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 sealevel 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 massflow 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.