Grimmen Formation

Studies of Toarcian strata near Grimmen began in 1874 with the discovery of fossiliferous clays in a railway cutting near Schönenwalde, 5 km north of Grimmen, initially misidentified as Middle Jurassic due to ammonite finds. In 1909, the succession was reclassified as Lower Toarcian (Lias epsilon), distinct from the Posidonia Shale Formation. Exploration wells (e.g., Reinberg 1E, Kb Barth 10, Kb Grambow 5) since the 1950s provided extensive core data, with Reinberg 1E designated as the reference section in 2025. == Sedimentology/Lithology ==

The Grimmen Formation comprises greenish to bluish claystones, siltstones, and fine-grained sandstones, with siderite and carbonate concretions, reaching up to 100 m thick in the eastern North German Basin. Pellet laminae, up to 5 mm thick, decrease upsection, while silty/sandy intercalations and siderite concretions increase, forming a coarsening-upward trend with pyrite-filled burrows. Glacially dislocated Eocene greensand and clay are locally intercalated. Organic-rich layers contain charcoal fragments, suggesting wildfires. Toarcian material found in glacial Erratics in Ahrensburg and the Hagen Forest have been in controversy due to its dubious origin, being linked with the Rya Formation and Sorthat Formation, as well this unit. They were originally considered or local or Baltic in derivation, but that changued with the recovery of erratic concretions in the Baltic sea cliffs near Lübeck, being found as part of the Weichselian Glacial Maximum. Liassic–Cretaceous sediments in the assemblage are most probably associated with the tectonic Sorgenfrei-Tornquist Zone. The origin of this erratics from southwestern Baltic, Poland or Danish archipelago is unlikely, as those zones are dominated by Late Cretaceous–Paleocene strata, suggesting that this Toarcian assamblages should come from south/SW between STZ–TTZ and the German Baltic coast. The most clear hint link this deposits with the Grimmen Fm, as they're identical in fauna and facies composition of Grimmen and Dobbertin, also affected by subglacial erosion and thrusting, suggesting a close stratigraphic and palaeogeographical origin. == Paleoenvironment ==

The Grimmen Formation was deposited in a prodelta to brackish-marine environment in the eastern North German Basin, influenced by sea-level changes and the Toarcian anoxic event. The Reinberg Member reflects a transition from organo-detrital Posidonia Shale Formation to brackish-marine clays, with high total organic carbon (TOC, ~3 wt%) at the base decreasing to 'Inoceramus' dubius are abundant, with adults in elegantulum and exaratum concretions, often in regurgitalites. Ostracods and foraminifera are absent in black shales but occur above the Dörnten Member. Calcareous nannoplankton (e.g., Rhombolithion) are diverse in the elegantulum Subzone, correlating with NJ5b–NJ6 zones. Palynofacies show a dinoflagellate cyst blackout in the elegantulum-exaratum Subzones, with amorphous organic matter (AOM) dominance and micro-charcoal. Megaspores (e.g., Paxillitriletes phyllicus'') from Fennoscandia indicate a humid climate. Reinberg Member Fossil Record The Reinberg Member, notably at Grimmen and Dobbertin, is a Konservatlagerstätte with exceptional marine and terrestrial fossils in exaratum concretions. Ammonites (e.g., Cleviceras exaratum), Coelodiscus minutus, and '''Inoceramus' dubius are preserved in calcite, with predation traces. Decapod crustaceans (e.g., Palaeoastacus-like), coleoid gladii, and fish (leptolepids, Grimmenodon aureum'') occur in calcium phosphate. Coprolites, likely from fish, sharks, and marine reptiles (e.g., ichthyosaurs, plesiosaurs), are common. Terrestrial insects, especially in exaratum concretions types 1 and 2, include well-preserved wings and rare 3D specimens, comparable to Dobbertin's fauna. These fossils, formed in anoxic, fine-grained limestone below the storm wave base, reflect a nutrient-rich environment fertilized by insect carcasses. == Biota ==

Foraminifera Ichnofossils Brachiopoda Bivalvia Gastropoda Cephalopoda Crustacea Small indeterminate shrimps, sometimes found associated in great numbers, are recovered on several layers at Grimmen. In Klein Lehmhagen insects are found as part of calcareous nodules in the exaratum-elegantulum subzones, with specimens also found in living chambers of Eleganticeras elegantulum macrochonchs and in fish coprolites which are the most frequent fossils at all. Vertebrates Fishes Amniotes Flora == See also ==