Observations sur les rides sous-marines du plateau continental

Observations on sand dunes on the continental shelf

¹ Centre Océanologique de Bretagne, Brest
² Rennes
³ C.N.R.S., Station biologique, Roscoff

Abstract

I. - Introduction

The growing economic interest shown in the shallower regions of the continental shelf explains the present need to know more about the contribution processes of the various sedimentary deposits.

II. - Morphological forms of sedimentary accumulation

The regions studied (Figure 1) permitted investigation of the various possible forms of accumulation, from the large sand bodies (Straits of Dover ; La Chapelle bank) to the sand ribbons present at the south-western end of the Channel. The large sand bodies of the Bassurelle running parallel to the principal tidal currents are flanked by large sand dunes whose crests are approximately parallel to the current. The ripple-marks seen on the surface of the dunes are due to present-day movement, as proved by underwater photographs. However, a colony of Lanice Conchilega of uniform age and short life cycle (2-2½ years) points to a period of stability following the probable occurence of a meteorological disturbance. In the south-western region of the Channel, measurement of tidal currents shows that currents are predominantly south-westerly, except to the south of the Bank ar Forest where a south-easterly direction has been observed. Two sectors (I 15 and I 20) illustrate the transition between the Paleozoic substratum and the Lutetian sedimentary platform covered by pebble deposits and sandbanks, e.g. Trezen Vraz and Bank ar Forest (Figure 6). Figure 9 gives an example of a "Barkhan"-type crescent dune found in the southern pan of the western Channel, to the north of the sand ribbon zone. The dunes on La Chapelle bank at a depth of 165 m (Figures 10 and 11) appear to be due to movement towards the south-west. The large sand bodies themselves are probably now quite stable.

Conclusion

Measurement of tidal currents and theoretical studies on the effect of waves in a number of sectors have provided an estimate (Table 1) of maximum probable sediment transport values (Bonneville, Sternberg). These are quite high, and range from 27.7 to 0.65 kg/m/s. It is clear that small differences in maximum current velocity give rise to large differences in the mass transport value.