The Miocene Lake Öhningen: lithofacies, geochronology, and the maar-paradigm

Lake Öhningen (Middle Miocene, Mammal Neogene Zone MN7) on the southern flank of the Schiener Berg (SW Germany) is one of the most iconic European Fossil-Lagerstätten, yet the depositional environment and age of its sediments have remained poorly constrained. In 2021, a new excavation in the historical upper quarry exposed 8 m of lacustrine sediments, representing the upper part of the known deposits. The succession comprises calcareous laminites, unstructured and laminated clays and silts, marls and limestones, a conspicuous interval of micaceous fine sands and calcarenites with well-developed ripple marks, and several tuffite layers. The laminites are largely unfossiliferous and lack petrographically resolvable varve couplets, contrasting sharply with the organic-rich, varved laminites of ‘textbook’ meromictic maar-lake Fossil-Lagerstätten. The rippled sandstones document phases of shallow, higher-energy conditions and significant input of reworked Molasse sand from the catchment, most likely via tributaries and weather-generated inflow. Together with the mixed facies pattern, this challenges the traditional interpretation of Öhningen as a classic maar lake. Two brown tuffites interbedded within the lacustrine succession yield apatite fission-track ages of 13.7 ± 0.5 Ma (lower Serravallian), providing the first robust absolute ages for the Öhningen Beds. These ages are slightly older than the Herrentisch Deckentuff (12.6 ± 0.4 Ma) and overlap with published ages of the Wangen tuff cone, placing Lake Öhningen within an early phase of Hegau volcanic activity and confirming its MN7 assignment from biochronology. We interpret Lake Öhningen as a small, stratified but hydrologically open lake influenced by episodic volcanic, fluvial, and storm events, and recommend the neutral term ‘Lake Öhningen’ rather than ‘Öhningen Maar’ for this depositional system.