Plio-Pleistocene evolution of the Var deep-sea fan off the French Riviera

Abstract

The Var deep-sea fan was deposited during the Pliocene and Quaternary seaward progradation of the Var delta within a steep-sided, flat-floored basin formed during the Messinian salinity crisis. The prominent palaeo-Var canyon was cut on the slope at the same time. Following the Early Pliocene transgression, Var sediments were initially trapped in the coastal ria formed by sea-level incursion of the head of the Messinian valley. By Middle Pliocene time Gilbert-type deltas had prograded to the break of slope (modern coastline) allowing coarse sediment to reach the basin. Quaternary coastal uplift and sediment supply from the glaciated Alps resulted in a supply of coarse sediment to the fan by the braided Var River throughout the Quaternary, even during sea-level highstands such as the present. In the earliest Pliocene, hemipelagic clays accumulated while coarse sediment was trapped in the Var ria. Through the rest of the Pliocene, a leveed fan valley extended southward from the Var canyon, which periodically switched locations through avulsion to the east. At about the beginning of the Quaternary, this eastward spillover channel became the main distributary and re-occupation of the older south-trending channels was blocked by rapid levee progradation and aggradation of mud waves by overbanking of turbidity flows on the Var sedimentary ridge. During the Quaternary, channel shifting to the north occurred. The Quaternary Var fan has an unusually steep concave longitudinal profile compared with most passive margin fans. The Laurentian fan has a similar profile to the Var; both have a high levee and a gravel and pebble-floored fan valley that is essentially a bypass zone, compared with the aggradational channel-levee systems (with lower levees) in most passive margin fans. This is a consequence of the constant supply of coarse sand, gravel and pebbles to the fan, even during highstands of sea level. The erosive capability of this coarse sediment in ignitive turbidity currents resulted in flushing out of the upper and middle fan valley system. Thick muddy turbidity currents, probably resulting from slumping caused by canyon incision, produced the prominent mud waves on the levee. The concave longitudinal profile led to rapid sediment deposition at critical points in the valley system as a result of turbidity flow expansion; this in turn promoted the development of a left hook in the valley system.