Organo- and lithofacies variations of the Pliensbachian-Toarcian (Early Jurassic) sedimentary succession in the Hils Synline, Lower Saxony Basin: Implication on paleoenvironmental reconstruction

The widespread deposition of Lower Toarcian (Early Jurassic) black shales throughout NW Europe has often been associated with a global oceanic anoxia event, the Toarcian Oceanic Anoxic Event (T-OAE). However, sedimentary variations within the European sub-basins suggest a significant influence of regional effects, arousing interest in investigating regional variations in the NW European basin system. At the Hils syncline (Lower Saxony Basin), black shales of the Posdionienschiefer Formation have been studied extensively in recent decades, mainly focusing on the effect of thermal maturation on organic-rich sediments. However, under- and overlaying organic-lean clay- and mudstones are sparsely studied in this region.

This study investigates a continuous profile of Upper Pliensbachian to Upper Toarcian/Lower Aalenian deposits by combining samples from two boreholes from the Hils syncline, Wickensen and BO2.0, to establish a regional model for the deposition of Lower Toarcian black shales in this region. Nearly 200 samples were analyzed using a multi-disciplinary approach, including X-Ray fluorescence, C_org isotopic composition, Rock-Eval pyrolysis, and biomarker analysis.

The geochemical results suggest that the development of anoxia was closely linked to sea level and associated water circulation variations. Upper Pliensbachian and lowermost Toarcian sediments were deposited under oxic conditions associated with increased terrestrial influx and a low sea level. The overlying black shales (falciferum zone) are characterized by prevailing anoxia and photic zone anoxia and decreased terrestrial input, indicating a sea level rise. For the overlying Upper Toarcian Jurensismergel shales, ongoing sea-level rise is suggested, promoting less restricted water circulation and dysoxic conditions.