Palaeoceanography of Lower Cretaceous (Barremian–Lower Aptian) black shales from northwest Germany evidenced by calcareous nannofossils and geochemistry

Abstract

During the Barremian–Early Aptian, the Lower Saxony Basin (LSB) of northwest Germany was covered by a shallow epicontinental sea where restricted conditions resulted in the formation of black shales. This study focuses on a 103-m-thick succession of Lower Barremian–Lower Aptian dark claystones and black shales exposed in a temporary road cut in northwest Germany (A39). Calcareous nannofossils and the δ13Corg record have been used to determine the timing of black shale deposition (Hauptblätterton, Blätterton, Fischschiefer events). Redox-sensitive trace elements indicate anoxic conditions with free H2S causing the formation of black shales and dysoxic conditions during deposition of the dark claystones. The palaeoecological reconstructions based on calcareous nannofossils suggest that during the Lower Barremian Hauptblätterton event a stratification of water masses existed, caused by high surface water temperatures and reduced terrigenous run-off. During the deposition of the Upper Barremian clay-Blätterton alternation warm surface water prevailed, accompanied by meso-eutrophic surface water conditions. The formation of the Barremian black shales was mainly caused by restricted conditions related to the palaeogeography and palaeoceanography of the LSB. Global processes associated with the Oceanic Anoxic Event 1a most likely induced the deposition of the Lower Aptian Fischschiefer. High weathering rates, high primary production, and restricted conditions in the LSB led to increased burial of organic matter and thereby to the formation of the Fischschiefer.