Intrabasinal faults and differences in facies were previously regarded as evidence for a rift origin of the structure including the Eocene fossil site "Grube Messel". However, recent scientific drilling provided strong evidence for a phreatomagmatic origin of the former lake basin. A present study revisited those features previously interpreted as evidence for a rift basin.
Whereas the southern and southeastern margin of the Grube Messel is underlain by dioritic bedrock, partially covered by Permian deposits and clearly separated from eachother by a SE–NW striking Paleozoic fault, which was obviously reactivated in Eocene times. Due to the different basement on both sides of the fault, the maar eruption resulted in an asymmetric funnel-shaped basin with steeper slopes in the dioritic bedrock. After the eruption ceased, collapse of the southeastern diatreme wall has produced a meagabreccia with a thickness exceeding ten meters.
All intrabasinal deposits, pyroclastic material and lacustrine sediments, were subject to significant compaction. As the pyroclastic deposits have a thickness of up to 2,5 km in the underlying diatreme, especially their compaction resulted in a strong subsidence of the later basin fill, lowering the marker horizons in the basin center relative to the margins by more than 50 m. A sector in the SE and E of the basin is characterised by significant intrabasinal faults and a high inclination of the oil shale base. These characteristics as well as folding of the oil shale and a 9 m thick spiculite which is only occurring in the SE of the basin may be used as evidence for a comparatively strong subsidence of the respective sector. This can easily be explained by an asymmetric funnel-shaped structure of the diatreme.
Subsidence most likely continued after the filling up of the lake, as indicated from three trough-like synclinal structures in the NE, E and S of the Messel basin. According to older descriptions, these troughs contained coal, shale and reworked sediment. Remaining parts of the northeastern trough have recently been cored by scientific drillings. The cores obviously recall the old descriptions and show some disturbance, but detailled studies are needed to confirm potential effects of compaction.
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