Gas-hydrate occurrences along the Chilean margin have been widely documented, but the processes associated with fluid escapes caused by the dissociation of gas hydrates are still unknown. We report a seabed morphology growth related to fluid migration offshore Lebu associated with mud cones by analysing oxygen and deuterium stable water isotopes in pore water, bathymetric, biological and sedimentological data. A relief was observed at − 127 m water depth with five peaks. Enrichment values of δ18O (0.0–1.8‰) and δD (0.0–5.6‰) evidenced past hydrate melting. The orientation of the relief could be associated with faults and fractures, which constitute pathways for fluid migration. The benthic foraminifera observed can be associated with cold seep areas. We model that the mud cones correspond to mud growing processes related to past gas-hydrate dissociation. The integration of (i) the seismic data analysis performed in the surrounding area, (ii) the orientation of our studied relief, (iii) the infaunal foraminifera observed, (iv) the grain size and (v) the total organic matter and isotope values revealed that this area was formerly characterised by the presence of gas hydrates. Hence, this part of the Chilean margin represents a suitable area for investigating fluid-migration processes.

Modern fish are able to produce a plethora of different traces (both bioturbation and bioerosion structures) according to several behaviours, yet only five ichnotaxa have been interpreted as produced by the activity of fish in the fossil record. Many taphonomic factors may favour the non-fossilization of many of these traces and, even fossilized, they could have been misinterpreted.In this contribution, shallow and bilobed traces produced by the feeding activity of the perciform fish Diplodus vulgaris (Sparidae) in the estuary of the PiedrasRiver (Lepe, Huelva, SW Spain) are described. Neoichnological study and comparison of these bioturbation structures with the fossil record allow associating them as Cruziana-and Rusophycus-like traces, i.e. traces with features very similar to those of such ichnogenera. Since these ichnotaxa have been commonly interpreted as the result of the locomotion and resting of different kinds of invertebrates, in order to get a better understanding of the marine and continental fossil record, we also propose taking into account fish as potential producers of these kind of traces in future paleoichnological studies.

No abstract