Black shale deposition during the Early Jurassic: Geochemistry of Pliensbachian and Toarcian sedimentary rocks of the Hunzen Well, Hils Syncline, Northwest German Basin

Abstract The Toarcian stage of the Early Jurassic witnessed warm global temperatures, oceanic anoxia (Toarcian Oceanic Anoxic Events) and extensive deposition of organic‐rich black shales. While black shale deposition in the Southwest German Basin on the Northwest Tethyan shelf has been well‐studied, the palaeodepositional conditions in the Northwest German Basin remain poorly constrained. The present study aims to bridge that gap by examining a completely cored well covering Upper Pliensbachian to Upper Toarcian sediments in the Hils Syncline. Inorganic and organic geochemical, as well as biostratigraphic information, carbon isotope and organic petrological results are interpreted to reconstruct depositional conditions and organic matter accumulation in a shallow Jurassic Sea. The study results demonstrate a Late Pliensbachian regression leading to the deposition of claystone in shallow water under humid conditions with a high terrigenous and freshwater influx, and well‐oxygenated bottom water environments. A transgression in the Early Toarcian initiated the deposition of laminated and organic matter‐rich calcareous black shale, the so‐called Posidonia Shale. This climatic shift caused deepening of the water column, resulting in salinity stratification and bottom water anoxia that favoured organic matter preservation. Black shale deposition was probably triggered by low oxygen solubility in warm waters and intensified monsoonal rainfall. The lowermost part of the Posidonia Shale (basal Unit I) represents the time of most intense carbonate deposition and strictly anoxic bottom water. The middle part (Unit II) is characterised by periodical, short‐term oxygenation of the sea floor, while the thick upper part (Unit III) represents a time of re‐establishment of anoxic conditions. In the Late Toarcian and Aalenian, bottom water reoxygenation marked the deposition of claystones with moderate contents of marine organic matter, above a decimetre‐thick breccia. The Hunzen section thus offers detailed insights into late Early to Middle Jurassic palaeodepositional changes in central Europe.