PLIOPLEISTOCENE EVOLUTION OF THE VAR DEEP-SEA FAN OFF THE FRENCH RIVIERA

The Var deep-sea fan was deposited during the Pliocene and Quaternary seaward progradation of the Var delta within a steep-sided, flat-floor ed basin formed during the Messinian salinity crisis. The prominent pa laeo-Var canyon was cut on the slope at the same time. Following the E arly Pliocene transgression, Var sediments were initially trapped in t he coastal ria formed by sea-level incursion of the head of the Messin ian valley. By Middle Pliocene time Gilbert-type deltas had prograded to the break of slope (modern coastline) allowing coarse sediment to r each 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. T hrough the rest of the Pliocene, a leveed fan valley extended southwar d from the Var canyon, which periodically switched locations through a vulsion to the east. At about the beginning of the Quaternary, this ea stward spillover channel became the main distributary and re-occupatio n of the older south-trending channels was blocked by rapid levee prog radation and aggradation of mud waves by overbanking of turbidity flow s on the Var sedimentary ridge. During the Quaternary, channel shiftin g 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 (w ith lower levees) in most passive margin fans. This is a consequence o f the constant supply of coarse sand, gravel and pebbles to the fan, e ven during highstands of sea level. The erosive capability of this coa rse sediment in ignitive turbidity currents resulted in flushing out o f the upper and middle fan valley system. Thick muddy turbidity curren ts, probably resulting from slumping caused by canyon incision, produc ed the prominent mud waves on the levee. The concave longitudinal prof ile 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.