13. CONTINENTAL SHELF AND SLOPE 

 (CONTINENTAL MARGIN) 



A. GUILCHER 



1. Definitions 



The continental shelf is usually defined as the platform extending off the 

 continents down to 100 fm, or 200 m ; and the continental slope, according to 

 the International Committee on the Nomenclature of Ocean Bottom Features 

 (Wiseman and Ovey, 1953), is "the declivity from the outer edge of the con- 

 tinental shelf into great depths". If the foot of the slope is conventionally taken 

 at 2000 m, the shelf and the slope cover respectively 7.6 and 8.5% of the 

 ocean floor. Shelf and slope are parts of the continental margin, and they will 

 be described and discussed together in this section, since they combine to form 

 prominences off the continents and the problems involved are the same. 



In fact, the definitions of these features cannot be very accurate for several 

 reasons. First, the outer edge of the shelf does not always lie at the same depth. 

 Shepard (1948) has shown that the average depth is 72 fm, so that the conven- 

 tional definition is too comprehensive in most areas. But some continental 

 shelves, especially in high latitudes, extend to depths much greater than 

 100 fm. The most typical case is the shelf around Antarctica, which commonly 

 extends down to 300 fm or even more, especially in the southern Indian Ocean 

 (Jivago and Lissitzin, 1957 and 1958). Off Norway, the edge between 64° and 

 67° 20'N is found from 280 to 410 m (H. Holtedahl, 1955, p. 45), and ab- 

 normally large depths are also found along parts of Greenland and in the 

 Barents Sea (Frebold, 1950, p. 12). On the other hand, the shelf edge in the 

 Beaufort Sea and the East Chukchi Sea, which were apparently not covered by 

 ice-caps during the Pleistocene, is only 35 fm deep (Carsola, 1954). In other 

 regions,, two or several steps occur after a first edge is reached, and when the 

 steps are wide it is difficult to decide if the first break of slope or the second one 

 should be taken as the boundary of the shelf. The term marginal 'plateau was 

 introduced by Heezen, Tharp and Ewing (1959) for such large steps at depths 

 greater than 100 fm and less than 1200 fm (e.g. the Blake Plateau off Georgia, 

 and the marginal plateau off southern Argentina). 



Bourcart states in several papers (e.g. 1954) that no true edge exists in many 

 areas between the shelf and the slope, the former merging into the latter in a 

 convex curve. According to his investigations in the Mediterranean, this is 

 the general situation off southern France. A very smooth, ill-defined, convex 

 edge is also found along Porcupine Bank off Ireland (Hill, 1956). However, it 

 must be noticed that such a feature remains quite exceptional, and the edge of 

 the shelf, if not always very sharp, is generally easily distinguished on the 

 echograms. 



The shelves surrounding very large islands which lie in the deep sea, such as 

 Madagascar, are true continental shelves, because these islands are small 

 continents. 

 [MS received June, I960]. 281 



