The Bascharage Konservat Lagerstätte in Luxembourg: stratigraphy and palaeontology of a Lower Toarcian near-shore environment

Exceptionally preserved fossils from the Toarcian (Lower Jurassic) black shales of Bascharage in southeast Luxembourg have been known for decades, but stratigraphy and palaeontological implications of these deposits have remained poorly understood. Recent attempts to describe the litho- and biostratigraphy of these black shales have yielded controversial or ambiguous results. Here, we present and discuss new data collected in May 2022 during a large-scale scientific excavation at the Bommelscheuer industrial zone in Bascharage, a campaign that was led by the National Museum of Natural History Luxembourg. A previously unexplored area of 40 m x 80 m was decorticated layer by layer, with the aim to expose a continuous section and record a maximum of geological data. We provide here a high-resolution litho-, bio-and chemostratigraphic survey and assessment of the Bascharage succession, comprising the upper Tenuicostatum and lower Serpentinum Zones and therewith the period before and during the initiation of the Toarcian oceanic anoxic event (T-OAE). This critical endeavour enables stratigraphic correlations with coeval successions, in particular with the SW-German Posidonia Shale. The high-resolution bulk carbonate carbon isotope record shows the four-step negative excursion in the initial T-OAE reported for coeval successions in the Paris and the Swabian basins. The course of the delta C-13(carb) trend suggests that the investigated site at Bascharage represents a largely continuous and complete T-OAE succession. It furthermore suggests that the base of the Serpentinum Zone coincides with the 2nd step of the negative carbon isotope excursion, the latter therefore representing an important chemostratigraphic marker. The nodule layer - called the insect nodule horizon in the present paper - may represent the stratigraphic equivalent to the SW-German "Unterer Stein" and with that also to the "Whale Stones" of Yorkshire, providing therefore a lithological reference horizon across Europe. Palaeontological evidence suggests that the Bascharage site can be qualified as a Lagerstatte because of the abundance and extraordinary preservation of the fossils, including 3D and soft tissue preservation. The biotic spectrum shows a combination of features typically found in near-shore settings. Due to the abundance and excellent preservation of insect fossils, the Bascharage Lagerst & auml;tte ranks among the most important Lower Toarcian insect taphocoenoses in Europe. This succession was deposited in a marginal near-shore setting close to the emerged London-Brabant landmass but still within the Central European areas of black shale depositions. Seawater current indicators based on 32 occurrences of wood fragment accumulations around ammonite shells recorded in the lower 30 cm of the Serpentinum Zone and three wood logs recorded at different levels