Patterns and processes of sediment dispersal on the continental slope off Nice, SE France

Abstract

The distribution of surficial sediments and sediment dispersion patterns on the steep continental slope off Nice (SE France) have been studied using side-scan sonar, 3.5 kHz profiles, short piston cores and bottom photographs. The input of terrigenous material to the Baie des Anges, a submarine embayment bounded by two prominent ridges, is dominated by fluvial input from the Var River, the Paillon River being only a minor source. The Var River provides very coarse bedload material (gravel and cobble) directly to the head of the Var Canyon. Gravel and cobble deposits are found all along the Var Canyon and the Upper Fan Valley of the Var submarine fan and have been shaped into gravel waves. The fine particles (suspension load) are separated from the coarse bedload upon entering the sea and form up to 60-m thick deposits on the uppermost continental slope of the Baie des Anges. These deposits are formed by settling out of sediment plumes. The presence of silt and fine sand laminae that decrease in thickness and frequency away from the Var River mouth indicate the influence of meso- and hyperpycnal flows on these plume deposits. Areas outside the Baie des Anges are not connected to major fluvial input and receive only hemipelagic sediments. These primary deposits are highly unstable and sediment failure due to seismic loading, sedimentary loading or undercutting is frequent. Sediment failure produces secondary sediment gravity flows that export most of the material to the basin, but also produce turbidity-current over-spill deposits on ridges bounding the slope canyons and on terraces within the Var Canyon. Slump and debris-flow deposits are also observed. At least some of these secondary flows erode the continental slope as cross-cutting chutes on the upper continental slope and erosional scours in the Upper Fan Valley demonstrate. Modern sediment dispersal patterns on the continental slope off Nice are proposed as a modern analogue to lowstand conditions on continental margins. In fact, the absence of a continental shelf together with a steep slope strongly reduces the influence of sealevel on the physiography of the margin.