Toarcian Black Shales in the Dutch Central Graben: Energetic, Variable Depositional Conditions during an Oceanic Anoxic Event

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 mostly deposited by bottom currents rather than settling from pelagic suspension. These observations are in contrast with the traditional interpretation of the deposition of these black shales as having taken place under a stagnant, anoxic water column by suspension settling. 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 favouring the preservation of organic matter.The results highlight the need for more detailed sedimentological studies of black shales and underline the importance of multidisciplinary investigation of these lithologies.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California