Stratigraphy and palaeoecology of the Toarcian in NE Germany: organo-detrital and detrital sedimentation in response to the productivity of the planktic ecosystem

Abstract

The Toarcian Posidonia Shale and contemporaneously formed sediments document an interval of black shale sedimentation and organic matter accumulation in the Central European Basin System (CEBS), lasting a few hundred thousand years up to 7 myr. The remarkable stable organo-detrital depositional regime in the deeper basin was accompanied by repetitive changes from organo-detrital to detrital depositional regimes in the shallower eastern part of the North German Basin (NGB). Here, we present the first detailed study on the marginal Toarcian strata in NE Germany integrating palaeontological, stratigraphical and geochemical methods. The revision of ammonite data and new records contribute to an improved biostratigraphic control of transgressive–regressive sequences, carbon isotope excursions and palaeoecology. Proximal–distal correlations show two transgressive–regressive couplets of marine black shales and brackish green clays, which evolved in response to third-order sea-level fluctuations. Marine maxima in the lower falciferum and upper bifrons zones are correlated with basin-wide deposition of Posidonia Shale-type black shales. These black shales, particularly in basinal settings, show elevated total organic carbon (TOC) and carbonate contents, and are dominantly formed of zooplanktic Coelodiscus and ‘ Inoceramus’ dubius produced fecal pellets, pelitic detritus and organic matter, mainly entrapped in fecal pellets. The change to organo-detrital black shales coincides with the onset and core of the Toarcian-Carbon Isotope Excursion (δ 13 C falling limb and valley) in the semicelatum to elegantulum subzones, whereas the recovery (δ 13 C rising limb) in the exaratum Subzone is recorded in progradational green clays of the Grimmen Formation (former ‘Grüne Serie’). Early diagenetic concretions represent marine Konservatlagerstätten with exceptional fossil preservation and give insights into contrasting planktic ecosystems. The basic trophic levels of the Posidonia Shale Sea are represented by calcareous nannoplankton, organic-walled phytoplankton and zooplankton groups. Accelerated productivity of this planktic factory triggered nektic adaption in fishes and ammonites and resulted in increased export rates of organic matter and carbonate particles (pellet rain). The accumulation of biogenic pellet ooze in a low energy and oxygen-depleated soupy mud at the seafloor, preserved in pre-compactional carbonate concretions, gave rise to elevated TOC and carbonate contents of the Posidonia Shale. During sea-level falls and lowstands, progradational coasts increasingly supplied detritus and freshwater to the eastern NGB. This contributed to the decline of the Posidonia Shale plankton community and proliferation of the green algae-dominated community of the Grimmen Formation. The independent development of the eastern NGB was amplified by swells limiting the exchange with the western NGB, where black shales occur up to the aalensis Zone in places.

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