Grimmen Formation

The Grimmen Formation is a Lower Jurassic (Toarcian) geological formation in northeastern Germany, primarily exposed in the Grimmen and Klein Lehmhagen clay pits and documented in wells such as Reinberg 1E. Formally established in 2025, it was previously part of the informal “Green Series” of the Ciechocinek Formation. It represents a prodelta to brackish- marine depositional system in the eastern North German Basin (NGB), shaped by sea-level changes and the Toarcian anoxic event.

History

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. From 1959 to 1995, the Grimmen clay pit was excavated, revealing a glacially dislocated raft of Liassic clays and sands deformed by Pleistocene ice advances. 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. The clay pit, abandoned and water-filled since 1995, was established as the type section in 2025, with 2016 and 2020 excavations refining an 18 m composite log spanning the Pliensbachian-Toarcian boundary to the elegans Subzone.

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. The type section in the Grimmen clay pit forms an 18 m composite log, transitioning from Pliensbachian sandy deposits of the Wolgast Formation to Toarcian organo-detrital clays and heteroliths of the Lehmhagen Member (Posidonia Shale Formation), followed by the Reinberg Member of the Grimmen Formation. The Reinberg Member (~10 m thick) starts with bluish clays, grading to greenish, pellet-laminated clays with exaratum and lower elegans marker beds (marly limestones with fecal pellets). 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. The upper Grimmen Formation features greenish clays with thin silt/sand laminae in 8–15 m symmetric cycles, capped by the Glashütte Formation’s deltaic sandstones. Heavy minerals (zircon, rutile, tourmaline) and smectite-group clay minerals indicate volcanogenic input, while kaolinite and chlorite are common. 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 <1 wt% upsection, indicating a shift to dysoxic conditions. Basal bluish clays (exaratum Subzone) represent a restricted marine basin, followed by greenish clays with pellet laminae, suggesting increased fluvial input and suspension-load plumes during wetter phases. The coarsening-upward trend and onset of bioturbation (pyrite-filled burrows) reflect prodelta progradation from Fennoscandia, culminating in the Glashütte Formation’s deltaic systems. High kaolinite and smectite content indicate a warm, humid climate with biochemical weathering and volcanogenic influence, akin to modern tropical settings. The absence of major ripples suggests minimal wave action, with sedimentation driven by fluvial input and storm events. The formation extends from western Poland to the Ringkøbing-Fyn-Møn-Arkona High, interfingering with the Ciechocinek Formation in the east. The Grimmen Formation’s biostratigraphy is primarily based on ammonite zones, with the falciferum Zone (elegantulum, exaratum, elegans, falciferum subzones) for the lower Toarcian. The type section spans the Pliensbachian-Toarcian boundary (tenuicostatum Subzone) to the elegans Subzone, with key fossils preserved in concretions and clays.Ammonites define subzones, with ''Dactylioceras semicelatum'' and possible ''D. tenuicostatum'' at the boundary, followed by '' Lobolytoceras siemensi'' and '' Tiltoniceras antiquum'' in the Lehmhagen Member (semicelatum Subzone). Elegantulum concretions yield abundant '' Eleganticeras elegantulum'' and rare '' Hildaites murleyi'', while exaratum concretions in the Reinberg Member contain '' Cleviceras exaratum'', '' Phylloceras heterophyllum'', and ''Lytoceras crenatum''. Belemnites ('' Passaloteuthis bisulcata'') are common in the semicelatum Subzone but absent from the elegantulum Subzone due to anoxia. Teuthoids and coleoids occur in elegantulum and exaratum concretions.

Holoplanktonic gastropod ''Coelodiscus minutus'' and larval ''‘Inoceramus’ dubius'' are abundant, with adults in elegantulum and exaratum concretions, often in regurgitalite