252 HEEZEN AND MENARD [CHAP. 12 



Pacific. Thus, we may anticipate, just as the Appalachian geosynchne is now 

 part of the North American continent and as the East China Sea is now virtually 

 part of the Chinese continental shelf, that the Ja"i)an, Okhotsk and Bering Seas 

 will also become incorporated in the Asian continent in some future geologic 

 period. 



The Bering Sea is composed of two parts, the broad continental shelf ex- 

 tending from the mainland of Alaska, and the deeper southwestern part lying 

 north of the Aleutian Islands between Dutch Harbor and Attu Island. An 

 abyssal plain with sediment several kilometers thick lies in the deepest part 

 of the Bering Sea, bearing evidence that the sea is being filled by turbidity 

 currents. The floor of the Okhotsk Sea is similar to that of the Bering Sea except 

 that the deeper portion covers a much smaller area. It, too, is occupied by an 

 abyssal plain. The minor relief features of the Okhotsk Sea have been described 

 in some detail by Udintsev (1957), who has proposed a complex genetic 

 classification of the minor rehef forms of the floor of the Okhotsk Sea. 



i. Margins of a subcontinent 



The area east of Australia and north of New Zealand, extending to a few 

 degrees north of the equator, is part continental and part oceanic in 

 character. It lies from a few hundred to a few thousand meters below sea-level ; 

 the deeps have an oceanic crust and the banks have a continental one. Although 

 the region differs markedly in surface relief from the continents, its margins 

 with the ocean basins are equally sharp. 



To the east lie the Tonga and Kermadec Trenches, and the block-faulted 

 region is modified into a double island arc very similar to the Marianas 

 Islands except for curvature. To the north the edge of the region is marked 

 by a dense grouj^ of submerged atolls such as Turpie and Alexa Bank rising 

 from a region of relatively shallow depths. An apparently normal, smooth 

 continental rise exists along the east part of the northern boundary of the 

 region, but to the west, toward the Santa Cruz Islands, the base of the 

 slope is lined with deep troughs such as the Cape Johnson and Vitiaz Troughs. 

 This little known region may provide a perfect example of a transition between 

 an oceanic trench and a continental rise. 



j. The Mediterranean 



The Mediterranean Sea, which separates Europe and Africa, has many of the 

 characteristics of an ocean basin. The continental shelves are moderately well 

 developed in the western Mediterranean. The continental slopes, which seem to 

 be of the same form as those of the Atlantic, are cut by submarine canyons 

 that appear to have the same characteristics as submarine canyons found else- 

 where in the world. The floor of the western Mediterranean is occupied by 

 the Balearic Abyssal Plain. Judging from the gradients in depth of the basin, 

 the plain has been dominated by deposition from the Rhone and the submarine 

 canyons of Alpes-Maritimes. However, deposition from submarine canyons 

 along the Algerian coast has also occurred. 



