RIASSUNTO
ABSTRACT
Important quantities of sand and gravel lay on the French continental shelf, particularly in the Channel and on the Atlantic coast.
The extracting activities have been increasing in the last ten years. However, only very small companies using rather small dredges carry on their activities.
Mining licenses are delivered by the Ministry of Industry for prospecting, research and exploitation of marine aggregates.
Mining licenses and extraction permits require a unique file including an evaluation of the field of the consequences of the extraction on marine environment.
RESOURCES AND RESERVES OF THE FRENCH CONTINENTAL SHELF
With its 4000 km of coast and the extension of the economical area up to 200 nautical from its territory, France presents a wide maritime facade. It is only in 1968 that a legislation concerning submerged mineral has been elaborated. Among these substances, there is an important quantity of sand and gravel on the French continental shelf, particularly in Channel and Atlantic.
These formations composed of sand and gravel (siliceous and calcareous) in variable rate, in which clayed beds, full of fragmented tests of marine organisms may lay , represents two distinct kinds according to their continental or marine process of accumulation.
The alluvial deposits from fluvial origin have been created during the regressions of the Quaternaries-age. All-Important part of water being transformed into ice on the condiments, the continental shelves have been emerged. So, the rivers flow lengthened causing an important erosion for finding again the equalization profile. This phenomenon has allowed the important accumulation of deposits in the valleys which are now submerged by the marine transgression. At the present time, Terri gene detritus coming from the rivers on the shelves are mainly silt and fine sand. In this sand, we find an important fauna which produces calcareous particles; those particles disturb the grain size of the deposit and increase the carbonate percentage when gathering on the spot.
The marine deposits form the superficial part of the continental shelf and recover continental alluviums of pale valley. Those deposits come from the alteration by the sea, of deep-seated sediments and arc redistributed according to currents. This new structure may be carried out under the shape of a simple accumulation, a dune or a spreading system.
These formations may be thick and then create hydraulically dunes, some of which having an important part of shell fragments ; owing to this high calcareous percentage, the project of utilizing those sediments for concrete, has been elaborated.
CNEXO has undertaken the investigation and the evaluation of usable aggregates resources on the French continental shelf. The inveterate resources which collect all the accumulations located at less than 50 m (under the lowest level of waters) are estimated to 30 milliards of m3 of sand and gravel (siliceous and calcareous). These workable reserves are much weaker, several conditions being required in order to make resources economically interesting:
- the height of water above located deposits must allow~ the dredging with available equipments (35 m of water)