Ichnological analysis at the Fonte Coberta section (Lusitanian Basin, Portugal): Approaching depositional environment during the Toarcian oceanic anoxic event (T-OAE)

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE) is commonly related with an extended marine mass extinction and global oceanic anoxic conditions. However, recently it has been demonstrated that local palaeotopography/palaeogeography can determinate the absence of anoxic conditions, being other limiting environmental parameters of higher signifi cance. The detailed study of the Fonte Coberta section (Lusitanian Basin, Portugal) shows significant changes in the macrobenthic community at the lower Toarcian (Levisoni Zone) sediments during the event. In the study section, this interval presents a high degree of bioturbation with a relatively abundant and moderately diverse trace fossil assemblage dominated by Thalassinoides, with punctual Spongeliomorpha. The ichnotaxonomical study conducted on this trace fossil assemblage has allowed the recognition of a single species of Thalassinoides, T. suevicus Rieth, with two different morphotypes (I and II), as well the differentiation of Spongeliomorpha aff . iberica Saporta. The distribution of these ichnospecies, including the differentiated morphogroups, along the studied interval, corresponding to the T-OAE, allows refining depositional conditions as changes in the substrate consistency related to variations in relative sea level. Ichnological analysis reveals as a proxy to improve sequence stratigraphy characterization for the Lusitanian Basin during the T-OAE.