Toarcian black shales in the dutch central graben: Record of energetic, variable depositional conditions during an oceanic anoxic event

Abstract

The environmental conditions, mechanisms, and processes that resulted in the deposition of organic-matter-rich sediments during the early Toarcian oceanic anoxic event are still a matter of discussion. A petrographic and geochemical study was carried out using Posidonia Shale Formation (lower Toarcian) black shale samples from two wells located offshore The Netherlands in the Dutch Central Graben. This formation is a lateral equivalent of Toarcian black shale successions in northern Europe. The lower Toarcian black shales in the Dutch Central Graben exhibit a variety of depositional fabrics, sedimentary structures, and textures that indicate dynamic energetic conditions at the time of their deposition and appear to have been deposited mostly by bottom currents rather than settling from pelagic suspension. The Posidonia Shale Formation black shales are characterized by normally graded thin beds (, 10 mm thick) with erosional bases wherein cross-lamination is very common. The tops of these thin beds are commonly bioturbated. These observations are in contrast with the traditional interpretation of the deposition of lower Toarcian black shales as having taken place under a stagnant, anoxic water column via suspension settling. Black shales are commonly more heterogeneous than normally assumed and are the product of diverse sedimentary processes. Geochemical results, in combination with petrography, suggest that productivity and high accumulation rates of reactive organic matter were behind the establishment of sediment anoxia. The most organic-matter-rich shales, which are characterized by redox-element anomalies, are cross-laminated, thin-bedded shales, and each thin bed was deposited quickly, thus favoring the preservation of organic matter. The petrographic study of black shales complements geochemical data because geochemical analysis of sedimentary rocks, however high-resolution, always represents an averaging of environmental conditions at the time of deposition over at least several hundreds of years.