Formation of the Late Aptian Niveau Fallot black shales in the Vocontian Basin (SE France): evidence from foraminifera, palynomorphs, and stable isotopes

Abstract

We present paleoceanographic models for the formation of the marlstone facies and the most prominent black shale intervals of the Late Aptian Niveau Fallot black shale succession from the Vocontian Basin (SE France). In the lower part of the succession, the composition of benthic foraminiferal assemblages of the Niveau Fallot 2′ and 2″ black shales suggests an enhanced burial of organic matter due to more eutrophic conditions and resulting low oxygen conditions at the seafloor. In the upper part of the succession (including Niveau Fallot 3 and 4), a third-order sea-level fall, indicated by a decrease in the stable carbon isotope values, may have resulted in a reduced water mass exchange between the Vocontian Basin and the western Tethyan Ocean. This may have led to dysoxic conditions at the seafloor of the Vocontian Basin and may have favored the formation of organic-rich sediments as shown by benthic foraminiferal assemblages. Decreased evaporation, however, is proposed as the most important mechanism causing a restriction of deep water formation within the basin which finally led to the formation of the Niveau Fallot 3 and 4 black shales. Based on our investigations, the formation of the Niveau Fallot black shales was caused by different factors, the most prominent ones include sea-level fluctuations, increasing productivity, and changes in precipitation and evaporation rates. Furthermore, Niveau Fallot 3 can probably be correlated with the Thalmann black shale event in California and a dark horizon at the Mazagan Plateau (DSDP Site 545).