The Miocene Hegau Volcanic Suite, situated in the Autochthonous North Alpine Foreland of Germany (area of Singen northwest of the Bodensee), is related to intraplate magmatism of alkaline basaltic composition. The Hegau volcanism seems to be associated with extension within the NW–SE striking Hegau-Bodensee Graben structure and regional uplift of the Schwarzwald area. Volcanic activity is of Middle to Late Miocene age, with various episodes succeeding each other between some 14 and 7 Ma: pyroclastics (“Deckentuffe”), including extensive layers of volcanic ash (up to 100 m thick) as well as chimney and crater fills, came first, followed by olivine nephelinites, hornblende tuffs and finally phonolites.
The Upper Freshwater Molasse of northeastern Switzerland contains several levels of bentonite (fossil weathering products of volcanic ash deposits) radiometrically dated between 15.3 and 14.2 Ma (Gubler 2020 and ref. therein), the ages of which coincide with the onset of Hegau volcanism. A younger bentonite layer near Frauenfeld was dated by Rahn & Selbekk (2007) at around 11.5 Ma.

Vulkanische Tuffe (Deckentuffe), d.h. pyroklastische Gesteine aus gasreichen Eruptionen, die in Form von Aschen- und Lapillituffen abgelagert wurden.