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Apparent sea-level change at times of extraordinary sediment supply: integrated facies analysis and chemostratigraphy of Early Toarcian Oceanic Anoxic Event, Peniche, Portugal

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Hesselbo S. , Jenkyns H. , Duarte L. , Veiga de Oliviera L.

tom Vol. 4, no. 1

171-172

The Toarcian Oceanic Anoxic Event in the Early Jurassic was a time of major environmental change characterized by widespread deposition of organic-rich black shales in both deep- and shallow-marine settings. Facies evidence, mainly from the European area, has been used to argue for a major sea-level rise in concert with oceanic anoxia, extinction of certain groups, and perturbation of the carbon cycle. However, in a number of locations, such as in the Lusitanian Basin, Portugal, it has also been argued previously that the start of the event was characterized by sea-level fall rather than rise, as evidenced by the initiation of gravity-flow deposition. In this study we combine new marine and terrestrial carbon-isotope and lithological data to suggest an alternative model for development of facies patterns in this basin, whereby an abrupt increase in sediment flux from the hinterland led to slope instability and gravity-flow deposition that lasted as long as the carbon-cycle perturbation. This interpretation is supported by existing osmium and strontium isotope data, which have been interpreted to indicate a significant continental transient increase in continental weathering fluxes. In effect this event provides a good example from the rock record where the assumption of constant sediment supply is demonstrably false, but in which the facies patterns may still be predictable on a variety of scales.