Facies analysis and formation of insect-bearing marine black shales from the Lower Jurassic of Schandelah (Lower Saxony/Germany)

Black shales are often key archives of past life, thanks to the preservation of organogenic tissues in the absence of oxygen. However, various taphonomic constraints and processes can strongly alter the fossil assemblages and may lead to false palaeoecological conclusions. This talk discusses insect preservation in the Lower Jurassic Posidonia Shale of Schandelah (NW Germany), a window into Mesozoic entomofaunas of central Pangaea. For this purpose, we present a cm-scale analysis of lithofacies and fossil content of a c. 2 m thick and 7 m wide excavation section, stretching the siemensi- to borealis-ammonoid subzones. Emphasise is put on dm-sized polished slabs from intercalated early diagenetic calcite concretions. Consistent with previous studies, the marly strata formed in a fully marine setting close to small islands with limited fluvial input. However, water stagnation and depth seem to have increased over time, given the profile-up decrease in grain size, extent in bioturbation and abundance of macrobenthos in the peloidal concretions. This trend improved the preservation of insects sunken to the sea floor, as reflected by their taphonomy and occurrence: insects are absent in the lowermost siemensi concretions, while resistant coleopteran remains rarely occur in the capillatum and elegantulum concretions. In the uppermost borealis concretions, insects are most abundant, diverse and dominated by isolated wings. Increasing annihilation of anatomical detail in the insect fossils towards the concretion margins attest to chitin dissolution during diagenesis. These results call for more bed-by-bed documentations of fossil content and preservation, and disclose ways to reconstruct the Posidonia-Shale environment.