SECT. 3] ESTUAKIES, DELTAS, SHELF, SLOPE 641 



On the shelf off Trinidad, Venezuela and Western Guiana (Van Andel and 

 Postma, 1954; Koldewijn, 1958; Nota, 1958), the distribution is much the same 

 as to the east of the Mississippi delta (Fig. 18). Pelitic sediments, including 

 more than 30% of terrigenous particles finer than 50 [i and less than 5% of 

 sand, are found in the median part of the shelf off the Orinoco River and near 

 shore to the north of Trinidad and of the Peninsula of Paria, while sediments 

 including 5% to more than 30% of sand occur on the outer part of the shelf, down 

 to 100 fm at least. It is believed that the finer sediments are now being supplied 

 by the Orinoco River and transported to the west by the North Equatorial 

 Current, whereas offshore sands have been deposited during one or several 

 low glacial sea-levels; since that time, fine particles coming from the Orinoco 

 River have been added to the sands on the outer shelf, but they do not conceal 

 the older deposits. The study of the heavy minerals off Trinidad and Tobago 

 demonstrates that the coarser fraction on the outer shelf cannot have been 

 supplied at the present time, because the assortment of minerals is different from 

 that existing on Trinidad and Tobago beaches: it has come either from local 

 sources on the shelf or from the Orinoco, which flowed into the sea during the 

 lowerings of sea-level farther north than now. 



In the Mediterranean off south-eastern France (Blanc, 1958b; Nesteroff, 

 1958), the distribution resembles more, it is true, the classic pattern described 

 in old textbooks (Fig. 19). Terrigenous muds supplied by the continent occur 

 on an average deeper than 30 fm down to great depths, while, closer to the shore, 

 the sediments consist of terrigenous or shell sands, often covered with dense 

 meadows of sea-grasses, Cymodoceae or Posidoniae, which trap the particles 

 in their rhizomes. But it must be noted that, even in this area, the classic 

 distribution is disturbed by the occurrence of a large number of small pebbles, 

 widely scattered among the muds and muddy sands of the shelf and consisting 

 of quartz, sandstones, phyllites, variohtes, etc. They have been dredged mostly 

 from gently sloping grounds beginning at 40 fm and extending down to 60 fm, 

 but others have been discovered (Bourcart) at much greater depths. In the 

 Bays of Marseilles and Cassis, they have encrustations which are associated 

 with thanatocoenoses, usually associated with shallow waters. Although their 

 age is not well known, they might represent fiuviatile or coastal deposits 

 belonging to the Pliocene or Lower Pleistocene. Be that as it may, they are 

 neither of present nor of recent age, and they show that present-day sedimenta- 

 tion on the shelf has not been thick enough to bury them deeply. 



Obviously this remark also applies to other coarse sediments quoted above: 

 since old deposits still lie at the surface near the edge of the shelf, it must be 

 concluded that the rate of sedimentation is very low or nil there today. On the 

 other hand the deposition has been very considerable in many inner parts of 

 the shelf. In the Gulf of Paria, Venezuela, Van Andel and Postma conclude 

 tentatively that it ranges from 12 cm to 100 cm per century for the last 700 

 years, except at and near the inlets of the bay where scouring by currents has 

 kept the bottom free from deposition. Off" Trinidad, figures ranging from 1.2 

 cm to 27 cm per century have been found by radiocarbon analyses for the inner 



