Organic geochemistry and depositional history of the Barremian–Aptian boundary interval in the Lower Saxony Basin, northern Germany

Abstract

The Barremian–Aptian boundary interval in northern Germany is characterized by a major lithologic turnover from dark coloured, organic matter-rich towards light-coloured, organic matter-lean sedimentary rocks. To derive information on the respective environmental conditions and their development through time, organic matter from this interval was studied by petrographic and geochemical methods. The study was performed on five cored boreholes each recovering the Barremian–Aptian boundary interval, and also on two research wells in the same area covering the Late Albian for comparative purpose.The geochemical and petrographical data allow the differentiation to two organofacies. The lower succession is characterized by variable organic carbon contents which reach maximum values up to 7% Corgin the paper shale units (‘Blättertone’) and in the early Aptian Fish Shale (‘Fischschiefer’; a possible product/equivalent of the Oceanic Anoxic Event 1a). Petrographically, the organic matter is mainly alginite and bituminite. It is moderately well preserved according to Hydrogen Index (HI) values between 150 and 450 mg HC/g Corgfor most of the samples. Then-alkane distribution reveals a contribution of both marine algae and terrestrial higher land plants. To explain these analytical results, a depositional model has been developed which is based on limited water exchange with the adjacent oceans having caused oxygen deficiency and deposition of numerous organic carbon-rich sediment layers. As revealed by HI values above 300 mg HC/g Corgand by ann-alkane maximum atn-C15, the organic matter in the Fischschiefer is mostly of marine origin. Furthermore, the 28,30-dinor-hopane detected in only one sample probably indicates slight variation in redox conditions during Fischschiefer deposition.The major change of environmental conditions began before Fischschiefer deposition, i.e., in the earliest Aptian. A decrease in organic carbon content and HI values and a higher ratio of reworked vitrinite and inertinite over primary vitrinite indicate gradually changing conditions. The observed environmental change is related to the Aptian transgression leading to a clear domination of terrigenous organic matter in the sediment above the Fischschiefer (HedbergellaMarl and above). Completely different oceanic conditions existed in the Lower Saxony Basin from then on until the Late Cretaceous, when organic matter-poor sediments were deposited under oxygen-rich water.