Life in a Gilbert-type delta system: Ichnoassociations of the Ventimiglia palaeovalley and their sequence stratigraphic significance (Pliocene, NW Italy)

Abstract

Gilbert-type deltas are inherently associated with stressful conditions for the benthic infauna, i.e. highly shifting substrates, coarse-grained sediments, traction sedimentation, supercritical flows, and salinity fluctuations. Nevertheless, trace fossils are locally abundant in the Gilbert-type delta system of Ventimiglia (NW Italy), representing the Pliocene infill of an incised valley that developed during the Messinian sea-level fall. The seismic-scale outcrops of the Ventimiglia delta system allow to link ichnological features (ichnotaxa, bioturbation intensity) to the major depositional environments of the deltaic system. Accordingly, rhizoliths characterize the delta plain, whereas delta top deposits are virtually unbioturbated. The delta slope is characterized by Planolites, Thalassinoides, Scolicia-like burrows, and Ophiomorpha. Proximal prodelta deposits show a characteristic Skolithos-Chondrites ichnoassociation, whereas Chondrites and sharp-walled burrows (Thalassinoides?) dominate the distal prodelta. Reworked beach deposits occasionally present bioerosional ichnotaxa (Entobia, Gastrochaenolites, and Maeandropolydora), which suggest the presence of a nearby rocky coast. Overall, bioturbation intensity increases with increasing distance from the delta-top setting, whereas ichnodiversity has its maximum in delta-toe and prodelta units. Moreover, a clear sequence-stratigraphic signature is observed for the Ventimiglia ichnoassociations. The ichnological evolution of the Ventimiglia delta system, with bioturbation intensity increasing in forced-regressive systems tracts of successive sequences, is here hypothesized as a signature of overfilled incised valleys. The Skolithos-Chondrites ichnoassociation is especially common in the highstand systems tract, possibly reflecting higher amounts of suspended matter during the sedimentation of the highstand systems tract itself. The basal surface of forced regression usually preserves passively filled burrows with a sharp unlined margin, which fits well with a sediment-starved seafloor.