present sedimentary prism dates from 6000 B.P when the sea-level was nearly stabilised. This prism is divided into three different sedimentological provinces (Aloisi and Monaco, 1975; Got et al.,
Littoral sands, or the inner shelf, supplied by
rivers and shore erosion.
b) The shelf-mud belt, with a location at the mid- dle shelf due to interaction between river input and prevailing long shore currents. In addition to this mud belt, in front of delta rivers a “pro-delta” is caused by deposition of muddy sediments with a laminated structure, alternating sandy and silty sed- iments that are rich in gas.
c) Relict sands at the outer shelf, deposited dur- ing the periods of low sea-level. In these zones the presence of relief is common due to erosion process- es with cemented sand and gravels and sedimentary structures (sand-dunes, sand-waves…) produced by bottom currents.
This pattern of the distribution of shelf deposits according to the “sedimentary prism” model is con- sistent with the whole main river-influenced conti- nental shelf mentioned above. The “sedimentary prism” model also emphasises two processes of sed- iment distribution: I) the role of sea-level change; and II) the relationship between residual currents and deposition of fine-size sediments as the “mud belt”. The cyclic changes of climate between cold and warm periods and the subsequent sea-level changes greatly affect the continental shelf deposi- tion. During the last low sea-level stage, 100 m below the present sea-level 18000 years BP, the outer continental shelf was subaerially exposed and eroded and the sediment inputs were deposited directly within the upper slope and submarine canyons. The sea-level rise until the present high sea-level stage, 6000 years BP, led to a preferential deposition of fine river sediments in the coastal and inner shelf zones, and with the decreasing rate of sea-level rise the sediments were trapped on deltas and the continental shelf and the “sedimentary prism” was established.
After the works of Aloïsi et al. (1975, 1979, 1980, 1982 and 1986), the model of a system of sev- eral nepheloid layers (turbid layers with higher con- centrations of suspended particulate matter) was accepted. The importance and distribution of the nepheloid layers (surface, intermediate and bottom) are related to the hydrologic structure of the water column and the supply of terrigenous sediments. The bottom nepheloid layer (BNL) shows a 3D
10 F. SARDÀ et al.
internal structure, a vertical stratification with increasing concentrations from the upper limit to the base and a seaward decreasing grain size. This bot- tom nepheloid layer can connect the inner shelf to the submarine canyon heads and the shelfbreak (Aloïsi et al., 1982; Calafat, 1993; Puig and Palan- ques, 1999). The BNL at the coastal and inner shelf zones has a composition rich in labile organic mat- ter and nutrients of fluvial origin (Naudin and Cauwet, 1997) that are useful for feeding by benth- ic organisms. However, the high organic carbon content and the high sedimentation rates lead to a limited oxygen penetration in the surficial sediment and the formation of sapropels or layers with high organic carbon contents.
On the outer shelf and shelfbreak zones, the BNL is also fed by organic detritus and carbonate debris of pelagic origin (Naudin and Cauwet, 1997). This nepheloid layer has become the main food supply for benthic organisms (Fèral et al., 1990).
The continental slope represents the step zone between the outer shelf and base of slope or the basin plain. The steepness of the slope and its rela- tion to the continental shelf serves to define the three main types of continental margins: progressive, intermediate and abrupt (Stanley, 1977). Margins located in front of or near to the main rivers are of the progressive type, showing a continuity of seis- mic reflectors.
Deposits on the continental slope are mainly fine-grained sediments. These sediments form a variety of deposits caused by various transport processes, from settling of suspensions to mass- gravity flows. Stanley and Maldonado (1981) pro- pose a by-pass model of fine-grained sediments deposited by gravitational processes, which produce a progressively less dense flow (from slumps to low concentration turbid layers) and a final sediment
characterised by a textural (unifites) on the basin plains.
Landsliding is of prime importance in shaping and mobilising huge quantities of sediment downs- lope on both active and passive continental margins. Landsliding modifies the distribution pattern of canyon-channel systems and channel-levee com- plexes and thus the sedimentary architecture of con- tinental margins (Canals et al., 2000).
Swath bathymetry techniques in addition to very high resolution reflection seismic profiles, side-