A variety of features, like intrabasinal faults and differences in facies were regarded previously as evidence for a rift origin of the Eocene fossil site "Grube Messel". However, recent scientific drilling provided strong evidence that the former lake developed in a maar crater. Those features that were previously interpreted as evidence to a rift origin rather reflect the influence of bedrock geology on the symmetry of evolving diatreme and subsequent posteruptive compaction.
Whereas the southern and southeastern rim of Grube Messel is underlain by dioritic bedrocks, its northwestern and western rim consists of granodiorites. The different magmatites are partially covered by Permian Rotliegend deposits and clearly separated by a SE-NW striking Paleozoic fault zone, which was obviously reactivated in Eocene times. Due to the different basement on both sides of the fault, the maar eruption resulted in an asymmetrically funnel-shaped basin with significantly steeper slopes in the dioritic bedrock. After the eruption ceased, the southeastern diatreme wall has collapsed at least once and produced a megabreccia 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, especially their compaction resulted in strong subsidence of the later sedimentary basin fill, lowering the marker horizons in the basin center relative to the margins by more than 50 m. The SE and E of Grube Messel are characterised by significant intrabasinal faults, a high inclination of the oil shale base and a folding of the sediment above. These characteristics of the SE and E may be used as evidence for a comparatively strong subsidence of this area and can easily be attributed to the proposed asymmetric funnel-shaped structure of the diatreme. In an area, where maximum subsidence is expected, a spiculitic facies of the oil shale, which is represented by a thin layer in most of the basin, has a thickness of about 9 m.
Subsidence most likely continued after the filling up of the lake, as indicated from three trough-like structures in the NE, E and S of former lake Messel. According to older descriptions, these troughs were up to 30 m deep and contained coal, shales and reworked sediment. They have almost completely been removed by mining activities, though some remains of the northeastern trough still exist and have been cored by three scientific drillings in 2004 and 2005, respectively. By throwing light on the late basin fill, the cores will help to elucidate if the formation of the troughs was in fact ruled by compactional processes.
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