Incidence of bioturbation on grain size redistributionPreventing misinterpretations in sedimentary analysis

Résumé

Bioturbational disturbance by trace fossil is revealed as a significant source of vertical redistribution of sediment particles, and then a potential cause of misinterpretations in sedimentary analysis. Core SEL08-02 from A Selva, NW of the Iberian Peninsula, is being studied following a multidisciplinary approach, including ichnological analysis, as a tool to interpret Mediterranean Outflow Water dynamics and Contourite Depositional System phenomena in the Galicia continental margin. Two main parts are determined, a sandy one in the upper part and a muddy part below. High-resolution digital images and Computed Tomography images were treated for trace fossil analysis. Zoophycos structures have been observed mainly localized at 35-50 cm below the upper surface, showing infilling material consisting of alternating host sediment and sediment similar in color to that from the upper part. The interval with concentration of Zoophycos is characterized by coarse sediments, similar sand percentage in sand to that in the upper part of the core, which could be interpreted as originated by similar sedimentary processes. However, in this case, increase in grain size is not produced by primary depositional processes but it is the consequence of secondary redistribution of particles by Zoophycos tracemaker.