created when the salt dome and the overlying overlying rock collapsed as a result of groundwater-related salt leaching. The Zwischenahner Sea was formed in the cavity created in this way. Other examples of waters created by sinkholes are the Sager Sea, the Maujahn Moor, the Arendsee and the Seeburger See. These lakes are characterized by their great depth in relation to their diameter. The Zwischenahner Meer, however, is quite flat and seems rather untypical. However, steeper edges can be detected on the Zwischenahner and Rostruper Ufer. However, they are heavily blurred by the thick layer of sludge.
In 1949, drillings were carried out to prove a salt dome, the result of which was a map showing the location of the salt dome under the lake. Accordingly, this salt dome belongs to a salt line that stretches from Delmenhorst via Leer and Jemgum to Bunde.
Proof of the lake formation as a sinkhole lake began in 1956 when the State Museum of Natural History and Prehistory created a model of the Zwischenahner Sea, which with the help of a double relief depicted the surface of the digested sludge filling and the mineral lake bed. Due to the transparent representation of the digested sludge layer in the model, the steep edges typical of a sinkhole lake were highlighted.
The apex of the salt dome is believed to be about 300 meters below the lake and is covered with clay layers. These layers ensured that the subrosions did not result in typical collapse funnels, but rather a less deep, more extensive depression with clear steep edges.
Boreholes in the digested sludge deposits (also called Mudde or Gyttja) revealed pollen that was found around 12,000 years ago (the end of the Vistula Ice Age in northern Germany, in this region characterized as a periglacial cold steppe).