Facies and processes in a Gilbert-delta-filled incised valley (Pliocene of Ventimiglia, NW Italy)

The incised valley of Ventimiglia, located along the Ligurian coast (NW Italy), was cut by deep river erosion during the Messinian sea-level fall and is connected seawards to a slope canyon. During Pliocene, the valley was flooded by the sea and transformed into a coastal embayment or ria. The infill sequence of the incised valley is up to 500 m thick. The paleovalley floor is locally paved by thin remnants of subaerial scree deposits, abruptly overlain by up to 150 m thick bathyal marls, above which a number of stacked prograding conglomerate Gilbert-type deltas constitute most of the valley fill. Gilbert deltas present 15°–25° dipping clinoforms, 50 to 250 m thick, and are alternated with up to 20–30 m thick marls intervals.This unusual character of incised-valley-fill sequence, can be accounted for by the rapid and high-amplitude eustatic sea-level rise that followed the Messinian event, and by the progradation occurring on a narrow and steep-gradient shelf, tectonically controlled by the tilting and collapse of the margin. High and coarse sediment supply was provided by the uplifting Alpine chain.A remarkable analogy in facies patterns and depositional setting is observed with the high-latitude Holocene fan-delta systems described by Prior and Bornhold [Prior, D.B., Bornhold, B.D., 1990. The underwater development of Holocene fan deltas. In: Colella, A., Prior, D.B. (Eds.), Coarse-Grained Deltas: Spec. Publs. Int. Ass. Sedim., vol. 10, pp. 75–90.] in the fjords of the British Columbia. Both examples are characterized by high rate of sea-level rise after the entrenchment stage, and predominance of mass-flow processes and debris-avalanching in the first stage of progradation, followed, in the later stages of delta progradation, by deposition of better-organized and stratified foreset beds on the delta slope predominantly by inertia and turbidity flows.A large facies variability is observed in the Gilbert-type delta complex, recording deposition under a wide range of physical conditions, both in individual and successive wedges. Long-term evolution of the valley fill shows a general trend from deep-water to shallow-water deltas and from fluvial-dominated to wave-influenced deltas.