SECT. 2] 



TOPOGKAPHY OF THE DEEP-SEA FLOOR 



237 



A. Continental Margins 



a. Physiographic provinces 



The continental margin inchides those provinces associated vv^ith the transi- 

 tion from continent to ocean floor. The continental margin in the Atlantic and 

 Indian Oceans is generally composed of continental shelf, continental slope, 

 and continental rise. A typical profile off northeastern United States is shown 

 in Fig. 5. Gradients on the continental shelf average 1 : 1000, while on the 

 continental slope, gradients range from 1 : 40 to 1:6, and occasionally local 

 slopes approach the vertical. The continental rise (or continental apron) lies at 

 the base of the continental slope. Continental rise gradients average 1 : 300, 

 but individual slope segments may be as low as 1 : 700, or as steep as 1 : 50. 



— I- 



n 



m 



-1000 — 



-2000fm 



'^3000 



-4000 



Sahara 



■ New York 



-Bay of Biscay 



- Blake Plateau 



Puerto Rico 



Fig. 6. Three categories of continental margin provinces. Category I provinces lie on the 

 continental block, Category II provinces form the side of the continental block, 

 and Category III provinces are the vipturned or depressed margins of the oceanic 

 depression. (After Heezen et al., 1959.) 



The continental slopes are cut by many submarine canyons. Some of the larger 

 canyons, such as the Hudson, Monterey and Congo, extend across the con- 

 tinental rise (Fig. 7). Submarine alluvial fans extend out from the seaward ends 

 of the larger canyons. The continental shelf and continental slope form the 

 upper and lateral surfaces of the continental terrace (Dietz and Menard, 1951). 

 The continental margin can be divided into three categories of provinces 

 (Fig. 6). Category I includes the continental shelf, marginal plateaus and 

 shallow epicontinental seas, all slightly submerged portions of the continental 

 block. Category II includes the continental slope, marginal escarpments and 

 the landward slopes of marginal trenches, all expressions of the outer edge of 

 the continental block. Category III includes the continental rise, marginal 

 basin-outer ridge complex and the marginal trench-outer ridge complex. 

 The continental slope of northeastern United States can be traced directly into 

 the marginal escarpment (Blake Escarpment) off southeastern United States 

 (Fig. 5) and the landward slope of the Antilles marginal trench (Puerto Rico 

 Trench). The lower continental rise off New England can be traced into the 



Antilles Outer Ridge. Seismic-refraction studies show that a trench filled with 

 9 — s. Ill 



