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A new scenario for the Domerian - Toarcian transition

¹ Institut de Géologie et Paléontologie, Université de Lausanne, BFSH-2, CH-1015 Lausanne, Suisse
² Université Paris VI, CNRS-FRE 2400, 4 Pl. Jussieu, 75352 Paris cedex 05, France
³ Shell International Exploration and Production, La Haye, Hollande
⁴ Laboratoire de Géologie, CNRS-FRE 2400, Muséum National d’Histoire Naturelle, 43 rue Buffon, F-75005 Paris, France

Abstract

In contrast to the majority of recently published hypotheses, we believe that the main trigger for early Toarcian anoxia is neither increased primary productivity during the Tenuicostatum and Falciferum Zones nor sudden methane hydrate degassing close to the transition between these two zones.

In our opinion, this peculiar paleoceanographic episode is linked to a major, though short-lived, regression at the end of Upper Domerian. Sea-level fall resulted from sudden cooling due to increased volcanic activity. This generated global thermal insulation and subsequent glaciation. The regression is responsible for a major hiatus over NW-European epicontinental seas and is later followed by the well-known Lower Toarcian transgression. The interval corresponding to this hiatus allowed vegetation to colonise vast newly emerged surfaces. The leaching and drowning of the accumulated organohumic matter then triggered the anoxic cycle at the transgressive maximum, concomitant with a global warming.

Key Words: Upper Pliensbachian • Domerian • Toarcian • Anoxic event • Stratigraphic gap